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Cache Volumes

Read this when:

  • repeated fresh leases spend most of their time rebuilding dependency caches;
  • a provider can attach persistent, rebuildable cache storage during warmup;
  • you need to decide whether a cache belongs in a volume, a checkpoint, an image, or the synced worktree.

Cache volumes are provider-backed persistent mount points for speed-only state. They are not checkpoints and not source storage. The synced worktree stays authoritative, and volume contents must be safe to delete and rebuild.

Contract

A cache volume has:

  • key: the provider cache identity;
  • path: the absolute mount path on the remote box;
  • name: an optional local label for humans;
  • sizeGB: an optional size hint for providers that support sizing;
  • required: whether Crabbox must fail instead of silently ignoring the volume.

Put provider cache paths outside the synced source tree. Prefer /var/cache/crabbox/<kind> for package-manager stores and other dependency caches. Do not store secrets, checkout state, build artifacts that are the result under test, proof bundles, screenshots, or logs in cache volumes.

Choose a key that changes whenever the cached bytes become incompatible. Include the repository, target OS, architecture, runtime, package manager, lockfile hash, and image or workflow generation when those values affect cache contents.

Configuration

Configure volumes under cache.volumes:

cache:
  volumes:
    - name: pnpm-store
      key: my-app-linux-amd64-node24-pnpm10-lockhash
      path: /var/cache/crabbox/pnpm
      sizeGB: 80
      required: false

An explicit empty list clears inherited volumes from lower-precedence config:

cache:
  volumes: []

The environment equivalent is comma-separated:

CRABBOX_CACHE_VOLUMES=pnpm-store=my-app-linux-amd64-node24-pnpm10-lockhash:/var/cache/crabbox/pnpm

CLI

For one-off lease creation, use repeatable --cache-volume flags:

crabbox warmup --provider blacksmith-testbox \
  --cache-volume pnpm-store=my-app-linux-amd64-node24-pnpm10-lockhash:/var/cache/crabbox/pnpm

Flag-provided volumes merge with configured volumes. If the same key:path already exists in config, the flag marks that existing volume required for the lease instead of duplicating it.

Use crabbox cache volumes to inspect the resolved config:

crabbox cache volumes
crabbox cache volumes --json

This command only reads local configuration. It does not connect to a lease.

Provider Support

Providers advertise cache volume support with the cache-volume feature. Blacksmith Testbox implements the feature by forwarding each resolved volume as a blacksmith testbox warmup --sticky-disk key:path argument. Local Container implements it with Docker named volumes mounted at the configured paths; the Docker volume name is derived from the cache key. Apple Container implements it with host cache directories under the local user cache directory mounted with Apple's --volume flag.

Providers that do not advertise cache-volume ignore non-required configured volumes. Required volumes fail early when the selected provider cannot honor them.

Existing Leases

Cache volumes are attached during warmup. Crabbox records the attached key:path specs in the local lease claim. A later crabbox run --id <lease> with required volumes is allowed only when the local claim proves those volumes were attached during warmup. If the claim is missing or does not include a required volume, warm a new lease.

Boundary With Images And Checkpoints

Use cache volumes for rebuildable, mutable speed state such as package-manager stores. Use provider images for stable machine setup, tools, runtimes, and OS packages. Use workspace checkpoints for reusable source/workspace state that should be restored or forked intentionally.