NAME

Server Life Cycle Handlers

Description

This chapter discusses server life cycle and the mod_perl handlers participating in it.

Server Life Cycle

The following diagram depicts the Apache 2.0 server life cycle and highlights which handlers are available to mod_perl 2.0:

Apache 2.0 starts by parsing the configuration file. After the configuration file is parsed, the PerlOpenLogsHandler handlers are executed if any. After that it's a turn of PerlPostConfigHandler handlers to be run. When the post_config phase is finished the server immediately restarts, to make sure that it can survive graceful restarts after starting to serve the clients.

When the restart is completed, Apache 2.0 spawns the workers that will do the actual work. Depending on the used MPM, these can be threads, processes and a mixture of both. For example the worker MPM spawns a number of processes, each running a number of threads. When each child process is started PerlChildInit handlers are executed. Notice that they are run for each starting process, not a thread.

From that moment on each working thread processes connections until it's killed by the server or the server is shutdown.

Startup Phases Demonstration Module

Let's look at the following example that demonstrates all the startup phases:

  file:MyApache/StartupLog.pm
  ---------------------------
  package MyApache::StartupLog;
  
  use strict;
  use warnings;
  
  use Apache::Log ();
  use Apache::ServerUtil ();
  
  use File::Spec::Functions;
  
  use Apache::Const -compile => 'OK';
  
  my $log_file = catfile "logs", "startup_log";
  my $log_fh;
  
  sub open_logs {
      my($conf_pool, $log_pool, $temp_pool, $s) = @_;
      my $log_path = Apache::server_root_relative($conf_pool, $log_file);
  
      $s->warn("opening the log file: $log_path");
      open $log_fh, ">>$log_path" or die "can't open $log_path: $!";
      my $oldfh = select($log_fh); $| = 1; select($oldfh);
  
      say("process $$ is born to reproduce");
      return Apache::OK;
  }
  
  sub post_config {
      my($conf_pool, $log_pool, $temp_pool, $s) = @_;
      say("configuration is completed");
      return Apache::OK;
  }
  
  sub child_init {
      my($child_pool, $s) = @_;
      say("process $$ is born to serve");
      return Apache::OK;
  }
  
  sub say {
      my($caller) = (caller(1))[3] =~ /([^:]+)$/;
      printf $log_fh "[%s] - %-11s: %s\n", scalar(localtime), $caller, $_[0];
  }
  
  END {
      say("process $$ is shutdown\n");
  }
  
  1;

And the httpd.conf configuration section:

  PerlModule            MyApache::StartupLog
  PerlOpenLogsHandler   MyApache::StartupLog::open_logs
  PerlPostConfigHandler MyApache::StartupLog::post_config
  PerlChildInitHandler  MyApache::StartupLog::child_init

When we perform a server startup followed by a shutdown, the logs/startup_log is created if it didn't exist already (it shares the same directory with error_log and other standard log files), and each stage appends to it its log information. So when we perform:

  % bin/apachectl start && bin/apachectl stop

the following is getting logged to logs/startup_log:

  [Thu Aug 22 15:57:08 2002] - open_logs  : process 21823 is born to reproduce
  [Thu Aug 22 15:57:08 2002] - post_config: configuration is completed
  [Thu Aug 22 15:57:09 2002] - END        : process 21823 is shutdown
  
  [Thu Aug 22 15:57:10 2002] - open_logs  : process 21825 is born to reproduce
  [Thu Aug 22 15:57:10 2002] - post_config: configuration is completed
  [Thu Aug 22 15:57:11 2002] - child_init : process 21830 is born to serve
  [Thu Aug 22 15:57:11 2002] - child_init : process 21831 is born to serve
  [Thu Aug 22 15:57:11 2002] - child_init : process 21832 is born to serve
  [Thu Aug 22 15:57:11 2002] - child_init : process 21833 is born to serve
  [Thu Aug 22 15:57:12 2002] - END        : process 21825 is shutdown

First of all, we can clearly see that Apache always restart itself after the first post_config phase is over. The logs show that the post_config phase is preceded by the open_logs phase. Only after Apache has restarted itself and has completed the open_logs and post_config phase again the child_init phase is run for each child process. In our example we have had the setting StartServers=4, therefore you can see four child processes were started.

Finally you can see that on server shutdown the END {} block has been executed by the parent server only.

Apache also specifies the pre_config phase, which is executed before the configuration files are parsed, but this is of no use to mod_perl, because mod_perl is loaded only during the configuration phase.

Now let's discuss each of the mentioned startup handlers and their implementation in the MyApache::StartupLog module in detail.

PerlOpenLogsHandler

The open_logs phase happens just before the post_config phase.

Handlers registered by PerlOpenLogsHandler are usually used for opening module-specific log files.

At this stage the STDERR stream is not yet redirected to error_log, and therefore any messages to that stream will be printed to the console the server is starting from (if such exists).

This phase is of type RUN_ALL.

The handler's configuration scope is SRV.

As we have seen in the MyApache::StartupLog::open_logs handler, the open_logs phase handlers accept four arguments: the configuration pool, the logging streams pool, the temporary pool and the server object:

  sub open_logs {
      my($conf_pool, $log_pool, $temp_pool, $s) = @_;
      my $log_path = Apache::server_root_relative($conf_pool, $log_file);
  
      $s->warn("opening the log file: $log_path");
      open $log_fh, ">>$log_path" or die "can't open $log_path: $!";
      my $oldfh = select($log_fh); $| = 1; select($oldfh);
  
      say("process $$ is born to reproduce");
      return Apache::OK;
  }

In our example the handler uses the function Apache::server_root_relative() to set the full path to the log file, which is then opened for appending and set to unbuffered mode. Finally it logs the fact that it's running in the parent process.

As you've seen in the example this handler is configured by adding to httpd.conf:

  PerlOpenLogsHandler MyApache::StartupLog::open_logs

PerlPostConfigHandler

The post_config phase happens right after Apache has processed the configuration files, before any child processes were spawned (which happens at the child_init phase).

This phase can be used for initializing things to be shared between all child processes. You can do the same in the startup file, but in the post_config phase you have an access to a complete configuration tree.

META: once mod_perl will have the API for that.

This phase is of type RUN_ALL.

The handler's configuration scope is SRV.

In our MyApache::StartupLog example we used the post_config() handler:

  sub post_config {
      my($conf_pool, $log_pool, $temp_pool, $s) = @_;
      say("configuration is completed");
      return Apache::OK;
  }

As you can see, its arguments are identical to the open_logs phase's handler. In this example handler we don't do much but logging that the configuration was completed and returning right away.

As you've seen in the example this handler is configured by adding to httpd.conf:

  PerlOpenLogsHandler MyApache::StartupLog::post_config

PerlChildInitHandler

The child_init phase happens immediately after the child process is spawned. Each child process (not a thread!) will run the hooks of this phase only once in their life-time.

In the prefork MPM this phase is useful for initializing any data structures which should be private to each process. For example Apache::DBI pre-opens database connections during this phase and Apache::Resource sets the process' resources limits.

This phase is of type VOID.

The handler's configuration scope is SRV.

In our MyApache::StartupLog example we used the child_init() handler:

  sub child_init {
      my($child_pool, $s) = @_;
      say("process $$ is born to serve");
      return Apache::OK;
  }

The child_init() handler accepts two arguments: the child process pool and the server object. The example handler logs the pid of the child process it's run in and returns.

As you've seen in the example this handler is configured by adding to httpd.conf:

  PerlOpenLogsHandler MyApache::StartupLog::child_init

PerlChildExitHandler

META: not implemented yet

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.

cpanm mod_perl

perl -MCPAN -e shell
install mod_perl