Shared Dependencies

Understanding shared dependencies helps you work with AWS Lambda confidently. Here you will learn the core ideas behind shared dependencies, see working code, and pick up best practices used on real teams.

Shared Dependencies Overview

Shared Dependencies is a building block you will reach for often in AWS Lambda. It keeps related logic together and makes your intent obvious to reviewers and future maintainers.

When you learn shared dependencies properly, you avoid the guesswork that leads to bugs and rework. The example below shows the shape you will use in most real AWS Lambda projects.

// build.mjs
import { build } from 'esbuild';

await build({
  entryPoints: ['src/handler.mjs'],
  bundle: true,
  platform: 'node',
  target: 'node20',
  format: 'esm',
  outfile: 'dist/handler.mjs',
  external: ['@aws-sdk/*'],
});

Bundling with esbuild produces a small deployment package and keeps cold starts fast.

Shared Dependencies Example

// handler.mjs
export const handler = async (event, context) => {
  // 1. read input from the event
  // 2. do the work
  // 3. return a response (or throw on error)
};

Start from a minimal Shared Dependencies example and grow it only as needed.
Keep configuration explicit so Shared Dependencies behaves the same in every environment.
Name things clearly so teammates understand your Shared Dependencies at a glance.
Add tests around Shared Dependencies early to lock in expected behaviour.

AWS Lambda Cheatsheet

Handy reference for working with shared dependencies in AWS Lambda and Node.js.

Task	Example	Purpose
Define handler	`export const handler = async (event) => {}`	Entry point AWS invokes
Read input	`event.body`, `event.Records`	Access request or trigger data
Return response	`{ statusCode, body }`	Reply through API Gateway
Reuse SDK client	`const c = new S3Client({})` (module scope)	Faster warm invocations
Env config	`process.env.TABLE_NAME`	Externalise settings
Log	`console.log(JSON.stringify(obj))`	Structured CloudWatch logs
Deploy	`sam deploy` / `serverless deploy`	Ship the function

How Shared Dependencies Works in AWS Lambda

Shared Dependencies runs inside the managed Lambda execution environment. AWS provisions a micro-VM, loads your Node.js code, runs any module-scope initialisation once, and then invokes your handler for each event.

Bundling with esbuild produces a small deployment package and keeps cold starts fast.

Handlers should be small and do one job well.
Initialise SDK clients and config outside the handler to reuse them on warm starts.
Return quickly and let event sources handle retries where possible.
Emit structured logs so CloudWatch and X-Ray can correlate activity.

Practical Guidance for Shared Dependencies

On real projects, shared dependencies works best when it is observable, secure, and cheap to run. Grant least-privilege IAM, validate every input, and keep the deployment package small.

Concern	Recommendation
Security	Least-privilege IAM role, validate all input
Performance	Reuse clients, right-size memory, avoid heavy cold starts
Reliability	Idempotent handlers, dead-letter queues for failures
Observability	Structured logs, metrics, and X-Ray tracing

Common Mistakes

Skipping error handling and edge cases when wiring up shared dependencies.
Leaving shared dependencies untested, so regressions slip into production.
Over-engineering shared dependencies before you actually need the extra flexibility.
Ignoring documentation, which makes shared dependencies hard for the next developer to change.

Key Takeaways

Shared Dependencies is a core part of working effectively with AWS Lambda.
Start small and keep shared dependencies focused on a single responsibility.
Apply consistent patterns so shared dependencies scales across your project.
Test and document shared dependencies to keep it maintainable over time.

Pro Tip

Pair shared dependencies with automated tests from day one. It is far cheaper to catch AWS Lambda regressions in CI than in production.