pixa/README.md

# pixa

pixa is a GPL-3.0-licensed Go web server by
[@sneak](https://sneak.berlin) that proxies images from upstream
sources, optionally resizing or transforming them, and serves the
results. Both source and transformed images are cached to disk so that
subsequent requests are served without origin fetches or additional
processing.

## Getting Started

```bash
# clone and build
git clone https://git.eeqj.de/sneak/pixa.git
cd pixa
make build

# run with a config file: copy the example and set a real signing key
# (the example placeholder is refused at startup), e.g. with
# openssl rand -base64 32
cp config.example.yml config.yml
$EDITOR config.yml   # replace the signing_key placeholder
./bin/pixad --config config.yml

# or build and run via Docker
make docker
docker run -p 8080:8080 -e PIXA_SIGNING_KEY="$(openssl rand -base64 32)" pixad:latest
```

A container is configured two ways. The signing key comes from the
`PIXA_SIGNING_KEY` environment variable, which the baked-in config
reads; if it is unset the container exits at startup naming the
variable. Everything else uses built-in defaults, so to change any
other setting mount your own file over `/etc/pixa/config.yml` (see
`config.example.yml` for the full set of keys).

## Rationale

Image-heavy web applications need a fast, caching reverse proxy that
can resize and transcode images on the fly. pixa fills that role as a
single, self-contained binary with no external runtime dependencies
beyond libvips. It supports HMAC-SHA256 signed URLs with expiration to
prevent abuse, and allowlisted source hosts for open access.

## Design

### Storage

- **Source content**:
  `<statedir>/cache/src-content/<ab>/<cd>/<sha256 of source content>`
- **Source metadata**:
  `<statedir>/cache/src-metadata/<hostname>/<sha256 of path>.json`
  (fetch time, original headers, request, content hash)
- **Database**: `<statedir>/state.sqlite3` (SQLite)
- **Output documents**:
  `<statedir>/cache/dst-content/<ab>/<cd>/<sha256 of output content>`

Multiple source paths may reference the same content blob; the
database tracks references rather than using filesystem refcounting.
In-process caching of request-to-output mappings targets 1-5k r/s.

### Routes

```
/v1/image/<host>/<path>/<size>.<format>?sig=<signature>&exp=<expiration>
```

Images are only fetched from origins using TLS with valid certificates.

- `<format>`: one of `orig`, `png`, `jpeg`, `webp`
- `<size>`: `orig` or `<width>x<height>` (e.g. `800x600`)

### Source Hosts

Source hosts may be allowlisted in the configuration. Non-allowlisted
hosts require an HMAC-SHA256 signature.

#### Signature Specification

Signatures use HMAC-SHA256 and include an expiration timestamp to
prevent replay attacks. Signatures are **exact match only**: every
component (host, path, query, dimensions, format, expiration) must
match exactly what was signed. No suffix matching, wildcard matching,
or partial matching is supported.

**Signed data format** (colon-separated):

```
HMAC-SHA256(secret, "host:path:query:width:height:format:expiration")
```

Where:

- `host` — source origin hostname (e.g. `cdn.example.com`)
- `path` — source path (e.g. `/photos/cat.jpg`)
- `query` — source query string, empty string if none
- `width` — requested width in pixels, `0` for original
- `height` — requested height in pixels, `0` for original
- `format` — output format (jpeg, png, webp, avif, gif, orig)
- `expiration` — Unix timestamp when signature expires

**Example:** resize
`https://cdn.example.com/photos/cat.jpg` to 800x600 WebP with
expiration 1704067200:

1. Build input:
   `cdn.example.com:/photos/cat.jpg::800:600:webp:1704067200`
2. Compute HMAC-SHA256 with your secret key
3. Base64URL-encode the result
4. URL:
   `/v1/image/cdn.example.com/photos/cat.jpg/800x600.webp?sig=<base64url>&exp=1704067200`

**Allowlist patterns:**

- **Exact match**: `cdn.example.com` — matches only that host
- **Suffix match**: `.example.com` — matches `cdn.example.com`,
  `images.example.com`, and `example.com`

### Configuration

Configured via YAML file (`--config`). Key settings:

- `access_control_allow_origin` — CORS origin
- `allowlist_hosts` — list of allowed upstream hosts
- `upstream_fetch_timeout` — timeout for origin requests
- `upstream_max_response_size` — max origin response size
- `downstream_timeout` — client response timeout
- `signing_key` — HMAC secret for URL signatures
- `cache_max_bytes` — disk cache size limit in bytes; `0` disables the
  disk cache entirely; omitted defaults to 75% of the free space on
  the filesystem containing `<state_dir>/cache/` (minimum 500 MiB)

See `config.example.yml` for all options with defaults.

### Architecture

- **Dependency injection**: Uber fx
- **HTTP router**: go-chi
- **Image processing**: govips (CGO wrapper for libvips)
- **Database**: SQLite via modernc.org/sqlite
- **Static assets**: embedded via `//go:embed`
- **Metrics**: Prometheus
- **Logging**: stdlib slog

## Entrypoints

This repository adheres to the
[Scripts to Rule Them All](https://github.com/github/scripts-to-rule-them-all)
standard: normalized scripts in `script/` are the entrypoints for the
development workflow, and the Makefile targets are thin shims that call
them. We provide:

- `script/bootstrap` — install all dependencies (idempotent)
- `script/setup` — make a fresh clone ready for development
  (bootstrap, then install-precommit)
- `script/projectname` — output the project name ("pixa")
- `script/test` — run the test suite
- `script/lint` — run golangci-lint
- `script/fmt` — format all code (writes)
- `script/fmt-check` — check formatting (read-only)
- `script/check` — run test, lint, and fmt-check
- `script/docker` — build the Docker image tagged via `script/projectname`
- `script/cibuild` — CI entrypoint: `docker build .` (the Dockerfile
  runs the checks, so a green build implies a green repo)
- `script/precommit` — pre-commit checks (`go mod tidy` guard, then
  `script/check`)
- `script/install-precommit` — install the git pre-commit hook that
  runs `script/precommit`

## TODO

See [TODO.md](TODO.md) for the full prioritized task list.

## License

GPL-3.0. See [LICENSE](LICENSE).

## Author

[@sneak](https://sneak.berlin)