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testland/serverless-integration-test-builder

Use when a serverless project (SAM, serverless.yml, Wrangler, Vercel or Netlify functions) has no integration tests yet. Inventories functions and event sources, picks a local emulator per function, generates test events, asserts cold-start and timeout budgets, and emits the tests and CI config.

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AWS Lambda timeout + billing budgets

AWS Lambda's wall-clock limit is 15 minutes (900 seconds) per invocation. Per docs.aws.amazon.com/lambda configuration-timeout: "The default value for this setting is 3 seconds, but you can adjust this in increments of 1 second up to a maximum value of 900 seconds (15 minutes)." For longer work use Step Functions, AWS Batch, or ECS Fargate - don't architect around the limit.

Timeout vs deadline at runtime

Per docs.aws.amazon.com/lambda python-context, the Context object exposes get_remaining_time_in_millis() (Python) / getRemainingTimeInMillis() (Node/JVM). Break out proactively:

def handler(event, context):
    while not_done:
        if context.get_remaining_time_in_millis() < 5000:
            save_checkpoint()
            return {"status": "partial", "checkpoint": ...}
        do_work_chunk()
    return {"status": "complete"}

The 5-second cushion lets the handler return cleanly; without it the Lambda is force-killed at timeout (no SIGTERM grace) and the caller gets a 504-equivalent.

Billing semantics

Per docs.aws.amazon.com/lambda lambda-pricing:

Cost component	Detail
Request charge	$0.20 per 1M requests (us-east-1)
Compute charge	Memory-class × GB-seconds (billed per ms)
Init duration	Free; not billed (per AWS; historically has changed - verify current docs)

GB-second formula: memory_GB * duration_seconds. A 512MB Lambda running 1000ms costs 0.5 * 1.0 * $0.0000166667 = $0.00000833; per million 1s invocations at 512MB: ~$8.54 including the request charge.

Memory ↔ CPU relationship

Per docs.aws.amazon.com/lambda configuration-memory: "The amount of CPU available to a function is proportional to the memory you allocate to it. At 1,769 MB, a function has the equivalent of one vCPU." Compute-bound workloads should size memory by CPU need; the sweet spot is usually the memory class where wall-clock time stops dropping (often 1024-2048MB).

Integration timeout cascade

The integration's timeout is often the operational ceiling:

Integration	Timeout	Lambda config
API Gateway (REST + HTTP API)	29 seconds (hard)	Lambda timeout MUST be < 29s
Application Load Balancer	4s default; configurable to 4000s	Configurable both sides
CloudFront (Lambda@Edge)	5s viewer functions; 30s origin	Tight viewer limit
SQS (event source)	Per-queue visibility timeout (default 30s)	Visibility > Lambda timeout × 6 (AWS recommendation)
DynamoDB Streams	6h batch window	Per-batch limit
EventBridge (async)	Retries on timeout	Idempotency required
Step Functions	Per-task timeout; default 60s	Per-task tuning

The API Gateway 29-second hard limit is the most-encountered surprise: a Lambda configured for 60s still times out at 29s because API Gateway gives up first.

Testable behaviours

Behaviour	Test
Completes within timeout under prod load	k6 / load run against the deployed function
Graceful return via remaining-time check	Inject slowness; assert "partial", not 504
API Gateway 29s budget honoured	Long-running endpoint via API GW URL; assert ≤ 29s
SQS visibility > Lambda timeout	Force a timeout; observe SQS re-delivery
Memory sweet spot found	Run at 256/512/1024/2048 MB; chart duration
Cost at p99 within budget	Latency × memory × invocations × price/GB-s at p99, not average

Anti-patterns

Anti-pattern	Why it fails	Fix
Timeout = 900s "for safety"	Stuck Lambdas burn 15min; concurrency limits hit	Match timeout to p99 + buffer
Lambda timeout > API Gateway's 29s	API GW kills first; Lambda runs unused	Timeout < 29s behind API GW
SQS visibility < Lambda timeout	In-flight message re-delivered → duplicates	Visibility > timeout × 6
No remaining-time check	Force-kill; no progress saved	Check + early-return
Sizing memory by RAM need only	Compute-bound work wastes wall-clock	Tune by p95 duration
Cost calculated from the average	p99 spikes blow the budget	Calculate against p99

Notes

Per-region pricing varies; the figures above are us-east-1.
Workers / Edge have different models: Cloudflare Workers 10ms CPU free / 50ms paid, 30s wall-clock; Vercel Edge Functions 30s wall-clock max.