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mod_perl internals: Apache 2.0 Integration


This document should help to understand the initialization, request processing and shutdown process of the mod_perl module. This knowledge is essential for a less-painful debugging experience. It should also help to know where a new code should be added when a new feature is added.

Internals of mod_perl-specific features are discussed in mod_perl internals: mod_perl-specific functionality flow.

Make sure to read also: Debugging mod_perl C Internals.


Apache starts itself and immediately restart itself. The following sections discuss what happens to mod_perl during this period.

The Link Between mod_perl and httpd

mod_perl.c includes a special data structure:

  module AP_MODULE_DECLARE_DATA perl_module = {


      modperl_config_dir_create, /* dir config creater */

      modperl_config_dir_merge,  /* dir merger --- default is to override */

      modperl_config_srv_create, /* server config */

      modperl_config_srv_merge,  /* merge server config */

      modperl_cmds,              /* table of config file commands       */

      modperl_register_hooks,    /* register hooks */


Apache uses this structure to hook mod_perl in, and it specifies six custom callbacks which Apache will call at various stages that will be explained later.

STANDARD20_MODULE_STUFF is a standard macro defined in httpd-2.0/include/http_config.h. Currently its main use is for attaching Apache version magic numbers, so the previously compiled module won't be attempted to be used with newer Apache versions, whose API may have changed.

modperl_cmds is a struct, that defines the mod_perl configuration directives and the callbacks to be invoked for each of these.

Configuration Tree Building

At the ap_read_config stage the configuration file is parsed and stored in a parsed configuration tree is created. Some sections are stored unmodified in the parsed configuration tree to be processed after the pre_config hooks were run. Other sections are processed right away (e.g., the Include directive includes extra configuration and has to include it as soon as it was seen) and they may or may not add a subtree to the configuration tree.

ap_build_config feeds the configuration file lines from to ap_build_config_sub, which tokenizes the input, and uses the first token as a potential directive (command). It then calls ap_find_command_in_modules() to find a module that has registered that command (remember mod_perl has registered the directives in the modperl_cmds command_rec array, which was passed to ap_add_module inside the perl_module struct?). If that command is found and it has the EXEC_ON_READ flag set in its req_override field, the callback for that command is invoked. Depending on the command, it may perform some action and return (e.g., User foo), or it may continue reading from the configuration file and recursively execute other nested commands till it's done (e.g., <Location ...>). If the command is found but the EXEC_ON_READ flag is not set or the command is not found, the current node gets added to the configuration tree and will be processed during the ap_process_config_tree() stage, after the pre_config stage will be over.

If the command needs to be executed at this stage as it was just explained, execute_now() invokes the corresponding callback with invoke_cmd.

Since LoadModule directive has the EXEC_ON_READ flag set, that directive is executed as soon as it's seen and the modules its supposed to load get loaded right away.

For mod_perl loaded as a DSO object, this is when mod_perl starts its game.

Enabling the mod_perl Module and Installing its Callbacks

mod_perl can be loaded as a DSO object at startup time, or be prelinked at compile time.

For statically linked mod_perl, Apache enables mod_perl by calling ap_add_module(), which happens during the ap_setup_prelinked_modules() stage. The latter is happening before the configuration file is parsed.

When mod_perl is loaded as DSO:

  <IfModule !mod_perl.c>

      LoadModule perl_module "modules/"


mod_dso's load_module first loads the shared mod_perl object, and then immediately calls ap_add_loaded_module() which calls ap_add_module() to enable mod_perl.

ap_add_module() adds the perl_module structure to the top of chained module list and calls ap_register_hooks() which calls the modperl_register_hooks() callback. This is the very first mod_perl hook that's called by Apache.

modperl_register_hooks() registers all the hooks that it wants to be called by Apache when the appropriate time comes. That includes configuration hooks, filter, connection and http protocol hooks. From now on most of the relationship between httpd and mod_perl is done via these hooks. Remember that in addition to these hooks, there are four hooks that were registered with ap_add_module(), and there are: modperl_config_srv_create, modperl_config_srv_merge, modperl_config_dir_create and modperl_config_dir_merge.

Finally after the hooks were registered, ap_single_module_configure() (called from mod_dso's load_module in case of DSO) runs the configuration process for the module. First it calls the modperl_config_srv_create callback for the main server, followed by the modperl_config_dir_create callback to create a directory structure for the main server. Notice that it passes NULL for the directory path, since we at the very top level.

If you need to do something as early as possible at mod_perl's startup, the modperl_register_hooks() is the right place to do that. For example we add a MODPERL2 define to the ap_server_config_defines here:

  *(char **)apr_array_push(ap_server_config_defines) =

      apr_pstrdup(p, "MODPERL2");

so the following code will work under mod_perl 2.0 enabled Apache without explicitly passing -DMODPERL2 at the server startup:

  <IfDefine MODPERL2>

      # 2.0 configuration

      PerlSwitches -wT


This section, of course, will see the define only if inserted after the LoadModule perl_module ..., because that's when modperl_register_hooks is called.

One inconvenience with using that hook, is that the server object is not among its arguments, so if you need to access that object, the next earliest function is modperl_config_srv_create(). However remember that it'll be called once for the main server and one more time for each virtual host, that has something to do with mod_perl. So if you need to invoke it only for the main server, you can use a s->is_virtual conditional. For example we need to enable the debug tracing as early as possible, but we need the server object in order to do that, so we perform this setting in modperl_config_srv_create():

  if (!s->is_virtual) {

      modperl_trace_level_set(s, NULL);


The pre_config Phase

After Apache processes its command line arguments, creates various pools and reads the configuration file in, it runs the registered pre_config hooks by calling ap_run_pre_config(). That's when modperl_hook_pre_config is called. And it does nothing.

Configuration Tree Processing

ap_process_config_tree calls ap_walk_config, which scans through all directives in the parsed configuration tree, and executes each one by calling ap_walk_config_sub. This is a recursive process with many twists.

Similar to ap_build_config_sub for each command (directive) in the configuration tree, it calls ap_find_command_in_modules to find a module that registered that command. If the command is not found the server dies. Otherwise the callback for that command is invoked with invoke_cmd, after fetching the current directory configuration:

  invoke_cmd(cmd, parms, dir_config, current->args);

The invoke_cmd command is the one that invokes mod_perl's directives callbacks, which reside in modperl_cmd.c. invoke_cmd knows how the arguments should be passed to the callbacks, based on the information in the modperl_cmds array that we have just mentioned.

Notice that before invoke_cmd is invoked, ap_set_config_vectors() is called which sets the current server and section configuration objects for the module in which the directive has been found. If these objects were't created yet, it calls the registered callbacks as create_dir_config and create_server_config, which are modperl_config_dir_create and modperl_config_srv_create for the mod_perl module. (If you write your custom module in Perl, these correspond to the DIR_CREATE and SERVER_CREATE Perl subroutines.)

The command callback won't be invoked if it has the EXEC_ON_READ flag set, because it was already invoked earlier when the configuration tree was parsed. ap_set_config_vectors() is called in any case, because it wasn't called during the ap_build_config.

So we have modperl_config_srv_create and modperl_config_dir_create both called once for the main server (at the end of processing the LoadModule perl_module ... directive), and one more time for each virtual host in which at least one mod_perl directive is encountered. In addition modperl_config_dir_create is called for every section and subsection that includes mod_perl directives (META: or inherits from such a section even though specifies no mod_perl directives in it?).

Virtual Hosts Fixup

After the configuration tree is processed, ap_fixup_virtual_hosts() is called. One of the responsibilities of this function is to merge the virtual hosts configuration objects with the base server's object. If there are virtual hosts, merge_server_configs() calls modperl_config_srv_merge() and modperl_config_dir_merge() for each virtual host, to perform this merge for mod_perl configuration objects.

META: is that's the place where everything restarts? it doesn't restart under debugger since we run with NODETACH I believe.

The open_logs Phase

After Apache processes the configuration it's time for the open_logs phase, executed by ap_run_open_logs(). mod_perl has registered the modperl_hook_init() hook to be called for this phase.

META: complete what happens at this stage in mod_perl

META: why is it called modperl_hook_init and not open_logs? is it because it can be called from other functions?

The post_config Phase

Immediately after open_logs, the post_config phase follows. Here ap_run_post_config() calls modperl_hook_post_config()

Request Processing

META: need to write


META: need to write


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

  • Stas Bekman <stas (at)>


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