📑 Cache Digest Immutable

Uses a service worker's fetch event and Cache API to set the cache-digest header. Webservers use this digest to check which files the client aleady has cached. This avoids server-pushing redundant data, providing atomic cache invalidation when used in combination with filename revving (aka cache busting).

As an explicit opt-in, to avoid overly aggressive caching, this implementation only includes, in the digest, responses that have the cache-control: immutable extension set.

Usage

1if ('serviceWorker' in navigator) {
2  addEventListener('load', function () {
3    navigator.serviceWorker.register('cache-digest-immutable.js')
4  }, {once: true})
5}

Note: This waits until everything else on the page has loaded. This is fine since the service worker only receives fetch events on future page loads.

Server Support

• H2O - Successfully tested with v2.1.0-beta3 (see config)
• http2server - Future support expected, see #6

Specifications

• Cache Digests for HTTP/2 draft-kazuho-h2-cache-digest-01
• HTTP Immutable Responses draft-mcmanus-immutable-00

Significant Decisions

• Written in ES2015+ which works fine as of Chrome 56 (Canary version at the time of starting this implementation).
• Uses importScripts(...) for native module loading within Service Workers. Not meant to be compatible with alternatives like CommonJS, ES2015 Modules (export/import), etc.
• Only caches if cache-control: immutable extension is set. This makes it opt-in for server configurations and avoids overly aggressive caching. Also minimises the cache digest size to only contain assets that will be cached long-term, as opposed to short-term ephemeral resources that require revalidation.
• Respects all caching directives by fully parsing the date/expires/cache-control headers (incl max-age=N/no-store/no-cache/must-revalidate).
• Manages cache by purging stale responses.
• Rewritten the URL value hashing code to use the browser's asynchronous SubtleCrypto API's SHA256 implementation.
• Rewritten the digest algo to closely follow the spec. Still uses the same (slightly tweaked) BitCoder implementation of Golomb coding.
• Memoises the digest to avoid recalculating on every single request. Huge performance improvement by throttling only one calculation per second.
• Rewrote Base64 encoding to use browser's native btoa function.

Related

• Unbundle - Originally developed for, and extracted from, this package.
• cache-digest.js - Passes the same test suite for guaranteed binary output compatibility.

Colophon

Made by Sebastiaan Deckers in Singapore 🇸🇬