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Reverse engineering GitHub Actions cache to make it fast
We reverse engineered GitHub Actions cache internals to transparently route cache requests through our faster, colocated cache. This delivered up to 10x faster cache performance for some of our customers, with no code changes required and no need to maintain forks of upstream actions.
After a few iterations of creative prompt engineering, we sniffed out the requests GitHub made to its control plane and came up with a proto definition of the actions service. While NGINX takes care of request routing for the GitHub Actions control plane, getting things to play nicely with the Azure SDK called for some serious kernel-level network gymnastics. Through testing, we quickly hit the limits: iptables was already doing heavy lifting for other subsystems inside our environment, and with each VM adding or removing its own set of rules, things got messy fast, and extremely flakey.
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