# Overview of new features in Apache 2.4

This document describes some of the major changes between the 2.2 and 2.4 versions of the Apache HTTP Server. For new features since version 2.0, see the 2.2 new features document.

# Core Enhancements

KeepAliveTimeout in milliseconds
  It is now possible to specify KeepAliveTimeout in milliseconds.

# Module Enhancements

mod_ssl: Support for OCSP validation of client certificates
  mod_ssl
  can now be configured to use an OCSP server to check the validation status of a client certificate. The default responder is configurable, along with the decision on whether to prefer the responder designated in the client certificate itself.

# Program Enhancements

# Module Developer Changes

Check Configuration Hook Added
  A new hook, `check_config`, has been added which runs between the `pre_config` and `open_logs` hooks. It also runs before the `test_config` hook when the `-t` option is passed to httpd. The `check_config` hook allows modules to review interdependent configuration directive values and adjust them while messages can still be logged to the console. The user can thus be alerted to misconfiguration problems before the core `open_logs` hook function redirects console output to the error log.
Expression Parser Added
  We now have a general-purpose expression parser, whose API is exposed in ap_expr.h. This is adapted from the expression parser previously implemented in mod_include.
Authorization Logic Containers
  Advanced authorization logic may now be specified using the Require directive and the related container directives, such as RequireAll, all provided by the mod_authz_core module.
Small-Object Caching Interface
  The ap_socache.h header exposes a provider-based interface for caching small data objects, based on the previous implementation of the mod_ssl session cache. Providers using a shared-memory cyclic buffer, disk-based dbm files, and a memcache distributed cache are currently supported.