This trace follows the actual state transitions behind the companion Designing APIs Around Explicit Semantics. It describes a common execution path; implementation details can vary, so keep the contract separate from the mechanism.
Step 1: Validate request shape
An API contract defines the meaning of operations, inputs, outputs, errors, and side effects. A good contract lets clients reason about retries and compatibility without depending on undocumented server behavior. HTTP methods communicate broad intent, while the application contract defines the resource-specific rules.
Step 2: Authorize the requested action
A PUT that replaces a resource can be retried safely when the same request produces the same target state. A POST that creates a payment may create duplicates unless the server recognizes an idempotency key. A response should distinguish validation failures, authorization failures, and transient server problems.
Step 3: Deduplicate a logical retry
For a retried side-effecting request, look up its idempotency key before applying the operation again and return the stored outcome for a duplicate attempt.
At this point, record the state that changed and check the invariant before advancing. If the operation repeats, make clear which values persist and which are recomputed.
Step 4: Apply the state transition
Backward compatibility includes behavior as well as field names: changing default ordering or error semantics can break clients. Adding an optional field is often safer than changing a required one, but strict client decoders may still reject it. Version only when compatibility policy and migration need justify it.
Step 5: Return a stable result
An endpoint creates an invoice but the client times out before receiving the response. Specify a request and storage design that lets the client retry without creating a second invoice.
The trace is complete when the result satisfies the stated contract. Compare this model with the concrete runtime or system you are studying before making a performance claim.