commit 4bc6c696aa7479c175b164914dfa2ff04d53974c
Author: Patrick Sy <patrick.sy@telekom.de>
Date:   Wed Feb 26 17:58:09 2025 +0100

    init

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+## Caching in MVP1
+
+### Requirements
+
+- We want to cache artifacts from upstream repositories in order to
+
+  - Avoid rate limiting (docker hub)
+  - Improve on download speed
+  - Improve on availability
+
+- We want to cache container images
+
+  - Docker Hub
+  - GCR
+  - Quay
+
+- We want to cache common software dependency artifacts of various programming languages
+
+  - Maven/Ivy Java
+  - Go
+  - NPM
+  - Rust
+  - PyPI
+
+- Must be easily configurable / manageable
+
+  - Static config
+  - API config (REST)
+
+- Must store artifacts permanently
+
+  - Resetting the cache (delete everything) should be easy, tho
+
+- Currently of out Scope
+  - Auth: Cache provides data to anyone who can reach it
+
+### Architectural Solutions
+
+#### File System-Based Caching
+
+- Re-using artifacts stored on the local file system
+
+  - e.g. backup and restore `node_modules` directory
+  - Setup within pipelines
+
+- Pro: Artifacts are downloaded directly from upstream, no further config needed
+  - Con: Does not address rate limiting concerns for initial cache warm up
+- Pro: No extra config needed in tooling apart of pipeline cache config
+- Has to be stored somewhere?
+  - GitHub Actions / GitLab typically manage this
+  - similar to local dev env
+- Con: State management
+  - Update the cache if new dependencies are used/requested
+  - Dirty state (looking at you, maven)
+  - Impure behaviour possible, creating side effects
+  - Integrity checks of package managers might be bypassed at this point
+- Con: Duplicate content
+- Con: Invalidation needed at some point
+
+#### Pull-Through Cache
+
+- Mirror/Proxy repo for upstream repo
+
+  - Downloads artifacts transparently from upstream when requested
+  - Downloaded artifacts are stored locally in the mirror for faster access
+
+- Pro: Can be re-used in pipelines, dev machines, cloud/prod environments
+- Pro: Little state management necessary if any
+- Con: Requires extra config in tooling, build tools, containerd, etc
+
+- Using only the pull-through cache should be fast enough for builds in CI
+  - Reproducible builds ftw
+
+### Solution Candidates
+
+#### Forgejo Runner Cache
+
+#### Nexus
+
+Open source / free version
+
+#### Artifactory
+
+Open source / free version
+
+#### Artipie
+
+- might be abandoned / low dev activity / needs new maintainer
+  - However, technically it looks extremely promising
+
+#### Pulp
+
+- Pull-Through Caches are only technical previews and might not work correctly
+
+#### kube-image-keeper
+
+#### 'Simple' Squid proxy (or similar)
+
+#### Harbor
+
+### Recommendation
+
+- Avoid using fs cache, i.e. forgejo runner cache, long term or at all
+  - Promote immutable infra and reproducible builds without side effects