FosterStack vs Harmonia

FosterStack is a self-hosted remote build cache server intended as a replacement for Develocity Build Cache Node. It supports Gradle and the Apache Maven Build Cache Extension remote HTTP mode, although Maven acceptance coverage is in progress. Size-capped LRU eviction frees space for new entries. HTTP Basic Auth over TLS uses constant-time credential comparison, and credentials are not logged. Prometheus metrics and a read-only status page report hit rates and cache size against its cap. Deployment options are a Docker image or static binary; release binaries are available for Linux and macOS on amd64 and arm64. Windows is not built or planned. The MIT-licensed core has signed releases with SLSA provenance, and security patches are included in the free tier. The maker says the free core makes no outbound connections and collects no telemetry or usage data. FosterStack identifies the product as v0.1 and says nobody outside its own work is yet running it in a production build pipeline. The full self-hosted server is free. Team, Business, and Compliance plans are described but not yet available, with listed prices of $49.00 per month, $199.00 per month, and $499.00 per year, respectively.

Harmonia is free, open-source software for serving a Nix store as a binary cache over HTTP. Written in Rust for Linux self-hosting, it supports HTTP range requests for NAR streaming and streaming build logs. The /serve// endpoint can provide package contents and discover index.html to serve websites directly from the Nix store. Harmonia can transparently compress content with zstd and provide TLS when no frontend web server is used. Documentation covers a NixOS module and nginx HTTPS frontend configuration for a public cache. Narinfo signing is disabled unless private signing-key paths are supplied. Prometheus metrics are available at /metrics, with a Grafana dashboard included in the repository. Since version 3.1.0, metadata is read directly from db.sqlite without a nix-daemon connection. Version 3.3.0 adds harmonia-gc for removing store garbage while nix-daemon builds. It uses the MIT License.