Apache HTTP Server Version 2.0

Apache Module perchild

Description: Multi-Processing Module allowing for daemon processes serving requests to be assigned a variety of different userids
Status: MPM
Module Identifier: mpm_perchild_module


This MPM does not currently work on most platforms. Work is ongoing to make it functional.

This Multi-Processing Module (MPM) implements a hybrid multi-process, multi-threaded web server. A fixed number of processes create threads to handle requests. Fluctuations in load are handled by increasing or decreasing the number of threads in each process.

A single control process launches the number of child processes indicated by the NumServers directive at server startup. Each child process creates threads as specified in the StartThreads directive. The individual threads then listen for connections and serve them when they arrive.

Apache always tries to maintain a pool of spare or idle server threads, which stand ready to serve incoming requests. In this way, clients do not need to wait for new threads to be created. For each child process, Apache assesses the number of idle threads and creates or destroys threads to keep this number within the boundaries specified by MinSpareThreads and MaxSpareThreads. Since this process is very self-regulating, it is rarely necessary to modify these directives from their default values. The maximum number of clients that may be served simultaneously is determined by multiplying the number of server processes that will be created (NumServers) by the maximum number of threads created in each process (MaxThreadsPerChild).

While the parent process is usually started as root under Unix in order to bind to port 80, the child processes and threads are launched by Apache as a less-privileged user. The User and Group directives are used to set the privileges of the Apache child processes. The child processes must be able to read all the content that will be served, but should have as few privileges beyond that as possible. In addition, unless suexec is used, these directives also set the privileges which will be inherited by CGI scripts.

MaxRequestsPerChild controls how frequently the server recycles processes by killing old ones and launching new ones.

See also: Setting which addresses and ports Apache uses.

In addition it adds the extra ability to specify that specific processes should serve requests under different userids. These processes can then be associated with specific virtual hosts.


AssignUserId Directive

Syntax: AssignUserID user_id group_id
Context: virtual host
Status: MPM
Module: perchild

Tie a virtual host to a specific child process. Requests addressed to the virtual host where this directive appears will be served by the process running with the specified user and group id.

ChildPerUserId Directive

Syntax: ChildPerUserID user_id group_id child_id
Context: server config
Status: MPM
Module: perchild

Specify a user id and group id for a specific child process. The number of children is set by the NumServers directive. For example, the default value for NumServers is 5 and that means children ids 1,2,3,4 and 5 are available for assigment. If a child does not have an associated ChildPerUserID, it inherits the User and Group settings from the main server

Apache HTTP Server Version 2.0