Caching

Pipeline caching saves dependency downloads or deterministic task outputs. Correct cache keys include lockfiles, source inputs, config, platform, runtime version, and any environment variable that affects output.

| Cache type | Key inputs | |------------|------------| | Dependencies | Lockfile hash, OS, package manager version | | Build output | Source hash, config, toolchain version | | Test cache | Fixture hash, shard id |

					- uses: actions/cache@v4
  with:
    path: ~/.npm
    key: npm-${{ runner.os }}-${{ hashFiles('package-lock.json') }}
				

Restore keys allow partial hits when the exact key misses. Monorepo tools (Turborepo, Nx) add remote caching with declared task inputs and outputs.

On interviews: dependency versus build caches, restore keys, cache poisoning, when to invalidate aggressively.

Common pitfalls: a fast wrong cache is worse than no cache; caching secrets or mutable generated outputs leaks data or hides failures.

The trade-off is feedback speed versus correctness when inputs are incomplete in the key.

Checklist:

• Hash all correctness inputs.
• Avoid caching secrets or unbounded mutable dirs.
• Measure hit rate and false-green patterns.
• Invalidate when toolchain or lockfile changes.