Help-Site Computer Manuals
Software
Hardware
Programming
Networking
  Algorithms & Data Structures   Programming Languages   Revision Control
  Protocols
  Cameras   Computers   Displays   Keyboards & Mice   Motherboards   Networking   Printers & Scanners   Storage
  Windows   Linux & Unix   Mac

NAME

NAME


NAME

Input and Output Filters


Description

This chapter discusses mod_perl's input and output filter handlers.

If all you need is to lookup the filtering API proceed directly to the Apache2::Filter|docs::2.0::api::Apache2::Filter and Apache2::FilterRec|docs::2.0::api::Apache2::FilterRec manpages.


Your First Filter

You certainly already know how filters work. That's because you encounter filters so often in real life. If you are unfortunate to live in smog-filled cities like Saigon or Bangkok you are probably used to wear a dust filter mask:

dust mask

If you are smoker, chances are that you smoke cigarettes with filters:

cigarrette filter

If you are a coffee gourmand, you have certainly tried a filter coffee:

coffee machine

The shower that you use, may have a water filter:

shower filter

When the sun is too bright, you protect your eyes by wearing sun goggles with UV filter:

sun goggles

If are a photographer you can't go a step without using filter lenses:

photo camera

If you love music, you might be unaware of it, but your super-modern audio system is literally loaded with various electronic filters:

LP player

There are many more places in our lives where filters are used. The purpose of all filters is to apply some transformation to what's coming into the filter, letting something different out of the filter. Certainly in some cases it's possible to modify the source itself, but that makes things unflexible, and but most of the time we have no control over the source. The advantage of using filters to modify something is that they can be replaced when requirements change Filters also can be stacked, which allows us to make each filter do simple transformations. For example by combining several different filters, we can apply multiple transformations. In certain situations combining several filters of the same kind let's us achieve a better quality output.

The mod_perl filters are not any different, they receive some data, modify it and send it out. In the case of filtering the output of the response handler, we could certainly change the response handler's logic to do something different, since we control the response handler. But this may make the code unnecessary complex. If we can apply transformations to the response handler's output, it certainly gives us more flexibility and simplifies things. For example if a response needs to be compressed before sent out, it'd be very inconvenient and inefficient to code in the response handler itself. Using a filter for that purpose is a perfect solution. Similarly, in certain cases, using an input filter to transform the incoming request data is the most wise solution. Think of the same example of having the incoming data coming compressed.

Just like with real life filters, you can pipe several filters to modify each other's output. You can also customize a selection of different filters at run time.

Without much further ado, let's write a simple but useful obfuscation filter for our HTML documents.

We are going to use a very simple obfuscation -- turn an HTML document into a one liner, which will make it harder to read its source without a special processing. To accomplish that we are going to remove characters \012 (\n) and \015 (\r), which depending on the platform alone or as a combination represent the end of line and a carriage return.

And here is the filter handler code:


  #file:MyApache2/FilterObfuscate.pm

  #--------------------------------

  package MyApache2::FilterObfuscate;

  

  use strict;

  use warnings;

  

  use Apache2::Filter ();

  use Apache2::RequestRec ();

  use APR::Table ();

  

  use Apache2::Const -compile => qw(OK);

  

  use constant BUFF_LEN => 1024;

  

  sub handler {

      my $f = shift;

  

      unless ($f->ctx) {

          $f->r->headers_out->unset('Content-Length');

          $f->ctx(1);

      }

  

      while ($f->read(my $buffer, BUFF_LEN)) {

          $buffer =~ s/[\r\n]//g;

          $f->print($buffer);

      }

  

      return Apache2::Const::OK;

  }

  1;

Next we configure Apache to apply the MyApache2::FilterObfuscate filter to all requests that get mapped to files with an ``.html'' extension:


  <Files ~ "\.html">

      PerlOutputFilterHandler MyApache2::FilterObfuscate

  </Files>

Filter handlers are similar to HTTP handlers, they are expected to return Apache2::Const::OK or Apache2::Const::DECLINED, but instead of receiving $r (the request object) as the first argument, they receive $f (the filter object).

The filter starts by unsetting of the Content-Length response header, because it modifies the length of the response body (shrinks it). If the response handler had set the Content-Length header and the filter hasn't unset it, the client may have problems receiving the response since it'd expect more data than it was sent.

The core of this filter is a read-modify-print expression in a while loop. The logic is very simple: read at most BUFF_LEN characters of data into $buffer, apply the regex to remove any occurences of \n and \r in it, and print the resulting data out. The input data may come from a response handler, or from an upstream filter. The output data goes to the next filter in the output chain. Even though in this example we haven't configured any more filters, internally Apache by itself uses several core filters to manipulate the data and send it out to the client.

As we are going to explain in great detail in the next sections, the same filter may be called many times during a single request, every time receiving a chunk of data. For example if the POSTed request data is 64k long, an input filter could be invoked 8 times, each time receiving 8k of data. The same may happen during response phase, where an upstream filter may split 64k output in 8 8k chunks. The while loop that we just saw is going to read each of these 8k in 8 calls, since it requests 1k on every read() call.

Since it's enough to unset the Content-Length header when the filter is called the first time, we need to have some flag telling us whether we have done the job. The method ctx() provides this functionality:


      unless ($f->ctx) {

          $f->r->headers_out->unset('Content-Length');

          $f->ctx(1);

      }

the unset() call will be made only on the first filter call for each request. Of course you can store any kind of a Perl data structure in $f->ctx|docs::2.0::api::Apache2::Filter/C_ctx_ and retrieve it later in subsequent filter invocations of the same request. We will show plenty of examples using this method in the following sections.

Of course the MyApache2::FilterObfuscate filter logic should take into account situations where removing new line characters will break the correct rendering, as is the case if there are multi-line <pre>...</pre> entries, but since it escalates the complexity of the filter, we will disregard this requirement for now.

A positive side effect of this obfuscation algorithm is in shortening the amount of the data sent to the client. If you want to look at the production ready implementation, which takes into account the HTML markup specifics, the Apache::Clean module, available from CPAN, does just that.

mod_perl I/O filtering follows the Perl's principle of making simple things easy and difficult things possible. You have seen that it's trivial to write simple filters. As you read through this tutorial you will see that much more difficult things are possible, even though a more elaborated code will be needed.


I/O Filtering Concepts

Before introducing the APIs, mod_perl provides for Apache Filtering, there are several important concepts to understand.

Two Methods for Manipulating Data

Apache 2.0 considers all incoming and outgoing data as chunks of information, disregarding their kind and source or storage methods. These data chunks are stored in buckets, which form bucket brigades. Input and output filters massage the data in bucket brigades. Response and protocol handlers also receive and send data using bucket brigades, though in most cases this is hidden behind wrappers, such as read() and print().

mod_perl 2.0 filters can directly manipulate the bucket brigades or use the simplified streaming interface where the filter object acts similar to a filehandle, which can be read from and printed to.

Even though you don't use bucket brigades directly when you use the streaming filter interface (which works on bucket brigades behind the scenes), it's still important to understand bucket brigades. For example you need to know that an output filter will be invoked as many times as the number of bucket brigades sent from an upstream filter or a content handler. Or you need to know that the end of stream indicator (EOS) is sometimes sent in a separate bucket brigade, so it shouldn't be a surprise that the filter was invoked even though no real data went through. As we delve into the filter details you will see that understanding bucket brigades, will help to understand how filters work.

Moreover you will need to understand bucket brigades if you plan to implement protocol modules.

HTTP Request Versus Connection Filters

HTTP request filters are applied when Apache serves an HTTP request.

HTTP request input filters get invoked on the body of the HTTP request only if the body is consumed by the content handler. HTTP request headers are not passed through the HTTP request input filters.

HTTP response output filters get invoked on the body of the HTTP response if the content handler has generated one. HTTP response headers are not passed through the HTTP response output filters.

Connection level filters are applied at the connection level.

A connection may be configured to serve one or more HTTP requests, or handle other protocols. Connection filters see all the incoming and outgoing data. If an HTTP request is served, connection filters can modify the HTTP headers and the body of request and response. If a different protocol is served over connection (e.g. IMAP), the data could have a completely different pattern, than the HTTP protocol (headers + body).

Apache supports several other filter types, which mod_perl 2.0 may support in the future.

Multiple Invocations of Filter Handlers

Unlike other Apache handlers, filter handlers may get invoked more than once during the same request. Filters get invoked as many times as the number of bucket brigades sent from an upstream filter or a content provider.

For example if a content generation handler sends a string, and then forces a flush, following by more data:


  # assuming buffered STDOUT ($|==0)

  $r->print("foo");

  $r->rflush;

  $r->print("bar");

Apache will generate one bucket brigade with two buckets (there are several types of buckets which contain data, one of them is transient):


  bucket type       data

  ----------------------

  1st    transient   foo

  2nd    flush

and send it to the filter chain. Then assuming that no more data was sent after print("bar"), it will create a last bucket brigade containing data:


  bucket type       data

  ----------------------

  1st    transient   bar

and send it to the filter chain. Finally it'll send yet another bucket brigade with the EOS bucket indicating that there will be no more data sent:


  bucket type       data

  ----------------------

  1st    eos

META: EOS buckets are valid for Request filters. For Connection filters, you will get one only in the response filters only at the end of the connection. See the trick how to workaround this in Apache2::Filter::HTTPHeadersFixup. Need to mention that in a few other places in this doc.

Notice that the EOS bucket may come attached to the last bucket brigade with data, instead of coming in its its own bucket brigade. Filters should never make an assumption that the EOS bucket is arriving alone in a bucket brigade. Therefore the first output filter will be invoked two or three times (three times if EOS is coming in its own brigade), depending on the number of bucket brigades sent by the response handler.

A user may install an upstream filter, and that filter may decide to insert extra bucket brigades or collect all the data in all bucket brigades passing through it and send it all down in one brigade. What's important to remember is when coding a filter, one should never assume that the filter is always going to be invoked once, or a fixed number of times. Neither one can make assumptions on the way the data is going to come in. Therefore a typical filter handler may need to split its logic in three parts.

Jumping ahead we will show some pseudo-code that represents all three parts. This is how a typical stream-oriented filter handler looks like:


  sub handler {

      my $f = shift;

  

      # runs on first invocation

      unless ($f->ctx) {

          init($f);

          $f->ctx(1);

      }

  

      # runs on all invocations

      process($f);

  

      # runs on the last invocation

      if ($f->seen_eos) {

          finalize($f);

      }

  

      return Apache2::Const::OK;

  }

  sub init     { ... }

  sub process  { ... }

  sub finalize { ... }

The following diagram depicts all three parts:

filter flow logic

Let's explain each part using this pseudo-filter.

  1. Initialization
  2. During the initialization, the filter runs all the code that should be performed only once across multiple invocations of the filter (this is during a single request). The filter context is used to accomplish that task. For each new request the filter context is created before the filter is called for the first time and its destroyed at the end of the request.
    
          unless ($f->ctx) {
    
              init($f);
    
              $f->ctx(1);
    
          }

    When the filter is invoked for the first time $f->ctx|docs::2.0::api::Apache2::Filter/C_ctx_ returns undef and the custom function init() is called. This function could, for example, retrieve some configuration data, set in httpd.conf or initialize some datastructure to its default value.

    To make sure that init() won't be called on the following invocations, we must set the filter context before the first invocation is completed:

    
              $f->ctx(1);

    In practice, the context is not just served as a flag, but used to store real data. For example the following filter handler counts the number of times it was invoked during a single request:

    
      sub handler {
    
          my $f = shift;
    
      
    
          my $ctx = $f->ctx;
    
          $ctx->{invoked}++;
    
          $f->ctx($ctx);
    
          warn "filter was invoked $ctx->{invoked} times\n";
    
      
    
          return Apache2::Const::DECLINED;
    
      }

    Since this filter handler doesn't consume the data from the upstream filter, it's important that this handler returns Apache2::Const::DECLINED, in which case mod_perl passes the current bucket brigade to the next filter. If this handler returns Apache2::Const::OK, the data will be simply lost. And if that data included a special EOS token, this may wreck havoc.

    Unsetting the Content-Length header for filters that modify the response body length is a good example of the code to be used in the initialization phase:

    
      unless ($f->ctx) {
    
          $f->r->headers_out->unset('Content-Length');
    
          $f->ctx(1);
    
      }

    We will see more of initialization examples later in this chapter.

  3. Processing
  4. The next part:
    
          process($f);

    is unconditionally invoked on every filter invocation. That's where the incoming data is read, modified and sent out to the next filter in the filter chain. Here is an example that lowers the case of the characters passing through:

    
      use constant READ_SIZE  => 1024;
    
      sub process {
    
          my $f = shift;
    
          while ($f->read(my $data, READ_SIZE)) {
    
              $f->print(lc $data);
    
          }
    
      }

    Here the filter operates only on a single bucket brigade. Since it manipulates every character separately the logic is really simple.

    In more complicated filters the filters may need to buffer data first before the transformation can be applied. For example if the filter operates on html tokens (e.g., '<img src=``me.jpg''>'), it's possible that one brigade will include the beginning of the token ('<img ') and the remainder of the token ('src=``me.jpg''>') will come in the next bucket brigade (on the next filter invocation). In certain cases it may involve more than two bucket brigades to get the whole token. In such a case the filter will have to store the remainder of unprocessed data in the filter context and then reuse it on the next invocation. Another good example is a filter that performs data compression (compression is usually effective only when applied to relatively big chunks of data), so if a single bucket brigade doesn't contain enough data, the filter may need to buffer the data in the filter context till it collects enough of it.

    We will see the implementation examples in this chapter.

  5. Finalization
  6. Finally, some filters need to know when they are invoked for the last time, in order to perform various cleanups and/or flush any remaining data. As mentioned earlier, Apache indicates this event by a special end of stream ``token'', represented by a bucket of type EOS. If the filter is using the streaming interface, rather than manipulating the bucket brigades directly, and it was calling read() in a while loop, it can check whether this is the last time it's invoked, using the $f->seen_eos method:
    
          if ($f->seen_eos) {
    
              finalize($f);
    
          }

    This check should be done at the end of the filter handler, because sometimes the EOS ``token'' comes attached to the tail of data (the last invocation gets both the data and EOS) and sometimes it comes all alone (the last invocation gets only EOS). So if this test is performed at the beginning of the handler and the EOS bucket was sent in together with the data, the EOS event may be missed and filter won't function properly.

    Jumping ahead, filters, directly manipulating bucket brigades, have to look for a bucket whose type is EOS to accomplish this. We will see examples later in the chapter.

Some filters may need to deploy all three parts of the described logic, others will need to do only initialization and processing, or processing and finalization, while the simplest filters might perform only the normal processing (as we saw in the example of the filter handler that lowers the case of the characters going through it).

Blocking Calls

All filters (excluding the core filter that reads from the network and the core filter that writes to it) block at least once when invoked. Depending on whether this is an input or an output filter, the blocking happens when the bucket brigade is requested from the upstream filter or when the bucket brigade is passed to the downstream filter.

First of all, the input and output filters differ in the ways they acquire the bucket brigades (which includes the data that they filter). Even though when a streaming API is used the difference can't be seen, it's important to understand how things work underneath. Therefore we are going to show examples of transparent filters, which pass data through them unmodified. Instead of reading the data in and printing it out the bucket brigades are now passed as is.

Here is a code for a transparent input filter:


  #file:MyApache2/FilterTransparent.pm (first part)

  #-----------------------------------------------

  package MyApache2::FilterTransparent;

  

  use Apache2::Filter ();

  

  use Apache2::Const -compile => qw(OK);

  use APR::Const     -compile => ':common';

  

  sub in {

      my ($f, $bb, $mode, $block, $readbytes) = @_;

  

      my $rv = $f->next->get_brigade($bb, $mode, $block, $readbytes);

      return $rv unless $rv == APR::Const::SUCCESS;

  

      return Apache2::Const::OK;

  }

When the input filter in() is invoked, it first asks the upstream filter for the next bucket brigade (using the get_brigade() call). That upstream filter is in turn going to ask for the bucket brigade from the next upstream filter in chain, etc., till the last filter (called core_in), that reads from the network is reached. The core_in filter reads, using a socket, a portion of the incoming data from the network, processes it and sends it to its downstream filter, which will process the data and send it to its downstream filter, etc., till it reaches the very first filter who has asked for the data. (In reality some other handler triggers the request for the bucket brigade, e.g., an HTTP response handler, or a protocol module, but for our discussion it's good enough to assume that it's the first filter that issues the get_brigade() call.)

The following diagram depicts a typical input filters chain data flow in addition to the program control flow.

input filter data flow

The black- and white-headed arrows show when the control is switched from one filter to another. In addition the black-headed arrows show the actual data flow. The diagram includes some pseudo-code, both for in Perl for the mod_perl filters and in C for the internal Apache filters. You don't have to understand C to understand this diagram. What's important to understand is that when input filters are invoked they first call each other via the get_brigade() call and then block (notice the brick wall on the diagram), waiting for the call to return. When this call returns all upstream filters have already completed finishing their filtering task.

As mentioned earlier, the streaming interface hides these details, however the first $f->read() call will block, as underneath it performs the get_brigade() call.

The diagram shows a part of the actual input filter chain for an HTTP request, the ... shows that there are more filters in between the mod_perl filter and http_in.

Now let's look at what happens in the output filters chain. Here the first filter acquires the bucket brigades containing the response data, from the content handler (or another protocol handler if we aren't talking HTTP), it then may apply some modification and pass the data to the next filter (using the pass_brigade() call), which in turn applies its modifications and sends the bucket brigade to the next filter, etc., all the way down to the last filter (called core) which writes the data to the network, via the socket the client is listening to. Even though the output filters don't have to wait to acquire the bucket brigade (since the upstream filter passes it to them as an argument), they still block in a similar fashion to input filters, since they have to wait for the pass_brigade() call to return.

Here is an example of a transparent output filter:


  #file:MyApache2/FilterTransparent.pm (continued)

  #-----------------------------------------------

  sub out {

      my ($f, $bb) = @_;

  

      my $rv = $f->next->pass_brigade($bb);

      return $rv unless $rv == APR::Const::SUCCESS;

  

      return Apache2::Const::OK;

  }

  1;

The out() filter passes $bb to the downstream filter unmodified and if you add debug prints before and after the pass_brigade() call and configure the same filter twice, the debug print will show the blocking call.

The following diagram depicts a typical output filters chain data flow in addition to the program control flow:

output filter data flow

Similar to the input filters chain diagram, the arrows show the program control flow and in addition the black-headed arrows show the data flow. Again, it uses a Perl pseudo-code for the mod_perl filter and C pseudo-code for the Apache filters, similarly the brick walls represent the waiting. And again, the diagram shows a part of the real HTTP response filters chain, where ... stands for the omitted filters.


mod_perl Filters Declaration and Configuration

Now let's see how mod_perl filters are declared and configured.

Filter Priority Types

When Apache filters are configured they are inserted into the filters chain according to their priority/type. In most cases when using one or two filters things will just work, however if you find that the order of filter invocation is wrong, the filter priority type should be consulted. Unfortunately this information is available only by consulting the source code, unless it's documented in the module man pages. Numerical definitions of priority types, such as AP_FTYPE_CONTENT_SET, AP_FTYPE_RESOURCE, can be found in include/util_filter.h.

As of this writing Apache comes with two core filters: DEFLATE and INCLUDES. For example in the following configuration:


  SetOutputFilter DEFLATE

  SetOutputFilter INCLUDES

the DEFLATE filter will be inserted in the filters chain after the INCLUDES filter, even though it was configured before it. This is because the DEFLATE filter is of type AP_FTYPE_CONTENT_SET (20), whereas the INCLUDES filter is of type AP_FTYPE_RESOURCE (10).

As of this writing mod_perl provides two kind of filters with fixed priority type (the type is defined by the filter handler's attribute):


  Handler's Attribute      Priority           Value

  -------------------------------------------------

  FilterRequestHandler     AP_FTYPE_RESOURCE    10

  FilterConnectionHandler  AP_FTYPE_PROTOCOL    30

Therefore FilterRequestHandler filters (10) will be always invoked before the DEFLATE filter (20), whereas FilterConnectionHandler filters (30) after it. The INCLUDES filter (10) has the same priority as FilterRequestHandler filters (10), and therefore it'll be inserted according to the configuration order, when PerlSetOutputFilter|/PerlSetOutputFilter or PerlSetInputFilter|/PerlSetInputFilter is used.

PerlInputFilterHandler

The PerlInputFilterHandler directive registers a filter, and inserts it into the relevant input filters chain.

This handler is of type VOID|docs::2.0::user::handlers::intro/item_VOID.

The handler's configuration scope is DIR|docs::2.0::user::config::config/item_DIR.

PerlInputFilterHandler handlers are automatically AutoLoad|docs::2.0::user::config::config/C_AutoLoad_ed, since they need to be compiled before the filter attributes can be accessed. Therefore if the filter handler subroutine is not called handler, you must preload the module containing the filter subroutine at the server startup. A filter handler can be configured not to be AutoLoad|docs::2.0::user::config::config/C_AutoLoad_ed, using the - prefix. For example:


  PerlInputFilterHandler -MyApache2::FilterTest::lc

The following sections include several examples that use the PerlInputFilterHandler handler.

PerlOutputFilterHandler

The PerlOutputFilterHandler directive registers a filter, and inserts it into the relevant output filters chain.

This handler is of type VOID|docs::2.0::user::handlers::intro/item_VOID.

The handler's configuration scope is DIR|docs::2.0::user::config::config/item_DIR.

The following sections include several examples that use the PerlOutputFilterHandler handler.

Similar to PerlInputFilterHandler|/C_PerlInputFilterHandler_ PerlOutputFilterHandler handlers are automatically AutoLoad|docs::2.0::user::config::config/C_AutoLoad_ed.

PerlSetInputFilter

The SetInputFilter directive, documented at http://httpd.apache.org/docs-2.0/mod/core.html#setinputfilter sets the filter or filters which will process client requests and POST input when they are received by the server (in addition to any filters configured earlier).

To mix mod_perl and non-mod_perl input filters of the same priority nothing special should be done. For example if we have an imaginary Apache filter FILTER_FOO and mod_perl filter MyApache2::FilterInputFoo, this configuration:


  SetInputFilter FILTER_FOO

  PerlInputFilterHandler MyApache2::FilterInputFoo

will add both filters, however the order of their invocation might be not the one that you've expected. To make the invocation order the same as the insertion order replace SetInputFilter with PerlSetInputFilter, like so:


  PerlSetInputFilter FILTER_FOO

  PerlInputFilterHandler MyApache2::FilterInputFoo

now FILTER_FOO filter will be always executed before the MyApache2::FilterInputFoo filter, since it was configured before MyApache2::FilterInputFoo (i.e., it'll apply its transformations on the incoming data last). Here is a diagram input filters chain and the data flow from the network to the response handler for the presented configuration:


       response handler

             /\

             ||

         FILTER_FOO

             /\

             ||

   MyApache2::FilterInputFoo

             /\

             ||

     core input filters

             /\

             ||

           network

As explained in the section Filter Priority Types this directive won't affect filters of different priority. For example assuming that MyApache2::FilterInputFoo is a FilterRequestHandler filter, the configurations:


  PerlInputFilterHandler MyApache2::FilterInputFoo

  PerlSetInputFilter DEFLATE

and


  PerlSetInputFilter DEFLATE

  PerlInputFilterHandler MyApache2::FilterInputFoo

are equivalent, because mod_deflate's DEFLATE filter has a higher priority than MyApache2::FilterInputFoo, thefore it'll always be inserted into the filter chain after MyApache2::FilterInputFoo, (i.e. the DEFLATE filter will apply its transformations on the incoming data first). Here is a diagram input filters chain and the data flow from the network to the response handler for the presented configuration:


      response handler

             /\

             ||

   MyApache2::FilterInputFoo

             /\

             ||

          DEFLATE

             /\

             ||

     core input filters

             /\

             ||

           network

SetInputFilter's ; semantics are supported as well. For example, in the following configuration:


  PerlInputFilterHandler MyApache2::FilterInputFoo

  PerlSetInputFilter FILTER_FOO;FILTER_BAR

MyApache2::FilterOutputFoo will be executed first, followed by FILTER_FOO and finally by FILTER_BAR (again, assuming that all three filters have the same priority).

The PerlSetInputFilter directives's configuration scope is DIR|docs::2.0::user::config::config/item_DIR.

PerlSetOutputFilter

The SetOutputFilter directive, documented at http://httpd.apache.org/docs-2.0/mod/core.html#setoutputfilter sets the filters which will process responses from the server before they are sent to the client (in addition to any filters configured earlier).

To mix mod_perl and non-mod_perl output filters of the same priority nothing special should be done. This configuration:


  SetOutputFilter INCLUDES

  PerlOutputFilterHandler MyApache2::FilterOutputFoo

will add all two filters to the filter chain, however the order of their invocation might be not the one that you've expected. To preserve the insertion order replace SetOutputFilter with PerlSetOutputFilter, like so:


  PerlSetOutputFilter INCLUDES

  PerlOutputFilterHandler MyApache2::FilterOutputFoo

now mod_include's INCLUDES filter will be always executed before the MyApache2::FilterOutputFoo filter. Here is a diagram input filters chain and the data flow from the response handler to the network for the presented configuration:


      response handler

             ||

             \/

          INCLUDES

             ||

             \/

   MyApache2::FilterOutputFoo

             ||

             \/

     core output filters

             ||

             \/

           network

SetOutputFilter's ; semantics are supported as well. For example, in the following configuration:


  PerlOutputFilterHandler MyApache2::FilterOutputFoo

  PerlSetOutputFilter INCLUDES;FILTER_FOO

MyApache2::FilterOutputFoo will be executed first, followed by INCLUDES and finally by FILTER_FOO (again, assuming that all three filters have the same priority).

Just as explained in the PerlSetInputFilter|/PerlSetInputFilter section, if filters have different priorities, the insertion order might be different. For example in the following configuration:


  PerlSetOutputFilter DEFLATE

  PerlSetOutputFilter INCLUDES

  PerlOutputFilterHandler MyApache2::FilterOutputFoo

mod_include's INCLUDES filter will be always executed before the MyApache2::FilterOutputFoo filter. The latter will be followed by mod_deflate's DEFLATE filter, even though it was configured before the other two filters. This is because it has a higher priority. And the corresponding diagram looks like so:


      response handler

             ||

             \/

          INCLUDES

             ||

             \/

   MyApache2::FilterOutputFoo

             ||

             \/

           DEFLATE

             ||

             \/

     core output filters

             ||

             \/

           network

The PerlSetOutputFilter directives's configuration scope is DIR|docs::2.0::user::config::config/item_DIR.

Adding OutputFilters Dynamically

If you have the need to add output filters dymically during the request, mod_perl 2.0 offers you the possibility to push filter callbacks at request time. For example here is how to add an output filter during the Fixup phase:


  <Files *\.html >

    PerlFixupHandler MyApache2::AddFilterDyn

  </Files>

And the corresponding module:


  #file:MyApache2/AddFilterDyn.pm

  #------------------------------

  package MyApache2::AddFilterDyn;

  

  use Apache2::Filter;

  use MyApache2::FilterObfuscate;

  

  use Apache2::Const -compile => qw(OK);

  

  sub handler {

      my $r = shift;

  

      $r->add_output_filter(\&MyApache2::FilterObfuscate::handler);

  

      return Apache2::Const::OK;

  }

  

  1;

You can also add connection filters dynamically. For more information refer to the Apache2::Filter|docs::2.0::api::Apache2::Filter manpage: add_input_filter|docs::2.0::api::Apache2::Filter/C_add_input_filter_ and add_output_filter|docs::2.0::api::Apache2::Filter/C_add_output_filter_.

HTTP Request vs. Connection Filters

mod_perl 2.0 supports connection and HTTP request filtering. mod_perl filter handlers specify the type of the filter using the method attributes.

HTTP request filter handlers are declared using the FilterRequestHandler attribute. Consider the following request input and output filters skeleton:


  package MyApache2::FilterRequestFoo;

  use base qw(Apache2::Filter);

  

  sub input  : FilterRequestHandler {

      my ($f, $bb, $mode, $block, $readbytes) = @_;

      #...

  }

  

  sub output : FilterRequestHandler {

      my ($f, $bb) = @_;

      #...

  }

  

  1;

If the attribute is not specified, the default FilterRequestHandler attribute is assumed. Filters specifying subroutine attributes must subclass Apache2::Filter, others only need to:


  use Apache2::Filter ();

The request filters are usually configured in the <Location> or equivalent sections:


  PerlModule MyApache2::FilterRequestFoo

  PerlModule MyApache2::NiceResponse

  <Location /filter_foo>

      SetHandler modperl

      PerlResponseHandler     MyApache2::NiceResponse

      PerlInputFilterHandler  MyApache2::FilterRequestFoo::input

      PerlOutputFilterHandler MyApache2::FilterRequestFoo::output

  </Location>

Now we have the request input and output filters configured.

The connection filter handler uses the FilterConnectionHandler attribute. Here is a similar example for the connection input and output filters.


  package MyApache2::FilterConnectionBar;

  use base qw(Apache2::Filter);

  

  sub input  : FilterConnectionHandler {

      my ($f, $bb, $mode, $block, $readbytes) = @_;

      #...

  }

  

  sub output : FilterConnectionHandler {

      my ($f, $bb) = @_;

      #...

  }

  

  1;

This time the configuration must be done outside the <Location> or equivalent sections, usually within the <VirtualHost> or the global server configuration:


  Listen 8005

  <VirtualHost _default_:8005>

      PerlModule MyApache2::FilterConnectionBar

      PerlModule MyApache2::NiceResponse

   

      PerlInputFilterHandler  MyApache2::FilterConnectionBar::input

      PerlOutputFilterHandler MyApache2::FilterConnectionBar::output

      <Location />

          SetHandler modperl

          PerlResponseHandler MyApache2::NiceResponse

      </Location>

   

  </VirtualHost>

This accomplishes the configuration of the connection input and output filters.

Notice that for HTTP requests the only difference between connection filters and request filters is that the former see everything: the headers and the body, whereas the latter see only the body.

mod_perl provides two interfaces to filtering: a direct bucket brigades manipulation interface and a simpler, stream-oriented interface. The examples in the following sections will help you to understand the difference between the two interfaces.

Filter Initialization Phase

There is one more callback in the filter framework. And that's FilterInitHandler. This init callback runs immediately after the filter handler is inserted into the filter chain, before it was invoked for the first time. Here is a skeleton of an init handler:


  sub init : FilterInitHandler {

      my $f = shift;

      #...

      return Apache2::Const::OK;

  }

The attribute FilterInitHandler marks the Perl function as suitable to be used as a filter initialization callback, which is called immediately after a filter is inserted to the filter chain and before it's actually called.

For example you may decide to dynamically remove a filter before it had a chance to run, if some condition is true:


  sub init : FilterInitHandler {

      my $f = shift;

      $f->remove() if should_remove_filter();

      return Apache2::Const::OK;

  }

Not all Apache2::Filter methods can be used in the init handler, because it's not a filter. Hence you can use methods that operate on the filter itself, such as remove()|docs::2.0::api::Apache2::Filter/C_remove_ and ctx()|docs::2.0::api::Apache2::Filter/C_ctx_ or retrieve request information, such as r()|docs::2.0::api::Apache2::Filter/C_r_ and c()|docs::2.0::api::Apache2::Filter/C_c_. But not methods that operate on data, such as read()|docs::2.0::api::Apache2::Filter/C_read_ and print()|docs::2.0::api::Apache2::Filter/C_print_.

In order to hook an init filter handler, the real filter has to assign this callback using the FilterHasInitHandler which accepts a reference to the callback function, similar to push_handlers(). The used callback function has to have the FilterInitHandler attribute. For example:


  package MyApache2::FilterBar;

  use base qw(Apache2::Filter);

  sub init   : FilterInitHandler { ... }

  sub filter : FilterRequestHandler FilterHasInitHandler(\&init) {

      my ($f, $bb) = @_;

      # ...

      return Apache2::Const::OK;

  }

While attributes are parsed during the code compilation (it's really a sort of source filter), the argument to the FilterHasInitHandler() attribute is compiled at a later stage once the module is compiled.

The argument to FilterHasInitHandler() can be any Perl code which when eval()'ed returns a code reference. For example:


  package MyApache2::OtherFilter;

  use base qw(Apache2::Filter);

  sub init  : FilterInitHandler { ... }

  

  package MyApache2::FilterBar;

  use MyApache2::OtherFilter;

  use base qw(Apache2::Filter);

  sub get_pre_handler { \&MyApache2::OtherFilter::init }

  sub filter : FilterHasInitHandler(get_pre_handler()) { ... }

Here the MyApache2::FilterBar::filter handler is configured to run the MyApache2::OtherFilter::init init handler.

Notice that the argument to FilterHasInitHandler() is always eval()'ed in the package of the real filter handler (not the init handler). So the above code leads to the following evaluation:


  $init_sub = eval "package MyApache2::FilterBar; get_pre_handler()";

though, this is done in C, using the eval_pv() C call.

META: currently only one initialization callback can be registered per filter handler. If the need to register more than one arises it should be very easy to extend the functionality.


All-in-One Filter

Before we delve into the details of how to write filters that do something with the data, lets first write a simple filter that does nothing but snooping on the data that goes through it. We are going to develop the MyApache2::FilterSnoop handler which can snoop on request and connection filters, in input and output modes.

But first let's develop a simple response handler that simply dumps the request's args and content as strings:


  #file:MyApache2/Dump.pm

  #---------------------

  package MyApache2::Dump;

  

  use strict;

  use warnings;

  

  use Apache2::RequestRec ();

  use Apache2::RequestIO ();

  use Apache2::Filter ();

  use APR::Brigade ();

  use APR::Bucket ();

  

  use Apache2::Const -compile => qw(OK M_POST);

  

  sub handler {

      my $r = shift;

      $r->content_type('text/plain');

  

      $r->print("args:\n", $r->args, "\n");

  

      if ($r->method_number == Apache2::Const::M_POST) {

          my $data = content($r);

          $r->print("content:\n$data\n");

      }

  

      return Apache2::Const::OK;

  }

  

  use Apache2::Connection ();

  

  use Apache2::Const -compile => qw(MODE_READBYTES);

  use APR::Const     -compile => qw(SUCCESS BLOCK_READ);

  

  use constant IOBUFSIZE => 8192;

  

  sub content {

      my $r = shift;

  

      my $bb = APR::Brigade->new($r->pool, $r->connection->bucket_alloc);

  

      my $data = '';

      my $seen_eos = 0;

      do {

          $r->input_filters->get_brigade($bb, Apache2::Const::MODE_READBYTES,

                                         APR::Const::BLOCK_READ, IOBUFSIZE);

  

          for (my $b = $bb->first; $b; $b = $bb->next($b)) {

              if ($b->is_eos) {

                  $seen_eos++;

                  last;

              }

  

              if ($b->read(my $buf)) {

                  $data .= $buf;

              }

  

              $b->remove; # optimization to reuse memory

          }

      } while (!$seen_eos);

  

      $bb->destroy;

  

      return $data;

  }

  

 1;

which is configured as:


  PerlModule MyApache2::Dump

  <Location /dump>

      SetHandler modperl

      PerlResponseHandler MyApache2::Dump

  </Location>

If we issue the following request:


  % echo "mod_perl rules" | POST 'http://localhost:8002/dump?foo=1&bar=2'

the response will be:


  args:

  foo=1&bar=2

  content:

  mod_perl rules

As you can see it simply dumped the query string and the posted data.

Now let's write the snooping filter:


  #file:MyApache2/FilterSnoop.pm

  #----------------------------

  package MyApache2::FilterSnoop;

  

  use strict;

  use warnings;

  

  use base qw(Apache2::Filter);

  use Apache2::FilterRec ();

  use APR::Brigade ();

  use APR::Bucket ();

  use APR::BucketType ();

  

  use Apache2::Const -compile => qw(OK DECLINED);

  use APR::Const     -compile => ':common';

  

  sub connection : FilterConnectionHandler { snoop("connection", @_) }

  sub request    : FilterRequestHandler    { snoop("request",    @_) }

  

  sub snoop {

      my $type = shift;

      my ($f, $bb, $mode, $block, $readbytes) = @_; # filter args

  

      # $mode, $block, $readbytes are passed only for input filters

      my $stream = defined $mode ? "input" : "output";

  

      # read the data and pass-through the bucket brigades unchanged

      if (defined $mode) {

          # input filter

          my $rv = $f->next->get_brigade($bb, $mode, $block, $readbytes);

          return $rv unless $rv == APR::Const::SUCCESS;

          bb_dump($type, $stream, $bb);

      }

      else {

          # output filter

          bb_dump($type, $stream, $bb);

          my $rv = $f->next->pass_brigade($bb);

          return $rv unless $rv == APR::Const::SUCCESS;

      }

  

      return Apache2::Const::OK;

  }

  

  sub bb_dump {

      my ($type, $stream, $bb) = @_;

  

      my @data;

      for (my $b = $bb->first; $b; $b = $bb->next($b)) {

          $b->read(my $bdata);

          push @data, $b->type->name, $bdata;

      }

  

      # send the sniffed info to STDERR so not to interfere with normal

      # output

      my $direction = $stream eq 'output' ? ">>>" : "<<<";

      print STDERR "\n$direction $type $stream filter\n";

  

      my $c = 1;

      while (my ($btype, $data) = splice @data, 0, 2) {

          print STDERR "    o bucket $c: $btype\n";

          print STDERR "[$data]\n";

          $c++;

      }

  }

  1;

This package provides two filter handlers, one for connection and another for request filtering:


  sub connection : FilterConnectionHandler { snoop("connection", @_) }

  sub request    : FilterRequestHandler    { snoop("request",    @_) }

Both handlers forward their arguments to the snoop() function that does the real job. We needed to add these two subroutines in order to assign the two different attributes. Plus the functions pass the filter type to snoop() as the first argument, which gets shifted off @_ and the rest of the @_ are the arguments that were originally passed to the filter handler.

It's easy to know whether a filter handler is running in the input or the output mode. The arguments $f and $bb are always passed, whereas the arguments $mode, $block, and $readbytes are passed only to input filter handlers.

If we are in the input mode, in the same call we retrieve the bucket brigade from the previous filter on the input filters stack and immediately link it to the $bb variable which makes the bucket brigade available to the next input filter when the filter handler returns. If we forget to perform this linking our filter will become a black hole in which data simply disappears. Next we call bb_dump() which dumps the type of the filter and the contents of the bucket brigade to STDERR, without influencing the normal data flow.

If we are in the output mode, the $bb variable already points to the current bucket brigade. Therefore we can read the contents of the brigade right away. After that we pass the brigade to the next filter.

Let's snoop on connection and request filter levels in both directions by applying the following configuration:


  Listen 8008

  <VirtualHost _default_:8008>

      PerlModule MyApache2::FilterSnoop

      PerlModule MyApache2::Dump

  

      # Connection filters

      PerlInputFilterHandler  MyApache2::FilterSnoop::connection

      PerlOutputFilterHandler MyApache2::FilterSnoop::connection

  

      <Location /dump>

          SetHandler modperl

          PerlResponseHandler MyApache2::Dump

          # Request filters

          PerlInputFilterHandler  MyApache2::FilterSnoop::request

          PerlOutputFilterHandler MyApache2::FilterSnoop::request

      </Location>

  

  </VirtualHost>

Notice that we use a virtual host because we want to install connection filters.

If we issue the following request:


  % echo "mod_perl rules" | POST 'http://localhost:8008/dump?foo=1&bar=2'

We get the same response, when using MyApache2::FilterSnoop, because our snooping filter didn't change anything. Though there was a lot of output printed to error_log. We present it all here, since it helps a lot to understand how filters work.

First we can see the connection input filter at work, as it processes the HTTP headers. We can see that for this request each header is put into a separate brigade with a single bucket. The data is conveniently enclosed by [] so you can see the new line characters as well.


  <<< connection input filter

      o bucket 1: HEAP

  [POST /dump?foo=1&bar=2 HTTP/1.1

  ]

  

  <<< connection input filter

      o bucket 1: HEAP

  [TE: deflate,gzip;q=0.3

  ]

  

  <<< connection input filter

      o bucket 1: HEAP

  [Connection: TE, close

  ]

  

  <<< connection input filter

      o bucket 1: HEAP

  [Host: localhost:8008

  ]

  

  <<< connection input filter

      o bucket 1: HEAP

  [User-Agent: lwp-request/2.01

  ]

  

  <<< connection input filter

      o bucket 1: HEAP

  [Content-Length: 14

  ]

  

  <<< connection input filter

      o bucket 1: HEAP

  [Content-Type: application/x-www-form-urlencoded

  ]

  

  <<< connection input filter

      o bucket 1: HEAP

  [

  ]

Here the HTTP header has been terminated by a double new line. So far all the buckets were of the HEAP type, meaning that they were allocated from the heap memory. Notice that the HTTP request input filters will never see the bucket brigades with HTTP headers, as it has been consumed by the last core connection filter.

The following two entries are generated when MyApache2::Dump::handler reads the POSTed content:


  <<< connection input filter

      o bucket 1: HEAP

  [mod_perl rules]

  

  <<< request input filter

      o bucket 1: HEAP

  [mod_perl rules]

      o bucket 2: EOS

  []

as we saw earlier on the diagram, the connection input filter is run before the request input filter. Since our connection input filter was passing the data through unmodified and no other custom connection input filter was configured, the request input filter sees the same data. The last bucket in the brigade received by the request input filter is of type EOS, meaning that all the input data from the current request has been received.

Next we can see that MyApache2::Dump::handler has generated its response. However we can see that only the request output filter gets run at this point:


  >>> request output filter

      o bucket 1: TRANSIENT

  [args:

  foo=1&bar=2

  content:

  mod_perl rules

  ]

This happens because Apache hasn't sent yet the response HTTP headers to the client. The request filter sees a bucket brigade with a single bucket of type TRANSIENT which is allocated from the stack memory.

The moment the first bucket brigade of the response body has entered the connection output filters, Apache injects a bucket brigade with the HTTP headers. Therefore we can see that the connection output filter is filtering the brigade with HTTP headers (notice that the request output filters don't see it):


  >>> connection output filter

      o bucket 1: HEAP

  [HTTP/1.1 200 OK

  Date: Tue, 07 Mar 2006 10:59:08 GMT

  Server: Apache/2.0.55 (Unix) mod_perl/2.000002

  Perl/v5.8.4 mod_ssl/2.0.55 OpenSSL/0.9.7c DAV/2

  Connection: close

  Transfer-Encoding: chunked

  Content-Type: text/plain; charset=ISO-8859-1

  

  ]

and followed by the first response body's brigade:


  >>> connection output filter

      o bucket 1: TRANSIENT

  [2b

  ]

      o bucket 2: TRANSIENT

  [args:

  foo=1&bar=2

  content:

  mod_perl rules

  

  ]

      o bucket 3: IMMORTAL

  [

  ]

If the response is large, the request and connection filters will filter chunks of the response one by one.

META: what's the size of the chunks? 8k?

Finally, Apache sends a series of the bucket brigades to finish off the response, including the end of stream meta-bucket to tell filters that they shouldn't expect any more data, and flush buckets to flush the data, to make sure that any buffered output is sent to the client:


  >>> connection output filter

      o bucket 1: IMMORTAL

  [0

  

  ]

      o bucket 2: EOS

  []

  

  >>> connection output filter

      o bucket 1: FLUSH

  []

  

  >>> connection output filter

      o bucket 1: FLUSH

  []

This module helps to understand that each filter handler can be called many time during each request and connection. It's called for each bucket brigade.

Also it's important to mention that HTTP request input filters are invoked only if there is some POSTed data to read and it's consumed by a content handler.


Input Filters

mod_perl supports Connection and HTTP Request input filters:

Connection Input Filters

Let's say that we want to test how our handlers behave when they are requested as HEAD requests, rather than GET. We can alter the request headers at the incoming connection level transparently to all handlers.

This example's filter handler looks for data like:


  GET /perl/test.pl HTTP/1.1

and turns it into:


  HEAD /perl/test.pl HTTP/1.1

The following input filter handler does that by directly manipulating the bucket brigades:


  #file:MyApache2/InputFilterGET2HEAD.pm

  #-----------------------------------

  package MyApache2::InputFilterGET2HEAD;

  

  use strict;

  use warnings;

  

  use base qw(Apache2::Filter);

  

  use APR::Brigade ();

  use APR::Bucket ();

  

  use Apache2::Const -compile => qw(OK DECLINED);

  use APR::Const     -compile => ':common';

  

  sub handler : FilterConnectionHandler {

      my ($f, $bb, $mode, $block, $readbytes) = @_;

  

      return Apache2::Const::DECLINED if $f->ctx;

  

      my $rv = $f->next->get_brigade($bb, $mode, $block, $readbytes);

      return $rv unless $rv == APR::Const::SUCCESS;

  

      for (my $b = $bb->first; $b; $b = $bb->next($b)) {

          $b->read(my $data);

          warn("data: $data\n");

  

          if ($data and $data =~ s|^GET|HEAD|) {

              my $nb = APR::Bucket->new($bb->bucket_alloc, $data);

              $b->insert_after($nb);

              $b->remove; # no longer needed

              $f->ctx(1); # flag that that we have done the job

              last;

          }

      }

  

      Apache2::Const::OK;

  }

  1;

The filter handler is called for each bucket brigade, which in turn includes buckets with data. The gist of any input filter handler is to request the bucket brigade from the upstream filter, and return it downstream filter using the second argument $bb. It's important to remember that you can call methods on this argument, but you shouldn't assign to this argument, or the chain will be broken. You have two techniques to choose from to retrieve-modify-return bucket brigades:

  1. Create a new empty bucket brigade $ctx_bb, pass it to the upstream filter via get_brigade() and wait for this call to return. When it returns, $ctx_bb is populated with buckets. Now the filter should move the bucket from $ctx_bb to $bb, on the way modifying the buckets if needed. Once the buckets are moved, and the filter returns, the downstream filter will receive the populated bucket brigade.

  2. Pass $bb to get_brigade() to the upstream filter, so it will be populated with buckets. Once get_brigade() returns, the filter can go through the buckets and modify them in place, or it can do nothing and just return (in which case, the downstream filter will receive the bucket brigade unmodified).

Both techniques allow addition and removal of buckets. Though the second technique is more efficient since it doesn't have the overhead of create the new brigade and moving the bucket from one brigade to another. In this example we have chosen to use the second technique, in the next example we will see the first technique.

Our filter has to perform the substitution of only one HTTP header (which normally resides in one bucket), so we have to make sure that no other data gets mangled (e.g. there could be POSTED data and it may match /^GET/ in one of the buckets). We use $f->ctx|docs::2.0::api::Apache2::Filter/C_ctx_ as a flag here. When it's undefined the filter knows that it hasn't done the required substitution, though once it completes the job it sets the context to 1.

To optimize the speed, the filter immediately returns Apache2::Const::DECLINED when it's invoked after the substitution job has been done:


    return Apache2::Const::DECLINED if $f->ctx;

In that case mod_perl will call get_brigade() internally which will pass the bucket brigade to the downstream filter. Alternatively the filter could do:


    my $rv = $f->next->get_brigade($bb, $mode, $block, $readbytes);

    return $rv unless $rv == APR::Const::SUCCESS;

    return Apache2::Const::OK if $f->ctx;

but this is a bit less efficient.

[META: the most efficient thing to do is to remove the filter itself once the job is done, so it won't be even invoked after the job has been done.


  if ($f->ctx) {

      $f->remove;

      return Apache2::Const::DECLINED;

  }

However, this can't be used with Apache 2.0.49 and lower, since it has a bug when trying to remove the edge connection filter (it doesn't remove it). Most likely that problem will be not fixed in the 2.0 series due to design flows. I don't know if it's going to be fixed in 2.1 series.]

If the job wasn't done yet, the filter calls get_brigade, which populates the $bb bucket brigade. Next, the filter steps through the buckets looking for the bucket that matches the regex: /^GET/. If that happens, a new bucket is created with the modified data (s/^GET/HEAD/. Now it has to be inserted in place of the old bucket. In our example we insert the new bucket after the bucket that we have just modified and immediately remove that bucket that we don't need anymore:


            $b->insert_after($nb);

            $b->remove; # no longer needed

Finally we set the context to 1, so we know not to apply the substitution on the following data and break from the for loop.

The handler returns Apache2::Const::OK indicating that everything was fine. The downstream filter will receive the bucket brigade with one bucket modified.

Now let's check that the handler works properly. For example, consider the following response handler:


  #file:MyApache2/RequestType.pm

  #---------------------------

  package MyApache2::RequestType;

  

  use strict;

  use warnings;

  

  use Apache2::RequestIO ();

  use Apache2::RequestRec ();

  use Apache2::Response ();

  

  use Apache2::Const -compile => 'OK';

  

  sub handler {

      my $r = shift;

  

      $r->content_type('text/plain');

      my $response = "the request type was " . $r->method;

      $r->set_content_length(length $response);

      $r->print($response);

  

      Apache2::Const::OK;

  }

  

  1;

which returns to the client the request type it has issued. In the case of the HEAD request Apache will discard the response body, but it'll will still set the correct Content-Length header, which will be 24 in case of the GET request and 25 for HEAD. Therefore if this response handler is configured as:


  Listen 8005

  <VirtualHost _default_:8005>

      <Location />

          SetHandler modperl

          PerlResponseHandler +MyApache2::RequestType

      </Location>

  </VirtualHost>

and a GET request is issued to /:


  panic% perl -MLWP::UserAgent -le \

  '$r = LWP::UserAgent->new()->get("http://localhost:8005/";); \

  print $r->headers->content_length . ": ".  $r->content'

  24: the request type was GET

where the response's body is:


  the request type was GET

And the Content-Length header is set to 24.

However if we enable the MyApache2::InputFilterGET2HEAD input connection filter:


  Listen 8005

  <VirtualHost _default_:8005>

      PerlInputFilterHandler +MyApache2::InputFilterGET2HEAD

  

      <Location />

          SetHandler modperl

          PerlResponseHandler +MyApache2::RequestType

      </Location>

  </VirtualHost>

And issue the same GET request, we get only:


  25:

which means that the body was discarded by Apache, because our filter turned the GET request into a HEAD request and if Apache wasn't discarding the body on HEAD, the response would be:


  the request type was HEAD

that's why the content length is reported as 25 and not 24 as in the real GET request.

HTTP Request Input Filters

Request filters are really non-different from connection filters, other than that they are working on request and response bodies and have an access to a request object.

Bucket Brigade-based Input Filters

Let's look at the request input filter that lowers the case of the request's body: MyApache2::InputRequestFilterLC:


  #file:MyApache2/InputRequestFilterLC.pm

  #-------------------------------------

  package MyApache2::InputRequestFilterLC;

  

  use strict;

  use warnings;

  

  use base qw(Apache2::Filter);

  

  use Apache2::Connection ();

  use APR::Brigade ();

  use APR::Bucket ();

  

  use Apache2::Const -compile => 'OK';

  use APR::Const     -compile => ':common';

  

  sub handler : FilterRequestHandler {

      my ($f, $bb, $mode, $block, $readbytes) = @_;

  

      my $c = $f->c;

      my $bb_ctx = APR::Brigade->new($c->pool, $c->bucket_alloc);

      my $rv = $f->next->get_brigade($bb_ctx, $mode, $block, $readbytes);

      return $rv unless $rv == APR::Const::SUCCESS;

  

      while (!$bb_ctx->is_empty) {

          my $b = $bb_ctx->first;

  

          if ($b->is_eos) {

              $bb->insert_tail($b);

              last;

          }

  

          my $len = $b->read(my $data);

          $b = APR::Bucket->new($bb->bucket_alloc, lc $data) if $len;

  

          $b->remove;

          $bb->insert_tail($b);

      }

  

      Apache2::Const::OK;

  }

  

  1;

As promised, in this filter handler we have used the first technique of bucket brigade modification. The handler creates a temporary bucket brigade (ctx_bb), populates it with data using get_brigade(), and then moves buckets from it to the bucket brigade $bb, which is then retrieved by the downstream filter when our handler returns.

This filter doesn't need to know whether it was invoked for the first time or whether it has already done something. It's a state-less handler, since it has to lower case everything that passes through it. Notice that this filter can't be used as the connection filter for HTTP requests, since it will invalidate the incoming request headers; for example the first header line:


  GET /perl/TEST.pl HTTP/1.1

will become:


  get /perl/test.pl http/1.1

which messes up the request method, the URL and the protocol.

Now if we use the MyApache2::Dump response handler, we have developed before in this chapter, which dumps the query string and the content body as a response, and configure the server as follows:


  <Location /lc_input>

      SetHandler modperl

      PerlResponseHandler    +MyApache2::Dump

      PerlInputFilterHandler +MyApache2::InputRequestFilterLC

  </Location>

When issuing a POST request:


 % echo "mOd_pErl RuLeS" | POST 'http://localhost:8002/lc_input?FoO=1&BAR=2'

we get a response:


  args:

  FoO=1&BAR=2

  content:

  mod_perl rules

indeed we can see that our filter has lowercased the POSTed body, before the content handler received it. You can see that the query string wasn't changed.

Stream-oriented Input Filters

Let's now look at the same filter implemented using the stream-oriented API.


  #file:MyApache2/InputRequestFilterLC2.pm

  #-------------------------------------

  package MyApache2::InputRequestFilterLC2;

  

  use strict;

  use warnings;

  

  use base qw(Apache2::Filter);

  

  use Apache2::Const -compile => 'OK';

  

  use constant BUFF_LEN => 1024;

  

  sub handler : FilterRequestHandler {

      my $f = shift;

  

      while ($f->read(my $buffer, BUFF_LEN)) {

          $f->print(lc $buffer);

      }

  

      Apache2::Const::OK;

  }

  1;

Now you probably ask yourself why did we have to go through the bucket brigades filters when this all can be done so much simpler. The reason is that we wanted you to understand how the filters work underneath, which will assist a lot when you will need to debug filters or optimize their speed. In certain cases a bucket brigade filter may be more efficient than the stream-oriented. For example if the filter applies transformation to selected buckets, certain buckets may contain open filehandles or pipes, rather than real data. And when you call read() the buckets will be forced to read that data in. But if you didn't want to modify these buckets you could pass them as they are and let Apache do faster techniques for sending data from the file handles or pipes.

The logic is very simple here, the filter reads in loop, and prints the modified data, which at some point will be sent to the next filter. This point happens every time the internal mod_perl buffer is full or when the filter returns.

read() populates $buffer to a maximum of BUFF_LEN characters (1024 in our example). Assuming that the current bucket brigade contains 2050 chars, read() will get the first 1024 characters, then 1024 characters more and finally the remaining 2 characters. Notice that even though the response handler may have sent more than 2050 characters, every filter invocation operates on a single bucket brigade so you have to wait for the next invocation to get more input. In one of the earlier examples we have shown that you can force the generation of several bucket brigades in the content handler by using rflush(). For example:


  $r->print("string");

  $r->rflush();

  $r->print("another string");

It's only possible to get more than one bucket brigade from the same filter handler invocation if the filter is not using the streaming interface and by simply calling get_brigade() as many times as needed or till EOS is received.

The configuration section is pretty much identical:


 <Location /lc_input2>

      SetHandler modperl

      PerlResponseHandler    +MyApache2::Dump

      PerlInputFilterHandler +MyApache2::InputRequestFilterLC2

  </Location>

When issuing a POST request:


 % echo "mOd_pErl RuLeS" | POST 'http://localhost:8002/lc_input2?FoO=1&BAR=2'

we get a response:


  args:

  FoO=1&BAR=2

  content:

  mod_perl rules

indeed we can see that our filter has lowercased the POSTed body, before the content handler received it. You can see that the query string wasn't changed.


Output Filters

mod_perl supports Connection and HTTP Request output filters:

Connection Output Filters

Connection filters filter all the data that is going through the server. Therefore if the connection is of HTTP request type, connection output filters see the headers and the body of the response, whereas request output filters see only the response body.

META: for now see the request output filter explanations and examples, connection output filter examples will be added soon. Interesting ideas for such filters are welcome (possible ideas: mangling output headers for HTTP requests, pretty much anything for protocol modules).

HTTP Request Output Filters

As mentioned earlier output filters can be written using the bucket brigades manipulation or the simplified stream-oriented interface.

First let's develop a response handler that sends two lines of output: numerals 1234567890 and the English alphabet in a single string:


  #file:MyApache2/SendAlphaNum.pm

  #-------------------------------

  package MyApache2::SendAlphaNum;

  

  use strict;

  use warnings;

  

  use Apache2::RequestRec ();

  use Apache2::RequestIO ();

  

  use Apache2::Const -compile => qw(OK);

  

  sub handler {

      my $r = shift;

  

      $r->content_type('text/plain');

  

      $r->print(1..9, "0\n");

      $r->print('a'..'z', "\n");

  

      Apache2::Const::OK;

  }

  1;

The purpose of our filter handler is to reverse every line of the response body, preserving the new line characters in their places. Since we want to reverse characters only in the response body, without breaking the HTTP headers, we will use the HTTP request output filter.

Stream-oriented Output Filters

The first filter implementation is using the stream-oriented filtering API:


  #file:MyApache2/FilterReverse1.pm

  #----------------------------

  package MyApache2::FilterReverse1;

  

  use strict;

  use warnings;

  

  use base qw(Apache2::Filter);

  

  use Apache2::Const -compile => qw(OK);

  

  use constant BUFF_LEN => 1024;

  

  sub handler : FilterRequestHandler {

      my $f = shift;

  

      while ($f->read(my $buffer, BUFF_LEN)) {

          for (split "\n", $buffer) {

              $f->print(scalar reverse $_);

              $f->print("\n");

          }

      }

  

      Apache2::Const::OK;

  }

  1;

Next, we add the following configuration to httpd.conf:


  PerlModule MyApache2::FilterReverse1

  PerlModule MyApache2::SendAlphaNum

  <Location /reverse1>

      SetHandler modperl

      PerlResponseHandler     MyApache2::SendAlphaNum

      PerlOutputFilterHandler MyApache2::FilterReverse1

  </Location>

Now when a request to /reverse1 is made, the response handler MyApache2::SendAlphaNum::handler() sends:


  1234567890

  abcdefghijklmnopqrstuvwxyz

as a response and the output filter handler MyApache2::FilterReverse1::handler reverses the lines, so the client gets:


  0987654321

  zyxwvutsrqponmlkjihgfedcba

The Apache2::Filter module loads the read() and print() methods which encapsulate the stream-oriented filtering interface.

The reversing filter is quite simple: in the loop it reads the data in the readline() mode in chunks up to the buffer length (1024 in our example), and then prints each line reversed while preserving the new line control characters at the end of each line. Behind the scenes $f->read() retrieves the incoming brigade and gets the data from it, and $f->print() appends to the new brigade which is then sent to the next filter in the stack. read() breaks the while loop, when the brigade is emptied or the end of stream is received.

In order not to distract the reader from the purpose of the example the used code is oversimplified and won't handle correctly input lines which are longer than 1024 characters and possibly using a different line termination token (could be ``\n'', ``\r'' or ``\r\n'' depending on a platform). Moreover a single line may be split between across two or even more bucket brigades, so we have to store the unprocessed string in the filter context, so it can be used on the following invocations. So here is an example of a more complete handler, which does takes care of these issues:


  sub handler {

      my $f = shift;

  

      my $leftover = $f->ctx;

      while ($f->read(my $buffer, BUFF_LEN)) {

          $buffer = $leftover . $buffer if defined $leftover;

          $leftover = undef;

          while ($buffer =~ /([^\r\n]*)([\r\n]*)/g) {

              $leftover = $1, last unless $2;

              $f->print(scalar(reverse $1), $2);

          }

      }

  

      if ($f->seen_eos) {

          $f->print(scalar reverse $leftover) if defined $leftover;

      }

      else {

          $f->ctx($leftover) if defined $leftover;

      }

  

      return Apache2::Const::OK;

  }

The handler uses the $leftover variable to store unprocessed data as long as it fails to assemble a complete line or there is an incomplete line following the new line token. On the next handler invocation this data is then prepended to the next chunk that is read. When the filter is invoked on the last time, it unconditionally reverses and flushes any remaining data.

Bucket Brigade-based Output Filters

The following filter implementation is using the bucket brigades API to accomplish exactly the same task as the first filter.


  #file:MyApache2/FilterReverse2.pm

  #--------------------------------

  package MyApache2::FilterReverse2;

  

  use strict;

  use warnings;

  

  use base qw(Apache2::Filter);

  

  use APR::Brigade ();

  use APR::Bucket ();

  

  use Apache2::Const -compile => 'OK';

  use APR::Const     -compile => ':common';

  

  sub handler : FilterRequestHandler {

      my ($f, $bb) = @_;

  

      my $bb_ctx = APR::Brigade->new($f->c->pool, $f->c->bucket_alloc);

  

      while (!$bb->is_empty) {

          my $b = $bb->first;

  

          $b->remove;

  

          if ($b->is_eos) {

              $bb_ctx->insert_tail($b);

              last;

          }

  

          if ($b->read(my $data)) {

              $data = join "",

                  map {scalar(reverse $_), "\n"} split "\n", $data;

              $b = APR::Bucket->new($bb->bucket_alloc, $data);

          }

  

          $bb_ctx->insert_tail($b);

      }

  

      my $rv = $f->next->pass_brigade($bb_ctx);

      return $rv unless $rv == APR::Const::SUCCESS;

  

      Apache2::Const::OK;

  }

  1;

and the corresponding configuration:


  PerlModule MyApache2::FilterReverse2

  PerlModule MyApache2::SendAlphaNum

  <Location /reverse2>

      SetHandler modperl

      PerlResponseHandler     MyApache2::SendAlphaNum

      PerlOutputFilterHandler MyApache2::FilterReverse2

  </Location>

Now when a request to /reverse2 is made, the client gets:


  0987654321

  zyxwvutsrqponmlkjihgfedcba

as expected.

The bucket brigades output filter version is just a bit more complicated than the stream-oriented one. The handler receives the incoming bucket brigade $bb as its second argument. Since when the handler is completed it must pass a brigade to the next filter in the stack, we create a new bucket brigade into which we are going to put the modified buckets and which eventually we pass to the next filter.

The core of the handler is in removing buckets from the head of the bucket brigade $bb while there are some, reading the data from the buckets, reversing and putting it into a newly created bucket which is inserted to the end of the new bucket brigade. If we see a bucket which designates the end of stream, we insert that bucket to the tail of the new bucket brigade and break the loop. Finally we pass the created brigade with modified data to the next filter and return.

Similarly to the original version of MyApache2::FilterReverse1::handler, this filter is not smart enough to handle incomplete lines. However the exercise of making the filter foolproof should be trivial by porting a better matching rule and using the $leftover buffer from the previous section is trivial and left as an exercise to the reader.


Filter Applications

The following sections provide various filter applications and their implementation.

Handling Data Underruns

Sometimes filters need to read at least N bytes before they can apply their transformation. It's quite possible that reading one bucket brigade is not enough. But two or more are needed. This situation is sometimes referred to as an underrun.

Let's take an input filter as an example. When the filter realizes that it doesn't have enough data in the current bucket brigade, it can store the read data in the filter context, and wait for the next invocation of itself, which may or may not satisfy its needs. Meanwhile it must return an empty bb to the upstream input filter. This is not the most efficient technique to resolve underruns.

Instead of returning an empty bb, the input filter can initiate the retrieval of extra bucket brigades, until the underrun condition gets resolved. Notice that this solution is absolutely transparent to any filters before or after the current filter.

Consider this HTTP request:


  % perl -MLWP::UserAgent -le ' \

    $r = LWP::UserAgent->new()->post("http://localhost:8011/";, \

         [content => "x" x (40 * 1024 + 7)]); \

    print $r->is_success ? $r->content : "failed: " . $r->code'

  read 40975 chars

This client POSTs just a little bit more than 40kb of data to the server. Normally Apache splits incoming POSTed data into 8kb chunks, putting each chunk into a separate bucket brigade. Therefore we expect to get 5 brigades of 8kb, and one brigade with just a few bytes (a total of 6 bucket brigades).

Now let's say that the filter needs to have 1024*16 + 5 bytes to have a complete token and then it can start its processing. The extra 5 bytes are just so we don't perfectly fit into 8bk bucket brigades, making the example closer to real situations. Having 40975 bytes of input and a token size of 16389 bytes, we will have 2 full tokens and 8197 remainder.

Jumping ahead let's look at the filter debug output:


  filter called

  asking for a bb

  asking for a bb

  asking for a bb

  storing the remainder: 7611 bytes

  

  filter called

  asking for a bb

  asking for a bb

  storing the remainder: 7222 bytes

 

  filter called

  asking for a bb

  seen eos, flushing the remaining: 8197 bytes

So we can see that the filter was invoked three times. The first time it has consumed three bucket brigades, collecting one full token of 16389 bytes and has a remainder of 7611 bytes to be processed on the next invocation. The second time it needed only two more bucket brigades and this time after completing the second token, 7222 bytes have remained. Finally on the third invocation it has consumed the last bucket brigade (total of six, just as we have expected), however it didn't have enough for the third token and since EOS has been seen (no more data expected), it has flushed the remaining 8197 bytes as we have calculated earlier.

It is clear from the debugging output that the filter was invoked only three times, instead of six times (there were six bucket brigades). Notice that the upstread input filter (if any) wasn't aware that there were six bucket brigades, since it saw only three. Our example filter didn't do much with those tokens, so it has only repackaged data from 8kb per bucket brigade, to 16389 bytes per bucket brigade. But of course in real world some transformation is applied on these tokens.

Now you understand what did we want from the filter, it's time for the implementation details. First let's look at the response() handler (the first part of the module):


  #file:MyApache2/Underrun.pm

  #-------------------------

  package MyApache2::Underrun;

  

  use strict;

  use warnings;

  

  use constant IOBUFSIZE => 8192;

  

  use Apache2::Const -compile => qw(MODE_READBYTES OK M_POST);

  use APR::Const     -compile => qw(SUCCESS BLOCK_READ);

  

  sub response {

      my $r = shift;

  

      $r->content_type('text/plain');

  

      if ($r->method_number == Apache2::Const::M_POST) {

          my $data = read_post($r);

          #warn "HANDLER READ: $data\n";

          my $length = length $data;

          $r->print("read $length chars");

      }

  

      return Apache2::Const::OK;

  }

  

  sub read_post {

      my $r = shift;

  

      my $bb = APR::Brigade->new($r->pool, $r->connection->bucket_alloc);

  

      my $data = '';

      my $seen_eos = 0;

      do {

          $r->input_filters->get_brigade($bb, Apache2::Const::MODE_READBYTES,

                                         APR::Const::BLOCK_READ, IOBUFSIZE);

  

          for (my $b = $bb->first; $b; $b = $bb->next($b)) {

              if ($b->is_eos) {

                  $seen_eos++;

                  last;

              }

  

              if ($b->read(my $buf)) {

                  $data .= $buf;

              }

  

              $b->remove; # optimization to reuse memory

          }

      } while (!$seen_eos);

  

      $bb->destroy;

  

      return $data;

  }

The response() handler is trivial -- it reads the POSTed data and prints how many bytes it has read. read_post() sucks in all POSTed data without parsing it.

Now comes the filter (which lives in the same package):


  #file:MyApache2/Underrun.pm (continued)

  #-------------------------------------

  use Apache2::Filter ();

  

  use Apache2::Const -compile => qw(OK M_POST);

  

  use constant TOKEN_SIZE => 1024*16 + 5; # ~16k

  

  sub filter {

      my ($f, $bb, $mode, $block, $readbytes) = @_;

      my $ba = $f->r->connection->bucket_alloc;

      my $ctx = $f->ctx;

      my $buffer = defined $ctx ? $ctx : '';

      $ctx = '';  # reset

      my $seen_eos = 0;

      my $data;

      warn "\nfilter called\n";

  

      # fetch and consume bucket brigades untill we have at least TOKEN_SIZE

      # bytes to work with

      do {

          my $tbb = APR::Brigade->new($f->r->pool, $ba);

          my $rv = $f->next->get_brigade($tbb, $mode, $block, $readbytes);

          warn "asking for a bb\n";

          ($data, $seen_eos) = flatten_bb($tbb);

          $tbb->destroy;

          $buffer .= $data;

      } while (!$seen_eos && length($buffer) < TOKEN_SIZE);

  

      # now create a bucket per chunk of TOKEN_SIZE size and put the remainder

      # in ctx

      for (split_buffer($buffer)) {

          if (length($_) == TOKEN_SIZE) {

              $bb->insert_tail(APR::Bucket->new($ba, $_));

          }

          else {

              $ctx .= $_;

          }

      }

  

      my $len = length($ctx);

      if ($seen_eos) {

          # flush the remainder

          $bb->insert_tail(APR::Bucket->new($ba, $ctx));

          $bb->insert_tail(APR::Bucket::eos_create($ba));

          warn "seen eos, flushing the remaining: $len bytes\n";

      }

      else {

          # will re-use the remainder on the next invocation

          $f->ctx($ctx);

          warn "storing the remainder: $len bytes\n";

      }

  

      return Apache2::Const::OK;

  }

  

  # split a string into tokens of TOKEN_SIZE bytes and a remainder

  sub split_buffer {

      my $buffer = shift;

      if ($] < 5.007) {

          my @tokens = $buffer =~ /(.{@{[TOKEN_SIZE]}}|.+)/g;

          return @tokens;

      }

      else {

          # available only since 5.7.x+

          return unpack "(A" . TOKEN_SIZE . ")*", $buffer;

      }

  }

  

  sub flatten_bb {

      my ($bb) = shift;

  

      my $seen_eos = 0;

  

      my @data;

      for (my $b = $bb->first; $b; $b = $bb->next($b)) {

          $seen_eos++, last if $b->is_eos;

          $b->read(my $bdata);

          push @data, $bdata;

      }

      return (join('', @data), $seen_eos);

  }

  

  1;

The filter calls get_brigade() in a do-while loop till it reads enough data or sees EOS. Notice that it may get underruns for several times, and then suddenly receive a lot of data at once, which will be enough for more than one minimal size token, so we have to take care this into an account. Once the underrun condition is satisfied (we have at least one complete token) the tokens are put into a bucket brigade and returned to the upstream filter for processing, keeping any remainders in the filter context, for the next invocations or flushing all the remaining data if EOS has been seen.

Notice that this won't be possible with streaming filters where every invocation gives the filter exactly one bucket brigade to work with and provides not facilities to fetch extra brigades. (META: however this can be fixed, by providing a method which will fetch the next bucket brigade, so the read in a while loop can be repeated)

And here is the configuration for this setup:


  PerlModule MyApache2::Underrun

  <Location />

    PerlInputFilterHandler MyApache2::Underrun::filter

    SetHandler modperl

    PerlResponseHandler MyApache2::Underrun::response

  </Location>

Setting the Content-Length Header in Request Output Filters

Earlier we have stated that a filter that modifies the content's length must unset the Content-Length HTTP header. However sometimes it's desirable to have this header set, for example when dealing with proxies. Since the headers are sent before the data, all the data must be first buffered and processed. You cannot accomplish this task with the streaming filter API since it passes FLUSH buckets through. As soon as the FLUSH bucket is received by the core filter that sends the headers, it generates the headers and sends those out. Therefore the bucket brigade API must be used here to have a complete control over what's going through. Here is a possible implementation:


  #file:MyApache2/FilterChangeLength.pm

  #-------------------------------------

  package MyApache2::FilterChangeLength;

  

  use strict;

  use warnings FATAL => 'all';

  

  use Apache2::RequestRec ();

  

  use APR::Table ();

  use APR::Bucket ();

  use APR::Brigade ();

  

  use base qw(Apache2::Filter);

  

  use Apache2::Const -compile => qw(OK);

  use APR::Const     -compile => ':common';

  

  sub handler {

      my ($filter, $bb) = @_;

  

      my $ctx = $filter->ctx;

  

      # no need to unset the C-L header, since this filter makes sure to

      # correct it before any headers go out.

      #unless ($ctx) {

      #    $filter->r->headers_out->unset('Content-Length');

      #}

  

      my $data = exists $ctx->{data} ? $ctx->{data} : '';

      $ctx->{invoked}++;

      my ($bdata, $seen_eos) = flatten_bb($bb);

      $bdata =~ s/-//g;

      $data .= $bdata if $bdata;

  

      if ($seen_eos) {

          my $len = length $data;

          $filter->r->headers_out->set('Content-Length', $len);

          $filter->print($data) if $data;

      }

      else {

          # store context for all but the last invocation

          $ctx->{data} = $data;

          $filter->ctx($ctx);

      }

  

      return Apache2::Const::OK;

  }

  

  sub flatten_bb {

      my ($bb) = shift;

  

      my $seen_eos = 0;

  

      my @data;

      for (my $b = $bb->first; $b; $b = $bb->next($b)) {

          $seen_eos++, last if $b->is_eos;

          $b->read(my $bdata);

          push @data, $bdata;

      }

      return (join('', @data), $seen_eos);

  }

  

  1;

In this module we use flatten_bb() to read the data from the buckets and signal when the EOS is received. The filter simply collects the data, storing it in the filter context. When it receives EOS it sets the Content-Length header and sends the data out.

The configuration is straightforward:


  PerlOutputFilterHandler MyApache2::FilterChangeLength


Filter Tips and Tricks

Various tips to use in filters.

Altering the Content-Type Response Header

Let's say that you want to modify the Content-Type header in the request output filter:


  sub handler : FilterRequestHandler {

      my $f = shift;

      ...

      $f->r->content_type("text/html; charset=$charset");

      ...

Request filters have an access to the request object, so we simply modify it.


Writing Well-Behaving Filters

Filter writers must follow the following rules:

Connection Filters over KeepAlive Connections

Whenever a new HTTP request is processed, request filters get their context ($f->ctx|docs::2.0::api::Apache2::Filter/C_ctx_) reset. This is also true for connection filters context, as long as the connection is not keepalive|docs::2.0::api::Apache2::Connection/C_keepalive_). When the connection is kept alive, there could be many requests processed during a single connection and the same filter context will persist through all of them, until the maximum number of KeepAlive requests over the same connection is reached or the client breaks the connection.

Sometimes it's desirable to reset the whole context or parts of it before a HTTP request is processed. For example Apache2::Filter::HTTPHeadersFixup needs to know when it should start and stop processing HTTP headers. It keeps the state in the filter's context. The problem is that whenever a new HTTP request is coming in, it needs to be able to reset the state machine. If it doesn't, it'll process the HTTP headers of the first request and miss the rest of the requests.

So let's say we have a hypothetical module MyApache2::Filter::StateMachine which implements an input connection filter, which processes incoming data as long as the state flag is down. Once that flag goes up the filter switches to the pass-through-unmodified mode. Here is a skeleton of the module:


  #file:MyApache2/Filter/StateMachine.pm

  #------------------------------------

  package MyApache2::Filter::StateMachine;

  

  use base qw(Apache2::Filter);

  use Apache2::Connection ();

  

  use Apache2::Const -compile => qw(OK DECLINED CONN_KEEPALIVE);

  

  sub handler : FilterConnectionHandler {

      my ($f, $bb, $mode, $block, $readbytes) = @_;

  

      my $ctx = context($f);

  

      # pass through unmodified

      return Apache2::Const::DECLINED if $ctx->{state};

  

      # get data, do some processing, send it out

      process(); # your code comes here

  

      # change the state if some condition is reached

      $ctx->{state}++ if $done_condition;

  

      return Apache2::Const::OK;

  }

  

  sub context {

      my ($f) = shift;

  

      my $ctx = $f->ctx;

      unless ($ctx) {

          $ctx = {

              state => 0,

          };

  

          $f->ctx($ctx);

      }

  

      return $ctx;

  }

  1;

In order to make this module work properly over KeepAlive connections, we want to reset the state flag at the very beginning of the new request. To accomplish that all we need to do is to change the context wrapper to be:


  sub context {

      my ($f) = shift;

  

      my $ctx = $f->ctx;

      unless ($ctx) {

          $ctx = {

              state       => 0,

              keepalives  => $f->c->keepalives,

          };

  

          $f->ctx($ctx);

          return $ctx;

      }

  

      my $c = $f->c;

      if ($c->keepalive == Apache2::Const::CONN_KEEPALIVE &&

          $ctx->{state} && $c->keepalives > $ctx->{keepalives}) {

  

          $ctx->{state}      = 0;

          $ctx->{keepalives} = $c->keepalives;

      }

  

      return $ctx;

  }

The only difference from the previous implementation is that we maintain one more state, which stores the number of requests, served over the current connection. When Apache reports more served requests than we have in the context that means that we have a new request coming in. So we reset the state flag and store the new value of the served connections.

For a concrete implementation examples see: http://search.cpan.org/dist/Apache-Filter-HTTPHeadersFixup/

Adjusting HTTP Headers

The following information is relevant for HTTP filters

  • Unsetting the Content-Length header
  • HTTP response filters modifying the length of the body they process must unset the Content-Length header. For example, a compression filter modifies the body length, whereas a lowercasing filter doesn't; therefore the former has to unset the header, and the latter doesn't have to.

    The header must be unset before any output is sent from the filter. If this rule is not followed, an HTTP response header with incorrect Content-Length value might be sent.

    Since you want to run this code once during the multiple filter invocations, use the ctx() method to set the flag:

    
      unless ($f->ctx) {
    
          $f->r->headers_out->unset('Content-Length');
    
          $f->ctx(1);
    
      }

  • META: Same goes for last-modified/etags, which may need to be unset, ``vary'' might need to be added if you want caching to work properly (depending on what your filter does.

Other issues

META: to be written. Meanwhile collecting important inputs from various sources.

[

If a filter desires to store the incoming buckets for post processing. It must check whether the bucket type is transient. If it is -- the data must be copied away. If not -- the buckets may contain corrupted data when used later. The right thing is accomplished transparently by apr_bucket_setaside, for which we need to provide a perl glue.

]

[

This one will be expanded by Geoff at some point:

HTTP output filter developers are ought to handle conditional GETs properly... (mostly for the reason of efficiency?)

]

[

talk about issues like not losing meta-buckets. e.g. if the filter runs a switch statement and propagates buckets types that were known at the time of writing, it may drop buckets of new types which may be added later, so it's important to ensure that there is a default cause where the bucket is passed as is.

of course mention the fact where things like EOS buckets must be passed, or the whole chain will be broken. Or if some filter decides to inject an EOS bucket by itself, it should probably consume and destroy the rest of the incoming bb. need to check on this issue.

]

[

Need to document somewhere (concepts?) that the buckets should never be modified directly, because the filter can't know ho else could be referencing it at the same time. (shared mem/cache/memory mapped files are examples on where you don't want to modify the data). Instead the data should be moved into a new bucket.

Also it looks like we need to $b->destroy (need to add the API) in addition to $b->remove. Which can be done in one stroke using $b->delete (need to add the API).

]

[

Mention mod_bucketeer as filter debugging tool (in addition to FilterSnoop)

]


Writing Efficient Filters

META: to be written

[

As of this writing the network input filter reads in 8000B chunks (not 8192B!), and making each bucket 8000B in size, so it seems that the most efficient reading technique is:


  use constant BUFF_LEN => 8000;

  while ($f->read(my $buffer, BUFF_LEN)) {

      # manip $buffer

      $f->print($buffer);

  }

however if there is some filter in between, it may change the size of the buckets. Also this number may change in the future.

Hmm, I've also seen it read in 7819 chunks. I suppose this is not very reliable. But it's probably a good idea to ask at least 8k, so if a bucket brigade has < 8k, nothing will need to be stored in the internal buffer. i.e. read() will return less than asked for.

]

[

Bucket Brigades are used to make the data flow between filters and handlers more efficient. e.g. a file handle can be put in a bucket and the read from the file can be postponed to the very moment when the data is sent to the client, thus saving a lot of memory and CPU cycles. though filters writers should be aware that if they call $b->read(), or any other operation that internally forces the bucket to read the information into the memory (like the length() op) and thus making the data handling inefficient. therefore a care should be taken so not to read the data in, unless it's really necessary.

]


CPAN Modules

Some of the CPAN modules that implement mod_perl 2.0 filters:

Apache::Clean - Interface into HTML::Clean for mod_perl 2.0
http://search.cpan.org/dist/Apache-Clean/

Apache::Filter::HTTPHeadersFixup - Manipulate Apache 2 HTTP Headers
http://search.cpan.org/dist/Apache-Filter-HTTPHeadersFixup/


Maintainers

Maintainer is the person(s) you should contact with updates, corrections and patches.

  • Stas Bekman <stas (at) stason.org>


Authors

Only the major authors are listed above. For contributors see the Changes file.

Programminig
Wy
Wy
yW
Wy
Programming
Wy
Wy
Wy
Wy