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A content-addressable caching proxy for Go modules and NPM packages.

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# content-cache

A content-addressable caching proxy for Go modules, NPM packages, PyPI packages, Maven artifacts, RubyGems, OCI registries, Git repositories, direct download artefacts, and a generic HTTP build cache compatible with sccache and Gradle's HTTP Build Cache. Reduces build times and network bandwidth by caching package downloads and build artifacts locally with automatic deduplication and expiration policies.

## Problem

Development teams waste significant time and bandwidth re-downloading the same packages across builds, CI runs, and developer machines. A single `go mod download` or `npm install` can fetch hundreds of megabytes that were already downloaded yesterday. Network failures during package downloads break builds unpredictably.

## Solution

content-cache acts as a local caching proxy that:
- Stores packages once using content-addressable storage (BLAKE3 hashing)
- Serves cached packages in microseconds instead of milliseconds
- Deduplicates identical content across different package versions
- Coalesces concurrent requests for the same uncached resource into a single upstream fetch
- Continues serving cached packages when upstream registries are unavailable

## S3-FIFO Cache Eviction

Based on [FIFO Queues are All You Need for Cache Eviction (CMU-CS-24-149)](https://www.pdl.cmu.edu/ftp/Storage/CMU-CS-24-149-juncheny.pdf).

S3-FIFO achieves lower miss ratios than LRU by keeping one-hit-wonders out of the main cache. It uses three structures:

- **Small queue (10% of `--cache-max-size`)** — new blobs enter here. A blob that is accessed again while in this queue is promoted to main; one that is never re-accessed is evicted and recorded in the ghost set.
- **Main queue (90%)** — hot blobs. Each eviction candidate gets a second-chance pass for every cache hit it received since its last eviction check, then is evicted cold.
- **Ghost set** — a compact in-memory + on-disk set of recently evicted hashes (no blob data). When a client re-requests an evicted blob it is admitted directly to main, bypassing the small queue probation period.

### Memory and disk overhead

The ghost set stores only hashes (32 bytes each), not blob data. Its size is automatically capped at the current number of entries in the main queue. For a cache holding 100,000 blobs this amounts to roughly 3 MB of ghost state persisted in bbolt alongside the queue metadata.

### Configuration

Size eviction activates automatically when `--cache-max-size` is set. The GC continues to run in parallel and handles TTL expiry, unreferenced blobs, and orphan cleanup — S3-FIFO handles only the size limit.

| Flag | Default | Description |
|------|---------|-------------|
| `--cache-max-size` | `10737418240` (10 GB) | Total byte limit for cached blobs. Set to `0` to disable size eviction. |
| `--gc-interval` | `1h` | How often the background eviction safety-net tick fires. Real eviction is signal-driven and happens inline after each cache write. |

### Startup behaviour

Queue state is persisted in bbolt, so eviction is warm across restarts — the manager recomputes byte totals from the persisted queue on startup and resumes where it left off. Blobs that were written before S3-FIFO was first enabled (e.g. files left on disk from a previous deployment) will not appear in any queue and will not be subject to size eviction until they are naturally expired by TTL or cleaned up by GC.

## Quick Start

```bash
# Build and run the cache server
go build -o content-cache ./cmd/content-cache
./content-cache serve --listen :8080 --storage ./cache

# Configure Go to use the cache
export GOPROXY=http://localhost:8080/goproxy,direct

# Downloads are now cached locally
go get github.com/pkg/errors@v0.9.1  # First request: ~12ms (upstream)
go get github.com/pkg/errors@v0.9.1  # Second request: ~100µs (cache hit)

# Configure NPM to use the cache
npm config set registry http://localhost:8080/npm/

# NPM packages are now cached
npm install express  # First request: fetches from upstream
npm install express  # Second request: served from cache

# Use as an OCI registry mirror (prefix-based routing, default prefix: docker-hub)
docker pull localhost:8080/docker-hub/library/alpine:latest

# Configure pip to use the cache
pip install --index-url http://localhost:8080/pypi/simple/ requests

# Python packages are now cached
pip install requests  # First request: fetches from upstream
pip install requests  # Second request: served from cache

# Configure Maven to use the cache
# Create ~/.m2/settings.xml with:
#
# <?xml version="1.0" encoding="UTF-8"?>
# <settings xmlns="http://maven.apache.org/SETTINGS/1.2.0"
#           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
#           xsi:schemaLocation="http://maven.apache.org/SETTINGS/1.2.0 https://maven.apache.org/xsd/settings-1.2.0.xsd">
#   <mirrors>
#     <mirror>
#       <id>content-cache</id>
#       <mirrorOf>central</mirrorOf>
#       <url>http://localhost:8080/maven</url>
#     </mirror>
#   </mirrors>
# </settings>

# Maven artifacts are now cached
mvn dependency:get -Dartifact=org.apache.commons:commons-lang3:3.12.0

# Configure Gradle to use the cache (in settings.gradle.kts or build.gradle.kts)
# repositories {
#     maven {
#         url = uri("http://localhost:8080/maven")
#     }
# }

# Gradle uses the same Maven repository protocol - no separate handler needed
./gradlew build  # Dependencies are cached through the Maven endpoint

# Add Clojars (or any extra Maven-compatible repo) as a fallback upstream.
# Fetches try each URL in order and fall through on 404, with short-lived
# negative caching so misses don't repeat on every request.
content-cache --maven-upstream=https://repo.maven.apache.org/maven2 \
              --maven-upstream=https://repo.clojars.org

# Configure Bundler to use the cache (global mirror)
bundle config set --global mirror.https://rubygems.org http://localhost:8080/rubygems/

# Or configure gem command directly
gem sources --add http://localhost:8080/rubygems/
gem sources --remove https://rubygems.org/

# Ruby gems are now cached
bundle install   # First request: fetches from upstream
bundle install   # Second request: served from cache

# Use as a Git HTTPS caching proxy (requires --git-allowed-hosts)
./content-cache serve --listen :8080 --storage ./cache --git-allowed-hosts github.com

# Clone through the proxy
git clone http://localhost:8080/git/github.com/user/repo.git /tmp/repo

# Repeated clones with the same refs are served from cache
git clone http://localhost:8080/git/github.com/user/repo.git /tmp/repo2  # cache hit

# Or configure Git to transparently route through the proxy
git config --global url."http://localhost:8080/git/github.com/".insteadOf "https://github.com/"

# Now regular git commands work transparently through the cache
git clone https://github.com/user/repo.git  # routed through proxy automatically

# To undo
git config --global --unset url."http://localhost:8080/git/github.com/".insteadOf

# Cache mise aqua downloads from GitHub Releases plus other direct HTTPS assets
./content-cache serve --listen :8080 --storage ./cache \
  --fetch-allowed-hosts raw.githubusercontent.com,releases.hashicorp.com,nodejs.org,dl.google.com

# mise can rewrite release downloads to the cache server
cat >> ~/.config/mise/config.toml <<'EOF'
[settings.url_replacements]
"regex:^https://github\\.com/(.+/releases/download/.+)" = "http://localhost:8080/github-release/$1"
"regex:^https://raw\\.githubusercontent\\.com/(.+)" = "http://localhost:8080/fetch/raw.githubusercontent.com/$1"
"regex:^https://releases\\.hashicorp\\.com/(.+)" = "http://localhost:8080/fetch/releases.hashicorp.com/$1"
EOF

# Use as a shared Go build cache (GOCACHEPROG)
# The cacheprog subcommand implements the GOCACHEPROG protocol, backed by the server's /buildcache/ endpoint.
# Artifacts are stored on the server and a local copy is kept for DiskPath responses.
export GOCACHEPROG="content-cache cacheprog --server http://localhost:8080"
go build ./...  # First build: artifacts uploaded to server
go build ./...  # Subsequent builds on any machine: artifacts served from server cache

# Use as an sccache HTTP storage backend
# sccache and Gradle's HTTP Build Cache share the same simple GET/PUT protocol.
# Point sccache at the /httpcache/ endpoint:
export SCCACHE_HTTP_URL=http://localhost:8080/httpcache/
sccache --start-server
cargo build  # First build: artifacts uploaded to server
cargo build  # Subsequent builds: artifacts served from cache

# Use as a Gradle HTTP Build Cache
# In settings.gradle.kts:
# buildCache {
#     remote<HttpBuildCache> {
#         url = uri("http://localhost:8080/httpcache/")
#         isPush = true
#     }
# }
./gradlew build  # Build outputs are stored in and served from the HTTP cache
```

## Performance

| Operation | Upstream | Cached | Improvement |
|-----------|----------|--------|-------------|
| Module info | 12ms | 100µs | 120x faster |
| Module zip | 150ms | 1ms | 150x faster |

## Current Features

### Implemented
- **GOPROXY Protocol**: Full support for Go module proxy protocol (`/@v/list`, `.info`, `.mod`, `.zip`)
- **Go Checksum Database Proxy**: Caches `sum.golang.org` lookups at `/sumdb/` (also accessible at `/goproxy/sumdb/`); entries never expire since they are cryptographically immutable
- **Go Build Cache (GOCACHEPROG)**: `cacheprog` subcommand implements the `GOCACHEPROG` protocol, backed by the `/buildcache/` HTTP endpoint — enables shared, persistent build artifact caching across CI runners and developer machines
- **HTTP Build Cache (sccache / Gradle)**: Generic HTTP build cache at `/{httpcache-prefix}/` (default `/httpcache/`) — compatible with sccache's HTTP storage backend and Gradle's `HttpBuildCache`. Both tools use the same simple `GET`/`PUT` protocol: `GET` returns the cached blob or 404, `PUT` stores a blob keyed by an arbitrary string. No upstream fetch; this is a write-through cache only.
- **NPM Registry Protocol**: Complete NPM registry support with tarball caching and integrity verification
- **PyPI Simple API**: Full support for PEP 503/691 Simple Repository API with wheel and sdist caching
- **Maven Repository**: Full support for Maven Central with JAR, POM, and checksum caching
- **RubyGems Registry**: Full support for Compact Index and legacy specs API with gem caching and SHA256 verification
- **OCI Distribution v2**: Read-through cache for container registries with tag-to-digest resolution
- **Git Smart HTTP Proxy**: Caching proxy for `git clone`/`fetch` over HTTPS with pack-level caching, host allowlist, and singleflight deduplication
- **Direct HTTPS Fetch Cache**: Read-through cache for immutable release artefacts and mirrored downloads via `/github-release/*` and `/fetch/{host}/...`
- **Content-Addressable Storage**: BLAKE3 hashing with automatic deduplication
- **Filesystem Backend**: Atomic writes with sharded directory structure
- **Download Deduplication**: Singleflight-based coalescing of concurrent requests for the same uncached resource
- **Pull-Through Caching**: Fetches from upstream on cache miss, caches for future requests
- **Cache Expiration**: TTL-based expiration with S3-FIFO size-based eviction (lower miss ratios by filtering one-hit-wonders from polluting the main cache)
- **Inbound Authentication**: Static token auth (`--auth-token` / `--auth-token-file`) or OIDC token validation (`--oidc-policies`) with per-protocol permission policies for CI/CD pipelines (GitHub Actions, Buildkite, GitLab CI). Accepts both `Authorization: Bearer <token>` and `Authorization: Basic` (password = token), enabling tools like pip, Maven, and Bundler that cannot send Bearer headers. All endpoints except `/health` and `/metrics` are protected.
- **Upstream Credentials**: Template-based credentials file (`--credentials-file`) with routing tables for per-scope (NPM) and per-repo-prefix (Git) credential selection, plus multi-registry OCI auth. Supports pluggable secret providers (environment variables, files, 1Password CLI)
- **Routing Tables**: NPM scope-based and Git repo-prefix-based routing with catch-all fallback, validated at startup
- **Health & Stats Endpoints**: `/health` for liveness checks, `/stats` for cache statistics
- **OpenTelemetry Metrics**: Request counts, bytes served, and latency histograms with cache hit/miss breakdown
- **Prometheus Integration**: Optional `/metrics` endpoint for Prometheus scraping
- **Structured Logging**: JSON logs with protocol, endpoint, cache_result, and timing fields

### Planned
- S3 storage backend
- Compression (zstd)
- OpenTelemetry tracing

## Architecture

```mermaid
graph TD
    A[HTTP Server] --> B[GOPROXY Handler]
    A --> C[NPM Handler]
    A --> D[OCI Handler]
    A --> G[PyPI Handler]
    A --> H[Maven Handler]
    A --> I[RubyGems Handler]
    A --> J[Git Handler]
    A --> K[BuildCache Handler]
    A --> L[HTTPCache Handler]

B --> DL[Download Deduplication]
    C --> DL
    D --> DL
    G --> DL
    H --> DL
    I --> DL
    J --> DL
    K --> E
    L --> E

DL --> E[Content-Addressable Store]

E --> F[Storage Backend]

A -.-> A1["/goproxy/*"]
    A -.-> A9["/sumdb/*"]
    A -.-> A2["/npm/*"]
    A -.-> A3["/v2/*"]
    A -.-> A4["/pypi/*"]
    A -.-> A5["/maven/*"]
    A -.-> A6["/rubygems/*"]
    A -.-> A8["/git/*"]
    A -.-> A10["/buildcache/*"]
    A -.-> A11["/httpcache/*"]
    A -.-> A7["/health, /stats"]

E -.-> E1["blobs/{hash[0:2]}/{hash}"]
    E -.-> E2["TTL + S3-FIFO Eviction"]

F -.-> F1["Filesystem (implemented)"]
    F -.-> F2["S3 (planned)"]

style A fill:#e1f5ff
    style DL fill:#f3e5f5
    style E fill:#fff4e1
    style F fill:#e8f5e9
```

## Configuration

Configuration is available via command-line flags or environment variables. Environment variables take precedence over defaults but are overridden by explicit flags.

### Server Options

| Flag | Environment Variable | Default | Description |
|------|---------------------|---------|-------------|
| `--listen` | `LISTEN_ADDRESS` | `:8080` | HTTP server listen address |
| `--storage` | `CACHE_STORAGE` | `./cache` | Local storage directory path |
| `--tls-cert` | `TLS_CERT_FILE` | | Path to TLS certificate file (enables HTTPS) |
| `--tls-key` | `TLS_KEY_FILE` | | Path to TLS private key file (enables HTTPS) |

### Authentication Options

| Flag | Environment Variable | Default | Description |
|------|---------------------|---------|-------------|
| `--auth-token` | `AUTH_TOKEN` | | Static Bearer token for inbound authentication (mutually exclusive with `--oidc-policies`) |
| `--auth-token-file` | `AUTH_TOKEN_FILE` | | Path to file containing auth token (for k8s secret mounts) |
| `--oidc-policies` | `OIDC_POLICIES_FILE` | | Path to OIDC trust policies JSON file (mutually exclusive with `--auth-token`) |
| `--credentials-file` | `CREDENTIALS_FILE` | | Path to credentials template file for upstream auth |

When `--auth-token` is set, all requests (except `/health` and `/metrics`) require authentication. Both `Authorization: Bearer <token>` and `Authorization: Basic base64(username:token)` are accepted — the password field is treated as the token. Basic auth support enables tools like pip, Maven, and Bundler that cannot send Bearer headers. The token can also be provided via the `auth_token` field in the credentials file; the CLI flag takes precedence.

**Configuring tools with Basic auth** (use any string as the username; the token goes in the password field):

```bash
# pip — embed credentials in the index URL
pip install --index-url http://x-token:$CACHE_TOKEN@cache.example.com/pypi/simple/ requests

# pip — or set via environment variable (PEP 503)
export PIP_INDEX_URL=http://x-token:$CACHE_TOKEN@cache.example.com/pypi/simple/
pip install requests
```

```xml

<settings>
  <servers>
    <server>
      <id>content-cache</id>
      <username>x-token</username>
      <password>${env.CACHE_TOKEN}</password>
    </server>
  </servers>
  <mirrors>
    <mirror>
      <id>content-cache</id>
      <mirrorOf>central</mirrorOf>
      <url>http://cache.example.com/maven</url>
    </mirror>
  </mirrors>
</settings>
```

```bash
# Bundler — embed credentials in the mirror URL
bundle config set --global mirror.https://rubygems.org \
  http://x-token:$CACHE_TOKEN@cache.example.com/rubygems/
```

When `--oidc-policies` is set, requests must present a valid OIDC token whose claims match a trust policy that grants access to the requested protocol. Both Bearer and Basic auth (password = OIDC token) are accepted. See [OIDC Authentication](#oidc-authentication) below.

When `--credentials-file` is set, the file is parsed as a Go template that produces JSON. Template functions resolve secrets from environment variables (`env`), files (`file`), or external stores like 1Password CLI (`op`). See the [Credentials File](#credentials-file) section for the full schema.

### OIDC Authentication

OIDC authentication allows CI/CD pipelines to use short-lived identity tokens rather than long-lived static secrets. Tokens are validated against the issuer's JWKS endpoint and then matched against trust policies that control per-protocol access.

**Trust policy file** (`--oidc-policies`):

```json
{
  "trust_policies": [
    {
      "name": "buildkite-myorg",
      "issuer": "https://agent.buildkite.com",
      "audience": ["https://cache.example.com"],
      "required_claims": {
        "organization_slug": "myorg"
      },
      "permissions": ["goproxy", "npm", "buildcache"]
    },
    {
      "name": "github-actions-myorg",
      "issuer": "https://token.actions.githubusercontent.com",
      "audience": ["https://cache.example.com"],
      "required_claims": {
        "repository_owner": "myorg",
        "ref": "refs/heads/*"
      },
      "permissions": ["goproxy", "npm"]
    }
  ]
}
```

**Policy fields:**

| Field | Description |
|-------|-------------|
| `name` | Human-readable policy name (appears in logs) |
| `issuer` | OIDC issuer URL — must match the `iss` claim exactly |
| `audience` | Allowed audience values — must match the `aud` claim. Omitting this field skips audience validation (not recommended) |
| `required_claims` | Map of claim name → expected value. Supports `*` wildcard suffix (e.g. `"refs/heads/*"`) and lists |
| `permissions` | Protocol names this policy grants: `goproxy`, `npm`, `oci`, `pypi`, `maven`, `rubygems`, `git`, `fetch`, `sumdb`, `buildcache`, `httpcache`, `admin`. Use `"*"` to grant all. |

**Using OIDC tokens in CI/CD:**

Buildkite — request an OIDC token via the [Buildkite Agent OIDC plugin](https://buildkite.com/docs/agent/v3/cli-oidc) and pass it as a Bearer token:

```bash
TOKEN=$(buildkite-agent oidc request-token --audience https://cache.example.com)
export GOPROXY=http://cache.example.com/goproxy
export GONOSUMCHECK=*
curl -H "Authorization: Bearer $TOKEN" http://cache.example.com/goproxy/...
```

GitHub Actions — the `id-token: write` permission exposes `ACTIONS_ID_TOKEN_REQUEST_URL`. Most package managers support setting a Bearer token via environment variables or config files; pass the token from `${{ steps.auth.outputs.token }}`.

### Upstream Registry Options

| Flag | Environment Variable | Default | Description |
|------|---------------------|---------|-------------|
| `--go-upstream` | `GO_UPSTREAM` | `proxy.golang.org` | Upstream Go module proxy URL |
| `--npm-upstream` | `NPM_UPSTREAM` | `registry.npmjs.org` | Upstream NPM registry URL |
| `--oci-upstream` | `OCI_UPSTREAM` | `registry-1.docker.io` | Upstream OCI registry URL |
| `--pypi-upstream` | `PYPI_UPSTREAM` | `pypi.org/simple/` | Upstream PyPI Simple API URL |
| `--maven-upstream` | `MAVEN_UPSTREAM` | `repo.maven.apache.org/maven2` | Upstream Maven repository URLs (repeat or comma-separate to add fallbacks, e.g. Clojars) |
| `--rubygems-upstream` | `RUBYGEMS_UPSTREAM` | `rubygems.org` | Upstream RubyGems registry URL |

### Fetch Cache Options

| Flag | Environment Variable | Default | Description |
|------|---------------------|---------|-------------|
| `--fetch-allowed-hosts` | `FETCH_ALLOWED_HOSTS` | | Comma-separated list of allowed upstream hosts for `/fetch/{host}/...` |

### OCI Options

| Flag | Environment Variable | Default | Description |
|------|---------------------|---------|-------------|
| `--oci-prefix` | `OCI_PREFIX` | `docker-hub` | Routing prefix for the OCI registry (appears in URL path as `/v2/{prefix}/...`) |
| `--oci-tag-ttl` | `OCI_TAG_TTL` | `5m` | TTL for OCI tag→digest cache mappings |

OCI registry credentials (username/password) are configured via the credentials file. When the credentials file defines `oci.registries[]`, the `--oci-upstream` and `--oci-prefix` CLI flags are ignored.

### Metadata TTL Options

| Flag | Environment Variable | Default | Description |
|------|---------------------|---------|-------------|
| `--pypi-metadata-ttl` | `PYPI_METADATA_TTL` | `5m` | TTL for PyPI project metadata cache |
| `--maven-metadata-ttl` | `MAVEN_METADATA_TTL` | `5m` | TTL for maven-metadata.xml cache |
| `--rubygems-metadata-ttl` | `RUBYGEMS_METADATA_TTL` | `5m` | TTL for RubyGems metadata cache |
| `--fetch-metadata-ttl` | `FETCH_METADATA_TTL` | `24h` | TTL for direct download cache metadata under `/fetch` and `/github-release` |

### Git Proxy Options

| Flag | Environment Variable | Default | Description |
|------|---------------------|---------|-------------|
| `--git-allowed-hosts` | `GIT_ALLOWED_HOSTS` | | Comma-separated list of allowed Git upstream hosts (e.g., `github.com,gitlab.com`) |
| `--git-max-request-body` | `GIT_MAX_REQUEST_BODY` | `104857600` | Maximum git-upload-pack request body size in bytes (100MB) |
| `--git-upstream-auth-trusted-single-tenant` | `GIT_UPSTREAM_AUTH_TRUSTED_SINGLE_TENANT` | `false` | Allow GitHub App upstream Git credentials without repo-level caller authorization. Only safe for trusted single-tenant deployments. |

### HTTP Build Cache Options (sccache / Gradle)

| Flag | Environment Variable | Default | Description |
|------|---------------------|---------|-------------|
| `--httpcache-ttl` | `HTTPCACHE_TTL` | `24h` | TTL for cached build artifacts |

Point clients at `http://host/httpcache/`.

### Cache Management

| Flag | Environment Variable | Default | Description |
|------|---------------------|---------|-------------|
| `--blob-retention` | `BLOB_RETENTION` | `24h` | Minimum time to retain blobs after last access before GC may delete them (0 to disable) |
| `--cache-max-size` | `CACHE_MAX_SIZE` | `10737418240` | Maximum cache size in bytes (10GB, 0 to disable) |
| `--expiry-check-interval` | `EXPIRY_CHECK_INTERVAL` | `1h` | How often to check for expired content |
| `--gc-interval` | `GC_INTERVAL` | `1h` | How often to run garbage collection |
| `--gc-startup-delay` | `GC_STARTUP_DELAY` | `5m` | Delay before first GC run after startup |

### Logging Options

| Flag | Environment Variable | Default | Description |
|------|---------------------|---------|-------------|
| `--log-level` | `LOG_LEVEL` | `info` | Log level: `debug`, `info`, `warn`, `error` |
| `--log-format` | `LOG_FORMAT` | `text` | Log format: `text`, `json` |

### Metrics Options

The Prometheus `/metrics` endpoint is controlled by a flag. OTLP export is configured entirely via the [standard OpenTelemetry environment variables](https://opentelemetry.io/docs/specs/otel/protocol/exporter/) so content-cache behaves like any other OTel-instrumented application — and so the OpenTelemetry Operator's `instrumentation.opentelemetry.io/inject-sdk` annotation works out of the box.

| Flag | Environment Variable | Default | Description |
|------|---------------------|---------|-------------|
| `--metrics-prometheus` | `METRICS_PROMETHEUS` | `false` | Enable Prometheus `/metrics` endpoint |

**OTLP export** is enabled iff one of `OTEL_EXPORTER_OTLP_ENDPOINT` or `OTEL_EXPORTER_OTLP_METRICS_ENDPOINT` is set. Supported env vars (see the [OTLP exporter spec](https://opentelemetry.io/docs/specs/otel/protocol/exporter/) for full semantics):

| Variable | Default | Description |
|----------|---------|-------------|
| `OTEL_SERVICE_NAME` | `content-cache` | Service name resource attribute |
| `OTEL_RESOURCE_ATTRIBUTES` | | Additional resource attributes (e.g., `k8s.namespace.name=...`) |
| `OTEL_EXPORTER_OTLP_ENDPOINT` | | OTLP endpoint URL (applies to all signals) |
| `OTEL_EXPORTER_OTLP_METRICS_ENDPOINT` | | Metrics-specific OTLP endpoint (wins over the general one) |
| `OTEL_EXPORTER_OTLP_PROTOCOL` | `http/protobuf` | Transport: `grpc` or `http/protobuf` |
| `OTEL_EXPORTER_OTLP_METRICS_PROTOCOL` | | Metrics-specific protocol (wins over the general one) |
| `OTEL_EXPORTER_OTLP_HEADERS` | | Comma-separated `key=value` headers (e.g., auth tokens) |
| `OTEL_EXPORTER_OTLP_INSECURE` | `false` | Disable TLS for the OTLP connection |
| `OTEL_EXPORTER_OTLP_COMPRESSION` | | `gzip` to enable compression |
| `OTEL_EXPORTER_OTLP_TIMEOUT` | `10000` | Per-export timeout in milliseconds |
| `OTEL_METRIC_EXPORT_INTERVAL` | `60000` | Export interval in milliseconds |

### Debug Options

| Flag | Environment Variable | Default | Description |
|------|---------------------|---------|-------------|
| `--pprof-address` | `PPROF_ADDRESS` | | Address for the pprof HTTP server (e.g., `localhost:6060`); disabled if empty |

### Example: Command Line

```bash
# Basic usage with inbound auth
./content-cache serve \
  --listen :8080 \
  --storage /var/cache/content-cache \
  --auth-token-file /run/secrets/auth-token \
  --cache-ttl 336h \
  --log-level debug \
  --log-format json \
  --metrics-prometheus

# With upstream credentials via credentials file
./content-cache serve \
  --listen :8080 \
  --storage /var/cache/content-cache \
  --auth-token-file /run/secrets/auth-token \
  --credentials-file /etc/content-cache/credentials.json.tmpl \
  --git-allowed-hosts github.com,gitlab.com \
  --cache-ttl 336h \
  --log-format json \
  --metrics-prometheus
```

### Example: Container Deployment

```yaml
# docker-compose.yml
services:
  content-cache:
    image: content-cache:latest
    command: ["serve"]
    ports:
      - "8080:8080"
    volumes:
      - cache-data:/data
      - ./secrets/auth-token:/run/secrets/auth-token:ro
      - ./config/credentials.json.tmpl:/etc/content-cache/credentials.json.tmpl:ro
    environment:
      LISTEN_ADDRESS: ":8080"
      CACHE_STORAGE: "/data"
      CACHE_TTL: "336h"
      CACHE_MAX_SIZE: "21474836480"
      LOG_LEVEL: "info"
      LOG_FORMAT: "json"
      METRICS_PROMETHEUS: "true"
      AUTH_TOKEN_FILE: "/run/secrets/auth-token"
      CREDENTIALS_FILE: "/etc/content-cache/credentials.json.tmpl"
      GIT_ALLOWED_HOSTS: "github.com"

volumes:
  cache-data:
```

```yaml
# Kubernetes ConfigMap + Secret
apiVersion: v1
kind: ConfigMap
metadata:
  name: content-cache-config
data:
  LISTEN_ADDRESS: ":8080"
  CACHE_STORAGE: "/data"
  CACHE_TTL: "168h"
  LOG_LEVEL: "info"
  LOG_FORMAT: "json"
  METRICS_PROMETHEUS: "true"
  GIT_ALLOWED_HOSTS: "github.com"
---
apiVersion: v1
kind: Secret
metadata:
  name: content-cache-secrets
type: Opaque
stringData:
  auth-token: "my-inbound-auth-token"
```

## Credentials File

The credentials file is a Go `text/template` that produces JSON. Template functions resolve secrets from external stores at startup. All upstream credentials (NPM, Git, OCI) are configured here — there are no per-protocol credential CLI flags.

### Template Functions

| Function | Description | Example |
|----------|-------------|---------|
| `env "KEY"` | Read environment variable (error if unset) | `{{ env "NPM_TOKEN" \| json }}` |
| `envDefault "KEY" "fallback"` | Read env var with default | `{{ envDefault "REGION" "us-east-1" }}` |
| `file "/path"` | Read and trim file contents | `{{ file "/run/secrets/token" \| json }}` |
| `json` | JSON-encode a string value (pipe) | `{{ env "TOKEN" \| json }}` |
| `op "reference"` | Read from 1Password CLI | `{{ op "op://vault/item/field" \| json }}` |

### Example Credentials File

```json
{
  "auth_token": {{ env "AUTH_TOKEN" | json }},

"npm": {
    "routes": [
      {
        "match": { "scope": "@mycompany" },
        "registry_url": "https://npm.pkg.github.com",
        "token": {{ env "NPM_TOKEN" | json }}
      },
      {
        "match": { "any": true },
        "registry_url": "https://registry.npmjs.org"
      }
    ]
  },

"git": {
    "routes": [
      {
        "match": { "repo_prefix": "github.com/orgA/" },
        "username": "x-access-token",
        "password": {{ env "GIT_PAT" | json }}
      },
      {
        "match": { "repo_prefix": "github.com/orgB/" },
        "github_app": {
          "app_id": "12345",
          "installation_id": "67890",
          "private_key": {{ file "/run/secrets/github-app-private-key.pem" | json }},
          "token_scope": "requested_repo"
        }
      },
      {
        "match": { "any": true }
      }
    ]
  },

"oci": {
    "registries": [
      {
        "prefix": "docker-hub",
        "upstream": "https://registry-1.docker.io"
      },
      {
        "prefix": "ghcr",
        "upstream": "https://ghcr.io",
        "username": {{ env "GHCR_USER" | json }},
        "password": {{ env "GHCR_PASS" | json }}
      }
    ]
  }
}
```

### Routing Rules

- **NPM**: Routes match by package scope (e.g., `@mycompany`). The last route must have `"any": true` as a catch-all. Scopes must start with `@` and must not be duplicated.
- **Git**: Routes match by repo prefix (e.g., `github.com/orgA/`). Prefixes must end with `/` to prevent ambiguous matching. The last route must have `"any": true` as a catch-all. A route may use either static `username`/`password` credentials or a `github_app` block, not both. GitHub App routes only support `github.com` and request installation tokens scoped to the requested repo with `token_scope: "requested_repo"`.
- **OCI**: Uses prefix-based routing (e.g., `docker-hub`, `ghcr`). Each registry entry defines its own upstream URL and optional credentials.

All sections are optional — omit any protocol section to use the default upstream with no auth.

## Storage Layout

```
./cache/
├── blobs/                   # Content-addressable storage
│   └── 58/                  # Sharded by first 2 hex digits of hash
│       └── 5818f08e...      # Full BLAKE3 hash as filename
├── goproxy/                 # Go module index
│   └── github.com/
│       └── pkg/
│           └── errors/
│               └── @v/
│                   ├── list          # Available versions
│                   ├── v0.9.1.info   # Version metadata
│                   └── v0.9.1.mod    # go.mod content
├── npm/                     # NPM package index
│   └── express/
│       ├── metadata.json    # Package metadata
│       └── versions/
│           └── 4.18.2/
│               └── tarball  # Reference to blob
├── pypi/                    # PyPI package index
│   └── projects/
│       └── requests/
│           └── metadata.json  # Project files and hashes
├── maven/                   # Maven artifact index
│   ├── metadata/
│   │   └── org/apache/commons/
│   │       └── commons-lang3/
│   │           └── metadata.json  # maven-metadata.xml cache
│   └── artifacts/
│       └── org/apache/commons/
│           └── commons-lang3/
│               └── 3.12.0/
│                   └── commons-lang3-3.12.0.jar.json  # Artifact reference
├── rubygems/                # RubyGems index
│   ├── versions.json        # Cached /versions metadata
│   ├── versions             # Raw /versions file content
│   ├── info/
│   │   └── rails.json       # Per-gem metadata with checksums
│   ├── specs/
│   │   └── specs.4.8.gz     # Legacy specs files
│   └── gems/
│       └── rails-7.1.0.gem.json  # Gem file references
├── oci/                     # OCI image index (prefix-scoped)
│   └── docker-hub/
│       └── library/
│           └── alpine/
│               ├── manifests/
│               │   └── sha256:abc...    # Image manifests
│               └── blobs/
│                   └── sha256:def...    # Layer references
└── meta.db                  # BoltDB metadata index (git pack cache, etc.)
```

## Development

```bash
# Install the pinned toolchain
mise install

# Run tests
mise run test

# Run lint (matches CI)
mise run lint

# Run with debug logging
./content-cache serve --log-level debug

# Test the Go proxy endpoint
curl http://localhost:8080/goproxy/github.com/pkg/errors/@v/v0.9.1.info

# Test the NPM registry endpoint
curl http://localhost:8080/npm/express

# Test the PyPI Simple API endpoint
curl http://localhost:8080/pypi/simple/requests/

# Test the Maven repository endpoint
curl http://localhost:8080/maven/org/apache/commons/commons-lang3/maven-metadata.xml

# Test the RubyGems registry endpoint (Compact Index)
curl http://localhost:8080/rubygems/versions
curl http://localhost:8080/rubygems/info/rails

# Test the Git proxy endpoint (requires --git-allowed-hosts github.com)
git clone http://localhost:8080/git/github.com/buildkite/content-cache.git /tmp/test-clone

# Test the OCI registry endpoint (version check)
curl http://localhost:8080/v2/

# Test an OCI manifest request (uses prefix-based routing)
curl http://localhost:8080/v2/docker-hub/library/alpine/manifests/latest

# Test the HTTP build cache endpoint (sccache / Gradle)
curl -X PUT http://localhost:8080/httpcache/abc123 --data-binary "artifact data"
curl -v http://localhost:8080/httpcache/abc123   # 200 on hit
curl -v http://localhost:8080/httpcache/missing  # 404 on miss

# Health check
curl http://localhost:8080/health

# Check cache statistics (requires cache-ttl or cache-max-size to be set)
curl http://localhost:8080/stats

# View cached content size on disk
ls -lh ./cache/blobs/
```

## Metrics & Observability

content-cache exports OpenTelemetry metrics for monitoring cache effectiveness.

### Metrics Exported

| Metric | Type | Description |
|--------|------|-------------|
| `content_cache_http_requests_total` | Counter | Total requests by protocol, endpoint, cache_result, status |
| `content_cache_http_response_bytes_total` | Counter | Bytes served by protocol, endpoint, cache_result, status |
| `content_cache_http_request_duration_seconds` | Histogram | Request latency by protocol, endpoint, cache_result, status |

### Labels

- `protocol`: npm, pypi, goproxy, sumdb, maven, rubygems, oci, git, buildcache, httpcache
- `endpoint`: metadata, tarball, artifact, blob, manifest, etc.
- `cache_result`: hit, miss, bypass
- `status_class`: 2xx, 3xx, 4xx, 5xx

### Example Queries (PromQL)

```promql
# Cache hit rate by protocol
sum(rate(content_cache_http_requests_total{cache_result="hit"}[5m])) by (protocol)
/ sum(rate(content_cache_http_requests_total{cache_result=~"hit|miss"}[5m])) by (protocol)

# Bandwidth served from cache (bytes/sec)
sum(rate(content_cache_http_response_bytes_total{cache_result="hit"}[5m]))

# P95 latency comparison: cache hits vs misses
histogram_quantile(0.95, 
  sum(rate(content_cache_http_request_duration_seconds_bucket[5m])) by (le, cache_result)
)

# Total bandwidth saved (approximate, cumulative)
sum(content_cache_http_response_bytes_total{cache_result="hit"})
```

### Structured Logging

With `-log-format json`, logs include fields for analysis:

```json
{
  "level": "INFO",
  "msg": "http request",
  "protocol": "npm",
  "endpoint": "tarball",
  "cache_result": "hit",
  "status": 200,
  "status_class": "2xx",
  "bytes_sent": 528640,
  "duration_ms": 12,
  "request_id": "550e8400-e29b-41d4-a716-446655440000"
}
```

## Goals

- Simple and efficient content storage and retrieval
- Automatic deduplication (same content stored once)
- Content retrieval by BLAKE3 hash
- Multiple storage backends (filesystem, S3)
- TTL and S3-FIFO size-based eviction
- Compression support
- Observability (metrics, traces, logs)

## Disclosure

This project was developed with AI, specifically Claude, from [Anthropic](https://www.anthropic.com/).

## License

Apache License, Version 2.0 - Copyright [Mark Wolfe](mailto:mark.wolfe@buildkite.com)

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