Caching & URL Filters

Cache-extension and URL-management filters in mod_pagespeed 1.15: extend_cache for 1-year content-hashed caching, extend_cache_pdfs, rewrite_domains for CDN and sharding, and local_storage_cache.

Overview

These filters extend browser cache lifetimes and manage resource URLs across domains. Browsers cache resources for a year, but pick up updated content the moment a resource changes, because the URL carries a content hash. The extend_cache CoreFilter drives this: it sets a 1-year cache lifetime on CSS, JS, and image URLs.

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

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.

Resources that previously had short or no cache lifetimes gain a 1-year cache lifetime. Because the URL carries the content hash, a changed resource gets a new URL and is never served stale.

The sub-filters extend_cache_css, extend_cache_images, and extend_cache_scripts are included when you enable extend_cache, which turns on all three. Each can also be enabled individually with EnableFilters (for example, extend_cache_images alone).

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

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

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.