# API Changes in Apache HTTP Server 2.4 since 2.2

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

## 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
- Change ap_configfile_t, ap_cfg_getline(), ap_cfg_getc() to return error code, new ap_pcfg_strerror().

## 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.
- New function ap_state_query to determine if the server is in the initial configuration preflight phase or not. This is both easier to use and more correct than the old method of creating a pool userdata entry in the process pool.

## 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
- Modules that load other modules later than the EXEC_ON_READ config reading stage need to call ap_reserve_module_slots() or ap_reserve_module_slots_directive() in their pre_config hook.

## 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_cache_disk (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_disk_cache (renamed)

The mod_disk_cache module has been renamed to mod_cache_disk in order to be consistent with the naming of other modules within the server.

## 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, timed_callback, and get_name
- CHANGED interfaces: monitor hook, ap_reclaim_child_processes, ap_relieve_child_processes

## 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

Note: This is absolutely required for C++-language modules. It can be skipped for C-language modules, though that breaks module-specific log level support for files without it.

The number of parameters of the `ap_log_*` functions and the definition of `APLOG_MARK` has changed. Normally, the change is completely transparent. However, changes are required if a module uses `APLOG_MARK` as a parameter to its own functions or if a module calls `ap_log_*` without passing `APLOG_MARK`. A module which uses wrappers around `ap_log_*` typically uses both of these constructs.

The easiest way to change code which passes `APLOG_MARK` to its own functions is to define and use a different macro that expands to the parameters required by those functions, as `APLOG_MARK` should only be used when calling `ap_log_*` directly. In this way, the code will remain compatible with HTTPD 2.0 and 2.2.

Code which calls `ap_log_*` without passing `APLOG_MARK` will necessarily differ between 2.4 and earlier releases, as 2.4 requires a new third argument, `APLOG_MODULE_INDEX`.

/* code for httpd 2.0/2.2 */
ap_log_perror(file, line, APLOG_ERR, 0, p, "Failed to allocate dynamic lock structure");
/* code for httpd 2.4 */
ap_log_perror(file, line, APLOG_MODULE_INDEX, APLOG_ERR, 0, p, "Failed to allocate dynamic lock structure");

A `server_rec` pointer must be passed to `ap_log_error()` when called after startup. This was always appropriate, but there are even more limitations with a `NULL` `server_rec` in 2.4 than in previous releases. Beginning with 2.3.12, the global variable `ap_server_conf` can always be used as the `server_rec` parameter, as it will be `NULL` only when it is valid to pass `NULL` to `ap_log_error()`. `ap_server_conf` should be used only when a more appropriate `server_rec` is not available.

Consider the following changes to take advantage of the new `APLOG_TRACE1..8` log levels:

- Check current use of `APLOG_DEBUG` and consider if one of the `APLOG_TRACEn` levels is more appropriate.
- If your module currently has a mechanism for configuring the amount of debug logging which is performed, consider eliminating that mechanism and relying on the use of different `APLOG_TRACEn` levels. If expensive trace processing needs to be bypassed depending on the configured log level, use the `APLOGtracen` and `APLOGrtracen` macros to first check if tracing is enabled.

Modules sometimes add process id and/or thread id to their log messages. These ids are now logged by default, so it may not be necessary for the module to log them explicitly. (Users may remove them from the error log format, but they can be instructed to add it back if necessary for problem diagnosis.)

## If your module uses these existing APIs...

`ap_default_type()`
  This is no longer available; Content-Type must be configured explicitly or added by the application.
`ap_get_server_name()`
  If the returned server name is used in a URL, use `ap_get_server_name_for_url()` instead. This new function handles the odd case where the server name is an IPv6 literal address.
`ap_get_server_version()`
  For logging purposes, where detailed information is appropriate, use `ap_get_server_description()`. When generating output, where the amount of information should be configurable by ServerTokens, use `ap_get_server_banner()`.
`ap_graceful_stop_signalled()`
  Replace with a call to `ap_mpm_query(AP_MPMQ_MPM_STATE)` and checking for state `AP_MPMQ_STOPPING`.
`ap_max_daemons_limit`, `ap_my_generation`, and `ap_threads_per_child`
  Use `ap_mpm_query()` query codes `AP_MPMQ_MAX_DAEMON_USED`, `AP_MPMQ_GENERATION`, and `AP_MPMQ_MAX_THREADS`, respectively.
`ap_mpm_query()`
  Ensure that it is not used until after the register-hooks hook has completed. Otherwise, an MPM built as a DSO would not have had a chance to enable support for this function.
`ap_server_conf->process->pool` userdata
  Optional:
  - If your module uses this to determine which pass of the startup hooks is being run, use `ap_state_query(AP_SQ_MAIN_STATE)`.
  - If your module uses this to maintain data across the unloading and reloading of your module, use `ap_retained_data_create()` and `ap_retained_data_get()`.
`apr_global_mutex_create()`, `apr_proc_mutex_create()`
  Optional: See `ap_mutex_register()`, `ap_global_mutex_create()`, and `ap_proc_mutex_create()`; these allow your mutexes to be configurable with the Mutex directive; you can also remove any configuration mechanisms in your module for such mutexes
`CORE_PRIVATE`
  This is now unnecessary and ignored.
`dav_new_error()` and `dav_new_error_tag()`
  Previously, these assumed that `errno` contained information describing the failure. Now, an `apr_status_t` parameter must be provided. Pass 0/APR_SUCCESS if there is no such error information, or a valid `apr_status_t` value otherwise.
`mpm_default.h`, `DEFAULT_LOCKFILE`, `DEFAULT_THREAD_LIMIT`, `DEFAULT_PIDLOG`, etc.
  The header file and most of the default configuration values set in it are no longer visible to modules. (Most can still be overridden at build time.) `DEFAULT_PIDLOG` and `DEFAULT_REL_RUNTIMEDIR` are now universally available via `ap_config.h`.
`unixd_config`
  This has been renamed to ap_unixd_config.

## If your module interfaces with this feature...

suEXEC
  Optional: If your module logs an error when `ap_unixd_config.suexec_enabled` is 0, also log the value of the new field `suexec_disabled_reason`, which contains an explanation of why it is not available.
Extended status data in the scoreboard
  In previous releases, `ExtendedStatus` had to be set to `On`, which in turn required that mod_status was loaded. In 2.4, just set `ap_extended_status` to `1` in a pre-config hook and the extended status data will be available.

## Does your module...

Parse query args
  Consider if `ap_args_to_table()` would be helpful.
Parse form data...
  Use `ap_parse_form_data()`.
Check for request header fields `Content-Length` and `Transfer-Encoding` to see if a body was specified
  Use `ap_request_has_body()`.
Implement cleanups which clear pointer variables
  Use `ap_pool_cleanup_set_null()`.