Caching & URL Filters

Configure cache extension and URL management filters in mod_pagespeed 1.1. Cache-extended URLs, domain rewriting, and resource instrumentation.

Overview

mod_pagespeed 1.1 includes filters for extending browser cache lifetimes and managing resource URLs across domains. These filters ensure that browsers cache resources aggressively while still receiving updated content when resources change. The extend_cache filter is a CoreFilter and one of the most impactful optimizations available.

Quick reference

Filter	Core	On-by-default	Description	Safe
`extend_cache`	Yes	Yes	Content-hashed URLs with 1-year TTL	Yes
`extend_cache_pdfs`	No	No	Same cache extension for PDF links	Yes
`rewrite_domains`	No	No	Applies domain mappings to resource URLs	Test first
`local_storage_cache`	No	No	localStorage-based caching for inlined resources	Experimental

IIS syntax

On IIS, use the same filter names with the pagespeed prefix in pagespeed.config (no semicolons):

pagespeed EnableFilters extend_cache

See IIS Configuration for the full file format reference.

extend_cache {#extend_cache}

Core filter. Rewrites resource URLs (CSS, JS, images) to include a content hash, then serves the optimized resource with a 1-year Cache-Control: max-age header. When the original resource changes, the hash changes, generating a new URL that bypasses the browser cache.

This is one of mod_pagespeed’s most impactful filters. Resources that previously had short or no cache lifetimes gain aggressive browser caching without the risk of serving stale content.

The sub-filters extend_cache_css, extend_cache_images, and extend_cache_scripts are implied by extend_cache and cannot be enabled independently. Enabling extend_cache enables all three.

extend_cache_pdfs {#extend_cache_pdfs}

Not a CoreFilter. Applies the same content-hash-based cache extension to PDF file links. Enable this filter if your site serves PDFs that change infrequently.

ModPagespeedEnableFilters extend_cache_pdfs

rewrite_domains {#rewrite_domains}

Not a CoreFilter. Test before deploying. Rewrites resource URLs to use domains specified by MapRewriteDomain or ShardDomain directives. Useful for CDN integration or domain sharding.

ModPagespeedEnableFilters rewrite_domains

This filter only affects resources that mod_pagespeed does not otherwise optimize. Resources already rewritten by other filters (rewrite_css, rewrite_images, etc.) already have their domains set by those filters.

See Domain Configuration for MapRewriteDomain and ShardDomain setup.

local_storage_cache {#local_storage_cache}

Experimental. Stores inlined CSS and JavaScript in the browser’s localStorage on first visit, then loads from localStorage on subsequent visits instead of re-inlining. This reduces HTML payload on repeat views at the cost of JavaScript complexity and reliance on localStorage availability.

ModPagespeedEnableFilters local_storage_cache

Not recommended for most deployments. Browser localStorage has size limits (typically 5-10 MB per origin) and can be cleared by the user at any time. Sites with many inlined resources may exceed these limits.