APACHE 2.0A Look Under the Hood

CHUUG, June 2002

by Cliff Woolley

jwoolley@apache.org

Introduction

Assumptions The problems with the design of 1.3 How Apache 2.0 addresses them

Assumptions

You are somewhat familiar with configuring Apache 1.3

That you understand the concept of Apache modules

The problems with 1.3

Non-standard configuration scripts Porting to new and unusual platforms is

difficult Doesn’t scale well Modules can’t interact in particularly

interesting ways

How Apache 2.0 addresses these design problems

Configuration now uses GNU autoconf The Apache Portable Runtime (APR) Multi-Processing Modules (MPMs) I/O filtering “hooks”

The build environment:Using GNU autoconf

No more APACI – now it’s the real thing No more Configuration.tmpl – everything

uses ./configure arguments (hint: look at config.nice)

Autoconf’s feature tests are nice from a developer’s perspective

The build environment:The source tree layout

Modules categorized by function, not just lumped together

Platform-specific files hidden away Vendors can add their own module

directories

The Apache Portable Runtime

Platform Abstraction Resource Management Consistency, consistency, consistency

APR: Platform abstraction

Feature tests Native OS-specific data structures hidden

behind a consistent interface

APR: Resource management

Memory allocation handled for you Resource lifetimes arranged into a tree

that’s easy to prune

APR: All about consistency

…interface to the Operating System …resource handling Naming convention! (i.e., be ready for

renames)

Multi-Processing Modules

What are they? How do you configure them? Which one is best?

MPMs defined

A module that is specialized for managing the process/thread model used by Apache on a particular platform

Each has its own target OS and scalability goals

MPM configuration

# prefork MPM

# StartServers: number of server processes to start

# MinSpareServers: minimum number of server processes which are kept spare

# MaxSpareServers: maximum number of server processes which are kept spare

# MaxClients: maximum number of server processes allowed to start

# MaxRequestsPerChild: maximum number of requests a server process serves

<IfModule prefork.c>

StartServers 5

MinSpareServers 5

MaxSpareServers 10

MaxClients 150

MaxRequestsPerChild 0

</IfModule>

MPM configuration

# worker MPM

# StartServers: initial number of server processes to start

# MaxClients: maximum number of simultaneous client connections

# MinSpareThreads: minimum number of worker threads which are kept spare

# MaxSpareThreads: maximum number of worker threads which are kept spare

# ThreadsPerChild: constant number of worker threads in each server process

# MaxRequestsPerChild: maximum number of requests a server process serves

<IfModule worker.c>

StartServers 2

MaxClients 150

MinSpareThreads 25

MaxSpareThreads 75

ThreadsPerChild 25

MaxRequestsPerChild 0

</IfModule>

MPMs: How to choose

Benchmark!! (but don’t trust ab) Consider RAM usage vs. performance,

etc. Other tunability factors too, but this is the

big one

Filtered I/O

Bucket Brigades (my specialty :) Input Filters Output Filters

Bucket Brigades

A convenient abstract data type What do they look like? How are they used? What good are they?

Input filtering

Data is “pulled” from the client through the input filters

Each filter transforms the data it hands back to its caller in some way

Order is assigned at the beginning of each request

Output filtering

The most common form – interesting things happen when old-style “handlers” get converted into output filters

Data is “pushed” to the client through the output filters

Again, each filter transforms the data that passes through it

Apache modules

The module structure itself has changed:module MODULE_VAR_EXPORT foo_module = {

STANDARD_MODULE_STUFF, foo_init_Module, /* module initializer */ foo_config_perdir_create, /* create per-dir config structures */ foo_config_perdir_merge, /* merge per-dir config structures */ foo_config_server_create, /* create per-server config structures */ foo_config_server_merge, /* merge per-server config structures */ foo_config_cmds, /* table of config file commands */ foo_config_handler, /* [#8] MIME-typed-dispatched handlers */ foo_hook_Translate, /* [#1] URI to filename translation */ foo_hook_Auth, /* [#4] validate user id from request */ foo_hook_UserCheck, /* [#5] check if the user is ok _here_ */ foo_hook_Access, /* [#3] check access by host address */ NULL, /* [#6] determine MIME type */ foo_hook_Fixup, /* [#7] pre-run fixups */ NULL, /* [#9] log a transaction */ NULL, /* [#2] header parser */ foo_init_Child, /* child_init */ NULL, /* child_exit */ foo_hook_ReadReq, /* [#0] post read-request */};

Apache modules

The module structure itself has changed:

module AP_MODULE_DECLARE_DATA foo_module = {

STANDARD20_MODULE_STUFF,

foo_config_perdir_create, /* create per-dir config structures */

foo_config_perdir_merge, /* merge per-dir config structures */

foo_config_server_create, /* create per-server config structures */

foo_config_server_merge, /* merge per-server config structures */

foo_config_cmds, /* table of configuration directives */

foo_register_hooks /* register hooks */

};

Apache modules

What happened to all the other functions? How does a module register interest in

one of those functions?

Hooks

A new, more flexible replacement for most of the module_struct’s “phases”

Order is runtime-selectable (mostly) Any module can register its own hooks –

this allows a whole new level of inter-module cooperation

Hooks: example

static void register_hooks(apr_pool_t *p)

{

APR_REGISTER_OPTIONAL_FN(ap_ssi_get_tag_and_value);

APR_REGISTER_OPTIONAL_FN(ap_ssi_parse_string);

APR_REGISTER_OPTIONAL_FN(ap_register_include_handler);

ap_hook_post_config(include_post_config, NULL, NULL,

APR_HOOK_REALLY_FIRST);

ap_hook_fixups(include_fixup, NULL, NULL,

APR_HOOK_LAST);

ap_register_output_filter("INCLUDES", includes_filter,

AP_FTYPE_RESOURCE);

}

Hooks: example cont.

static int include_post_config(apr_pool_t *p, apr_pool_t *plog, apr_pool_t *ptemp, server_rec *s){ include_hash = apr_hash_make(p);

ssi_pfn_register = APR_RETRIEVE_OPTIONAL_FN(ap_register_include_handler);

if(ssi_pfn_register) { ssi_pfn_register("if", handle_if); ssi_pfn_register("set", handle_set); ssi_pfn_register("else", handle_else); } return OK;}

Conclusion

What will I get when upgrading to Apache 2.0?

What won’t I get (yet)? Future directions

Questions?