Build caching

Build caching stores deterministic task outputs keyed by sources, lockfiles, toolchain versions, config files, and environment variables that affect output. Local caches speed laptops; remote caches speed CI.

| Input class | Examples | |-------------|----------| | Source | `src/**`, package manifests | | Toolchain | Node version, compiler flags | | Env | `NODE_ENV`, feature flags baked into bundle |

Nondeterministic tasks (timestamps in bundles, network fetches without pins) should not be cached—or need normalized inputs.

On interviews: cache poisoning concerns, declaring `outputs`, invalidation when env changes, and secrets never entering cache keys or artifacts.

Common pitfalls: incomplete input sets reusing stale builds; caching integration tests with shared DB state; leaking env secrets into cached artifacts.

The trade-off is CI duration versus rigor maintaining correct invalidation.

Checklist:

• List every input that changes output.
• Cache only deterministic tasks.
• Test cache hit and miss paths.
• Keep secrets out of cache keys and outputs.