AWS Lambda Layers: The Dependency Management Pattern You Should Be Using

# Without layers:
# your-function.zip = your code (50KB) + dependencies (45MB)
# Every deployment = 45MB upload
# Every cold start = download + extract 45MB

# With layers:
# your-function.zip = your code only (50KB) 
# layer.zip = dependencies (45MB) — deployed ONCE, shared across functions
# Cold start: Lambda already has layer cached from previous invocation

# Real-world cold start reduction:
# Before layers: 2.8s cold start (downloading 45MB package)
# After layers:  1.1s cold start (60KB function + pre-cached layer)
# Improvement:  61% faster cold start

# Layer structure MUST follow this exact directory layout:
mkdir -p my-layer/nodejs
cd my-layer/nodejs

# Install only production dependencies
npm init -y
npm install lodash axios moment --production

# Package the layer
cd ..
zip -r my-layer.zip nodejs/

# Deploy via AWS CLI
aws lambda publish-layer-version \
  --layer-name "common-dependencies" \
  --description "Shared Node.js dependencies" \
  --zip-file fileb://my-layer.zip \
  --compatible-runtimes nodejs18.x nodejs20.x

# Response includes LayerVersionArn:
# arn:aws:lambda:us-east-1:123456789:layer:common-dependencies:1

// Lambda mounts layer contents at /opt/
// Node.js layer at /opt/nodejs/node_modules/
// Python layer at /opt/python/ or /opt/python/lib/python3.x/site-packages/

// In your function code — just require normally:
const _ = require('lodash');    // Loaded from /opt/nodejs/node_modules/lodash
const axios = require('axios'); // Same

// Your function ZIP contains ONLY your code:
// index.js (2KB) — not node_modules!

exports.handler = async (event) => {
  const data = await axios.get(event.url);
  return _.pick(data.data, ['id', 'name', 'email']);
};

# SAM template.yaml
AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31

Globals:
  Function:
    Layers:
      - !Ref CommonDependenciesLayer  # All functions share this layer
    Runtime: nodejs20.x
    Timeout: 30

Resources:
  CommonDependenciesLayer:
    Type: AWS::Serverless::LayerVersion
    Properties:
      LayerName: common-dependencies
      ContentUri: layers/common/
      CompatibleRuntimes:
        - nodejs20.x
      RetentionPolicy: Retain  # Keep old versions for rollback

  UserFunction:
    Type: AWS::Serverless::Function
    Properties:
      CodeUri: src/user/  # Only your code — no node_modules
      Handler: index.handler

  OrderFunction:
    Type: AWS::Serverless::Function
    Properties:
      CodeUri: src/order/  # Shares the same layer

# Layer size limits:
# Unzipped: 250MB per function (all layers combined)
# Zipped: 50MB per layer

# Best practice: split by update frequency
# Layer 1: slow-changing dependencies (aws-sdk, lodash) — update monthly
# Layer 2: fast-changing internal libs — update weekly
# Function: your code — update multiple times daily

# Share layers across accounts (useful for org-wide shared libs):
aws lambda add-layer-version-permission \
  --layer-name common-dependencies \
  --version-number 1 \
  --statement-id allow-org \
  --action lambda:GetLayerVersion \
  --principal "*" \
  --organization-id o-xxxxx

# Reference specific layer version for reproducibility:
# Never use :latest alias in production — always pin to version number