mod_cache
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| mod_cache | |
|---|---|
| Name | mod_cache |
| Type | Apache HTTP Server module |
| Author | Robert McCool, Apache Software Foundation contributors |
| Stable release | varies by distribution |
| Operating system | Cross-platform |
| License | Apache License 2.0 |
mod_cache
mod_cache is an HTTP caching module for the Apache HTTP Server that provides configurable, reusable response caching to reduce backend load and improve response latency. It integrates with Apache's request processing model and with storage backends to cache responses for repeatable requests. The module is commonly used in conjunction with reverse proxies, load balancers, and content delivery setups in production environments.
Overview
mod_cache implements storage and retrieval of HTTP responses inside the Apache HTTP Server process tree, allowing downstream components like Varnish (software), Squid, and Nginx to be complemented or replaced by an in-process cache. It cooperates with HTTP semantics defined by RFC 7234, RFC 7231, and related standards to respect cache-control directives from origins such as Mozilla-hosted resources, content from WordPress sites, or APIs served by Node.js backends. Administrators of infrastructure at organizations like Facebook, Wikipedia, and cloud providers often consider mod_cache when designing architectures that also include HAProxy, Kubernetes, or Amazon Web Services load distribution. The module exposes hooks that interact with Apache core phases and other modules like mod_headers, mod_proxy, and mod_rewrite.
Architecture and Components
mod_cache is layered to separate cache policy, storage, and lookup logic. The core integrates with Apache's filter chain and request handler phases used by modules such as mod_ssl and mod_proxy_fcgi. Key components include the cache provider interface, the cache policy engine, and the cache object serializer. Providers implement persistent or ephemeral stores such as file-system-based backends similar to DiskCache patterns used by Squid or memory-backed stores like those used by Memcached-based adapters. The cache policy engine evaluates headers and status codes in accordance with guidance from Internet Engineering Task Force standards and can be influenced by directives emitted by control panels like cPanel or orchestration systems such as Ansible. Serialization formats and locking strategies are designed to work on filesystems used by Linux, FreeBSD, and Windows Server deployments.
Configuration and Usage
Administrators configure mod_cache through Apache configuration files parsed alongside directives for modules like mod_mime, mod_authz_core, and mod_log_config. Typical directives include enabling the module for a virtual host, selecting a cache provider, and setting expiration rules that interact with semantics from RFC 2616 and its successors. In reverse-proxy roles paired with mod_proxy, rules often employ regular expressions similar to patterns used in Perl Compatible Regular Expressions implementations adopted by nginx and HAProxy. Integration workflows commonly appear in deployment guides produced by vendors such as Red Hat, Debian, and Ubuntu. Real-world usage examples include caching of static assets served by Apache HTTP Server installations that host sites built on Drupal or Joomla!, and API response caching for services built on Django or Ruby on Rails.
Cache Invalidation and Management
Cache invalidation strategies for mod_cache mirror techniques used across caching systems: time-based expiration, header-driven invalidation, and explicit purge operations. Administrators may implement purge endpoints or control-plane operations compatible with tooling from Puppet, SaltStack, or Chef to trigger removal of cached objects. The module respects cache-control and pragma directives specified by origins such as GitHub Pages or GitLab pages, and supports conditional requests using headers familiar from HTTP/1.1 clients like curl and libraries like libcurl. For cooperative invalidation with external CDNs provided by vendors such as Fastly or Cloudflare, orchestration commonly involves coordinated API calls to purge or ban specific paths alongside mod_cache purges. Locking and concurrency management is patterned after strategies from cache systems developed by projects like Varnish Software to avoid stampede effects.
Performance and Security Considerations
Performance tuning for mod_cache involves balancing memory utilization, disk I/O patterns, and metadata lookup costs, similar to considerations documented for Varnish (software), Squid, and Redis when used as cache metadata stores. Security considerations include ensuring sensitive responses from authentication systems like OAuth 2.0 providers or SSO platforms such as Okta are not inadvertently cached, enforcing header policies defined by OWASP and adjusting behavior with modules such as mod_security. Administrators should also be mindful of cache poisoning vectors described in security advisories from entities like CERT and apply strict host- and vary-based controls analogous to mitigations recommended for shared caches in IETF publications. Rate limiting and request filtering can be coordinated with mod_evasive or upstream appliances like F5 Networks products.
Implementations and Compatibility
Implementations of mod_cache in Apache distributions vary by version and packaging maintained by organizations like the Apache Software Foundation, Red Hat, Debian, and IIS alternatives in Microsoft ecosystems. Cache provider modules that ship or integrate with mod_cache include filesystem providers, database-backed adapters, and custom modules created by vendors or open-source projects. Compatibility considerations include interaction with HTTP/2 stacks implemented in nghttp2 or OpenSSL-secured configurations maintained by distributions such as CentOS and AlmaLinux. Portability and integration are often evaluated in benchmarking efforts conducted by research groups and companies like Netcraft and Cloudflare Research.