Domain 3 of 4 · Chapter 2 of 4

Development Environment Testing

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Included in this chapter:

  • The testing ladder: from in-process unit tests to a deployed test stack
  • Local emulation: the four sam local subcommands and generated events
  • Validate and lint the template before you deploy it
  • Mocking AWS calls, isolating test traffic, and CI execution
  • What SAM local cannot emulate, and exam pattern recognition
ApproachWhat it runsNeeds AWS deploy?CatchesBest for
Unit test (mocked SDK)Handler logic in-process with stubbed AWS clientsNoLogic bugs, branch coverageFast inner loop, no Docker
`sam local invoke` / `start-api`Function in a local Docker Lambda-like containerNoEvent-shape, handler/runtime, route mapping bugsLocal end-to-end before deploy
Deployed test stage / accountReal Lambda + real triggers in an isolated envYes (test stage/account)IAM, integration, latency, quota issuesIntegration tests against the real stack

Decision tree

Did the SAM/CFN template change? Yes sam validate --lint cfn-lint the template before deploy No Test depends on AWS services or triggers? No Unit test (mocked SDK) in-process, no Docker, fast inner loop Yes Need real IAM, latency, or account quotas? No Yes sam local invoke / sam local start-api local Docker container; event-shape, route bugs Deployed test stage / account real Lambda + triggers; isolated from prod Drive with a real payload sam local generate-event
Cheat sheet

Cheat sheet

  • Test serverless code on a fidelity ladder: unit, then SAM local, then a deployed stage
  • Every sam local subcommand runs the function in Docker, so Docker is a hard prerequisite
  • Use sam local invoke for a one-shot scripted run of a single function
  • Use sam local start-api when the test driver is an HTTP request to your API Gateway routes
  • Use sam local start-lambda when an SDK or the CLI invokes the function, not an HTTP call
  • Generate the test event with sam local generate-event instead of hand-writing the JSON
  • Save event JSON as a named test event in the Lambda console to re-run a deployed function
  • Run sam validate --lint to catch template errors before a deploy rolls back
  • cfn-lint runs standalone against any CloudFormation template, not just SAM ones
  • Mock (stub) the AWS SDK client so unit tests stay fast, offline, and deterministic
  • Isolate deployed test traffic with a separate API Gateway stage or a separate AWS account
  • Use the IAM policy simulator to check whether a policy allows an action without a real request
  • SAM local can't emulate IAM, real latency, or account quotas. Those need a deployed environment
  • Run the test ladder automatically in CodeBuild via buildspec.yml phases
  • Run the X-Ray daemon with -o (local mode) off EC2 to skip the metadata check
  • Point the SDK at a non-default daemon with AWS_XRAY_DAEMON_ADDRESS; bind it with -b for other containers
  • X-Ray annotations are indexed and searchable; metadata is not
  • Set AWS_XRAY_CONTEXT_MISSING to LOG_ERROR to avoid context-missing exceptions outside a request
  • Use sam local -d/--debug-port to attach an IDE debugger to a function
  • Use sam local --warm-containers EAGER to reuse containers between invocations
  • Point the AWS SDK at LocalStack via endpoint_url http://localhost:4566 with placeholder credentials
  • LocalStack does not enforce IAM, so validate permissions against real AWS
  • Cloud9 managed temporary credentials are restricted and disabled when a non-owner shares the environment

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References

  1. Introduction to testing with the sam local command
  2. Intro to sam local invoke
  3. Intro to sam local start-api
  4. Intro to sam local generate-event
  5. sam validate command reference
  6. Validate AWS SAM template files with AWS CloudFormation Linter
  7. Set up stages for a REST API in API Gateway
  8. Intro to sam local start-lambda
  9. Testing Lambda functions in the console
  10. IAM policy testing with the IAM policy simulator
  11. What is AWS CodeBuild?