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web: fix static-file caching & root redirects - #3656
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When deploying #3647 I realized that there is a problem with caching certain static assets.
Generally (before this PR):
/-/static/asset.css?build-slug=12345as URLbuild_static_routerdid set caching headers in a way that the CDN & the browser can cache forever.This worked, but only for all the places where we actually added the build-slug.
We forgot that in
/-/static/chartjs/chart.min.css/-/static/chartjs/chart.min.js/-/static/trigger-rebuild.png/opensearch.xmland/-/static/opensearch.xml/robots.txtand/-/static/robots.txt/favicon.icoand/-/static/favicon.icothat means when we would want to update any of these, we could only purge our CDN, but user browsers would have old versions.
new approach
The new approach here is:
/-/static/{path}will serve static files with CDN only caching. It uses a surrogate key we invalidate on each deploy automatically/-/static/v/{all-assets-hash}/{path}will serve the same files, but _with different caching headers (cache forever in CDN & browser)/robots.txt) will be changed to be temporary redirects, with cdn-only caching.I tried to convert all places to using the assets-hash URL, but even when I forgot a place, it won't break. Worst case is cdn-only caching.
Nice side-effect: if we use relative
@urlor other references in CSS, the browser will request the versioned URL when the CSS file was referenced with version. In one CSS file I changed absolute paths to relative ones because of that.alternatives
The classic alternative to this approach is to include content-hashes in the filenames. This would improve cache ratio even more, but is quite a bit of work, especially rewriting the CSS links.