CodeParkA cold start happens when Lambda has no free, ready environment for a request. Before your handler runs, Lambda must:
- create the environment
- download your code
- start the runtime (Node.js)
- run your init code, which is the code outside the handler
A warm request reuses an environment that already exists, so it skips all of that.
To make cold starts shorter:
- Keep the deployment bundle small. Bundle and tree-shake (remove unused code) with esbuild.
- Remove unused dependencies. Import only the AWS SDK v3 clients you need, not the whole SDK.
- Keep top-level init code cheap. Lazy-load modules you rarely use, which means load them only when you need them.
- Avoid heavy frameworks at startup. Starting a full NestJS app on every cold start is slow, unless you cache the app instance and reuse it.
- Give the function more memory. More memory also gives more CPU, so init runs faster.
For endpoints where speed matters, use provisioned concurrency. It keeps a set number of environments started and ready ahead of time. It costs extra.
- A cold start means: create the environment, load the code, start the runtime, then run init code.
- Make bundles smaller with esbuild and with only the SDK v3 clients you need.
- Keep init cheap, lazy-load rarely used code, and cache the NestJS app instance.
- More memory gives more CPU, which makes init faster.
- Provisioned concurrency keeps environments started and ready ahead of time.