API Changes in Apache HTTP Server 2.4 since 2.2

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This document describes changes to the Apache HTTPD API from version 2.2 to 2.4, that may be of interest to module/application developers and core hacks. At the time of writing, the 2.4 API is not finalised, and this document may serve to highlight points that call for further review.

API changes fall into two categories: APIs that are altogether new, and existing APIs that are expanded or changed. The latter are further divided into those where all changes are back-compatible (so existing modules can ignore them), and those that might require attention by maintainers. As with the transition from HTTPD 2.0 to 2.2, existing modules and applications will require recompiling and may call for some attention, but most should not require any substantial updating (although some may be able to take advantage of API changes to offer significant improvements).

For the purpose of this document, the API is split according to the public header files. These headers are themselves the reference documentation, and can be used to generate a browsable HTML reference with make docs.

Changed APIs
Specific information on upgrading modules from 2.2

Changed APIs

ap_expr (NEW!)

Introduces a new API to parse and evaluate boolean and algebraic expressions, including provision for a standard syntax and customised variants.

ap_listen (changed; back-compatible)

Introduces new API to enable apache child processes to serve different purposes.

ap_mpm (changed)

ap_mpm_run is replaced by a new mpm hook. Also ap_graceful_stop_signalled is lost, and ap_mpm_register_timed_callback is new.

ap_regex (changed)

In addition to the existing regexp wrapper, a new higher-level API ap_rxplus is now provided. This provides the capability to compile Perl-style expressions like s/regexp/replacement/flags and to execute them against arbitrary strings. Support for regexp backreference.

ap_slotmem (NEW!)

Introduces an API for modules to allocate and manage memory slots (normally) for shared memory.

ap_socache (NEW!)

API to manage a shared object cache.

heartbeat (NEW!)

common structures for heartbeat modules (should this be public API?)

http_config (changed)

Introduces per-module, per-directory loglevels, including macro wrappers.
New AP_DECLARE_MODULE macro to declare all modules.
New APLOG_USE_MODULE macro necessary for per-module loglevels in multi-file modules.
New API to retain data across module unload/load
New check_config hook
New ap_process_fnmatch_configs() to process wildcards

http_core (changed)

REMOVED ap_default_type, ap_requires, all 2.2 authnz API
Introduces Optional Functions for logio and authnz
New function ap_get_server_name_for_url to support ipv6 literals.
New function ap_register_errorlog_handler to register errorlog format string handlers.

httpd (changed)

Introduce per-directory, per-module loglevel
New loglevels APLOG_TRACEn
Introduce errorlog ids for requests and connections
Support for mod_request kept_body
Support buffering filter data for async requests
New CONN_STATE values
Function changes: ap_escape_html updated; ap_unescape_all, ap_escape_path_segment_buffer

http_log (changed)

Introduce per-directory, per-module loglevel
New loglevels APLOG_TRACEn
ap_log_*error become macro wrappers (fully back-compatible if APLOG_MARK macro is used)
piped logging revamped
module_index added to error_log hook
new function: ap_log_command_line

http_request (changed)

New auth_internal API and auth_provider API
New EOR bucket type
New function ap_process_async_request
New flags AP_AUTH_INTERNAL_PER_CONF and AP_AUTH_INTERNAL_PER_URI
New access_checker_ex hook to apply additional access control and/or bypass authentication.
New functions ap_hook_check_access_ex, ap_hook_check_access, ap_hook_check_authn, ap_hook_check_authz which accept AP_AUTH_INTERNAL_PER_* flags
DEPRECATED direct use of ap_hook_access_checker, access_checker_ex, ap_hook_check_user_id, ap_hook_auth_checker

When possible, registering all access control hooks (including authentication and authorization hooks) using AP_AUTH_INTERNAL_PER_CONF is recommended. If all modules' access control hooks are registered with this flag, then whenever the server handles an internal sub-request that matches the same set of access control configuration directives as the initial request (which is the common case), it can avoid invoking the access control hooks another time.

If your module requires the old behavior and must perform access control checks on every sub-request with a different URI from the initial request, even if that URI matches the same set of access control configuration directives, then use AP_AUTH_INTERNAL_PER_URI.

mod_auth (NEW!)

Introduces the new provider framework for authn and authz

mod_cache (changed)

Introduces a commit_entity() function to the cache provider interface, allowing atomic writes to cache. Add a cache_status() hook to report the cache decision. Remove all private structures and functions from the public mod_cache.h header file.

mod_core (NEW!)

This introduces low-level APIs to send arbitrary headers, and exposes functions to handle HTTP OPTIONS and TRACE.

mod_disk_cache (changed)

Changes the disk format of the disk cache to support atomic cache updates without locking. The device/inode pair of the body file is embedded in the header file, allowing confirmation that the header and body belong to one another.

mod_request (NEW!)

The API for mod_request, to make input data available to multiple application/handler modules where required, and to parse HTML form data.

mpm_common (changed)

REMOVES: accept, lockfile, lock_mech, set_scoreboard (locking uses the new ap_mutex API)
NEW API to drop privileges (delegates this platform-dependent function to modules)
NEW Hooks: mpm_query, mpm_note_child_killed, timed_callback, get_name, and function ap_mpm_note_child_killed

scoreboard (changed)

ap_get_scoreboard_worker is gratuitously made non-back-compatible as an alternative version is introduced. Additional proxy_balancer support. Child status stuff revamped.

util_cookies (NEW!)

Introduces a new API for managing HTTP Cookies.

util_ldap (changed)

I have yet to get a handle on this update.

util_mutex (NEW!)

A wrapper for APR proc and global mutexes in httpd.

util_script (changed)

NEW: ap_args_to_table

util_time (changed)

NEW: ap_recent_ctime_ex

Specific information on upgrading modules from 2.2

Logging

In order to take advantage of per-module loglevel configuration, any source file that calls the ap_log_* functions should declare which module it belongs to. If the module's module_struct is called foo_module, the following code can be used to remain backward compatible with HTTPD 2.0 and 2.2:

#include <http_log.h> #ifdef APLOG_USE_MODULE APLOG_USE_MODULE(foo); #endif

The number of parameters of the ap_log_* functions and the definition of APLOG_MARK has changed. Normally, the change is completely transparent. However, if a module implements wrapper functions for ap_log_* and uses APLOG_MARK when calling these wrappers, some adjustments are necessary. The easiest way is for the module to define and use a different macro that expands to the parameters required by the log wrapper functions. APLOG_MARK should only be used when calling ap_log_* without additional wrappers. In this way, the code will remain compatible with HTTPD 2.0 and 2.2.