AWS Lambda cold starts: causes and fixes

AWS Lambda cold starts: causes and how to fix them What happens on a cold start Cold: Download image → Start runtime → Init code → Execute handler Warm: Execute handler only Cold starts only happen when there's no warm container available. Cold start times by runtime Go: 50-200ms Node.js: 100-300ms Python: 100-400ms Java (JVM): 1000-4000ms ← painful outlier Fixes ranked by ROI 1. Slim the container image (biggest ROI) FROM node:18 AS builder WORKDIR /app; COPY package*.json ./; RUN npm ci --production FROM node:18-alpine WORKDIR /app COPY --from=builder /app/node_modules ./node_modules COPY . .; CMD ["node", "handler.js"] Large image (~1GB) → 3-8s cold start. Slimmed (~100MB) → 0.5-2s. 2. Move heavy init outside the handler # Good: runs once per container lifetime db = create_connection_pool() def handler(event, context): return process(event, db) # db already exists 3. Java SnapStart (reduces 2-4s to 100-200ms) resource "aws_lambda_function" "api" { runtime = "java21" snap_start { app