What is a serverless API?

The serverless API request path

A client resolves a domain, establishes a secure connection, and sends a request to an API front door. The API layer matches a route, authenticates or authorizes the caller, validates or transforms the request, and invokes a backend integration.

The backend might be a Lambda function, another HTTP service, or a direct AWS service integration. The response path must translate backend outcomes into a stable public contract without leaking internal errors or service-specific details.

Choose synchronous or asynchronous work

Keep work synchronous when the client needs an immediate outcome and the dependencies can meet the latency and availability target. Bound timeouts and define which failure is safe to expose or retry.

Accept work asynchronously when it can take longer, needs buffering, or must survive downstream pressure. Return an operation identifier or accepted status, persist the request safely, and provide a way to observe completion or failure.

Secure the API boundary

Authenticate the caller with a mechanism that matches the audience, then authorize the specific resource and action. Treat authorization as a business rule, not merely proof that a token is valid.

Validate payloads, constrain sizes and content types, protect sensitive fields, rate-limit abusive patterns, and ensure the integration role can reach only the backend actions and resources required by that route.

Operate the API as a customer path

Measure availability, latency percentiles, error classes, throttles, authentication failures, backend saturation, and the business outcome the route exists to produce. Correlation identifiers should connect gateway logs, function logs, traces, and downstream work.

Test timeouts, malformed input, dependency failure, quota pressure, duplicate requests, and deployment rollback. Managed scaling helps with capacity, but the complete request path is only as resilient as its weakest dependency and recovery decision.