Apache MPM perchild

Available Languages: en

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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
Source File:	perchild.c

Summary

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- This module is not functional. Development of this module is not - complete and is not currently active. Do not use - perchild unless you are a programmer willing to - help fix it. -

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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.

Directives

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Topics

How it works

How it works

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.

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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).

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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.

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

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Working with different user-IDs

The perchild MPM adds the extra ability to - specify that particular processes should serve requests under - different user-IDs. These user-IDs can then be associated with - specific virtual hosts. You have to use one ChildPerUserID directive for - every user/group combination you want to be run. Then you can tie - particular virtual hosts to that user and group IDs.

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The following example runs 7 child processes. Two of them are run - under user1/group1. The next four are run - under user2/group2 and the remaining - process uses the User and Group - of the main server:

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Global config

- NumServers 7 - ChildPerUserID user1 group1 2 - ChildPerUserID user2 group2 4 -

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Using unbalanced numbers of processes as above is useful, if the - particular virtual hosts produce different load. The assignment to - the virtual hosts is easily done as in the example below. In - conclusion with the example above the following assumes, that - server2 has to serve about twice of the hits of - server1.

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Example

- NameVirtualHost * - - <VirtualHost *> - - ServerName fallbackhost - # no assignment; use fallback - - </VirtualHost> - - <VirtualHost *> - - ServerName server1 - AssignUserID user1 group1 - - </VirtualHost> - - <VirtualHost *> - - ServerName server2 - AssignUserID user2 group2 - - </VirtualHost> -

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AssignUserID Directive

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Description:	Tie a virtual host to a user and group ID
Syntax:	`AssignUserID user-id group-id`
Context:	virtual host
Status:	MPM
Module:	perchild

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

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The user and group ID has to be assigned to a number of children - in the global server config using the ChildPerUserID directive. See the section above for a - configuration example.

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ChildPerUserID Directive

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Description:	Specify user ID and group ID for a number of child -processes
Syntax:	`ChildPerUserID user-id group-id -num-children`
Context:	server config
Status:	MPM
Module:	perchild

Specify a user ID and group ID for a number of child processes. - The third argument, num-children, is the number of child - processes to start with the specified user and group. It does - not represent a specific child number. In order to use this - directive, the server must be run initially as root. - If you start the server as a non-root user, it will fail to change - to the lesser privileged user.

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If the total number of child processes, found by totaling all of the - third arguments to all ChildPerUserID directives - in the config file, is less than NumServers, then all remaining children will inherit the - User and Group settings from the main server. - See the section above for a configuration - example.

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Security

Don't set user-id (or group-id) to - root unless you know exactly what you are doing, and - what the dangers are.

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MaxThreadsPerChild Directive

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Description:	Maximum number of threads per child process
Syntax:	`MaxThreadsPerChild number`
Default:	`MaxThreadsPerChild 64`
Context:	server config
Status:	MPM
Module:	perchild

This directive sets the maximum number of threads that will be - created in each child process. To increase this value beyond its - default, it is necessary to change the value of the ThreadLimit directive and stop and - re-start the server.

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NumServers Directive