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AWS Serverless

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AWS Serverless

Serverless means you don’t manage servers — AWS handles provisioning, scaling, and capacity. You write functions or deploy containers, and pay per execution/request.

Service Map

ServiceTypeUse Case
LambdaFunctionsEvent-driven compute, glue logic
API GatewayAPIREST/WebSocket/HTTP APIs, rate limiting
App RunnerContainersWeb apps, APIs without infra management
BedrockFoundation ModelsLLMs, RAG, agents

When to Use Serverless

Need to run code?
  │
  ├── Short-running (< 15 min) ──► Lambda (functions)
  │
  ├── Long-running (> 15 min)
  │   ├── Container workload ────► App Runner / ECS Fargate
  │   └── Batch job ────────────► Batch
  │
  └── HTTP/REST API ─────────────► API Gateway + Lambda
  │
  └── Web app (container) ───────► App Runner

Cold Starts

Request arrives
  │
  ├── Warm (instance cached) ──► Execute immediately ──► Response
  │
  └── Cold ──► Download code ──► Start runtime ──► Execute ──► Response
               │                                         │
               └── 500ms-2s latency                     └── Total time

Pricing Comparison

ServicePricing Model
Lambda0.0000166667/GB-second
API Gateway0.50/million (HTTP)
App Runner0.02/GB-hour
Fargate0.004442/GB-hour

References

Nuggets & Gotchas

  • Lambda cold starts can be 1-5 seconds — for latency-sensitive apps, keep functions warm: Use provisioned concurrency (pre-warmed instances) or scheduled pings to keep functions warm. Provisioned concurrency costs money but eliminates cold starts.
  • Serverless doesn’t mean scale-to-zero instantly — there’s always a brief provisioning delay: Even with Lambda’s near-infinite scale, the first request to a new instance after idle has a cold start. Design for this.
  • Lambda has a 15-minute maximum execution time — for longer jobs, use Step Functions or ECS/Fargate: If your job takes 30 minutes, Lambda will timeout at 15 minutes. Break into smaller steps or use a different service.
  • App Runner is NOT the same as Lambda — you manage the container image, not just code: App Runner runs containers, so you need a Dockerfile and container registry. Lambda lets you just upload code/zip. App Runner is for when you need full runtime control.
  • Serverless pricing can be unexpectedly high at scale — 100M Lambda invocations/month = $20K/month: At low volume, serverless is cheap. At high volume (millions of requests/minute), a persistent service (ECS/Fargate) is often cheaper. Model your costs before going all-in on serverless.

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