Reverse engineering GitHub Actions cache to make it fast

Blacksmith replaced its fork-based GitHub Actions cache alternative with a transparent, colocated caching system that requires no workflow changes and avoids the maintenance burden of modified actions. After GitHub moved its cache service to a Twirp- and Azure Blob Storage-based design, the team reverse engineered its protocols using network inspection, Protobuf definitions, and assistance from an LLM, then redirected cache traffic from ephemeral Firecracker VMs through in-VM NGINX and host-level proxies to a MinIO-backed S3-compatible store. Addressing Azure SDK hostname-dependent concurrency optimizations required Azure-like URLs, DNS remapping, and proxy translation, while nftables replaced unreliable iptables rules for per-VM traffic management. The Go-based proxy streams data efficiently and uses connection pooling, and the system treats degraded cache retrieval as a cache miss to prevent slow builds. Beta testing exposed compatibility differences among third-party cache actions, enabling refinements that reportedly delivered cache download speeds up to ten times faster than GitHub Actions, including a 114 MB cache retrieval at roughly 327.5 MB/s without user changes.

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