Browser Cache Partitioning: A Quick Guide

Hey, I’m Aditya, a frontend engineer, and today I’m breaking down browser cache partitioning—what it is, how it changes Chrome’s HTTP cache, and what it means for us. Let’s dive in!

Caching 101

Caching is like a browser’s memory trick: it saves web resources (images, scripts, CSS) locally so they load faster next time. The HTTP cache handles this, using rules like Cache-Control to decide what to store and for how long. Simple, speedy, and bandwidth-friendly.

How Chrome’s HTTP Cache Works (Pre-Partitioning)

Before Chrome 86, the HTTP cache was a shared pool. Resources were keyed by their URL—like https://cdn.example.com/doge.png.

If site-a.com and site-b.com used the same file, Chrome cached it once and reused it. Great for performance, but risky—sites could snoop on each other’s cache use, leaking user activity.

Cache Partitioning:

Since Chrome 86 (fully rolled out by 2025), cache partitioning locks that down. Now, Chrome adds a Network Isolation Key (top-level site + frame site) to the cache key. So doge.png on site-a.com is cached as { site-a.com, site-a.com, cdn.example.com/doge.png }, and on site-b.com, it’s a separate { site-b.com, site-b.com, cdn.example.com/doge.png }. Same file, different partitions.

Pros: Privacy boost—cross-site tracking is dead.

Cons: No more sharing. Identical resources get downloaded per site, increasing traffic (~4% per Google).

What It Means for Frontend Devs

CDNs Lose Shine: Shared assets like Google Fonts get fetched repeatedly. Consider self-hosting.

Optimize: Bundle files, cut requests—sharing’s off the table.

Test: Check load times with DevTools and adjust.

Partitioning trades speed for security. It’s a fair deal—privacy matters. Adapt, optimize, and keep building fast sites!

Thoughts? Hit me up!

If you found anything wrong in this content please let me know in the comments

Aditya