Designing and sharing APIs10% of the exam
Idempotent Requests and Concurrent Updates
Separate safe reads, repeatable request effects and conditional updates. Use HTTP validators to distinguish existence checks, lost-update protection and cache revalidation.
Identify idempotent HTTP methods and HTTP-native support for optimistic concurrency.
~6 min read · 6 practice questions
What the exam expects
- Identify HTTP methods with safe and idempotent semantics.
- Distinguish repeatable server effects from identical responses.
- Choose a representation-version precondition or an existence precondition.
- Apply strong comparison when evaluating If-Match.
- Separate conditional updates from cached-response validation.
Safe means the client requests read-only behavior; idempotent means repeating an identical request has the same intended server effect; an ETag identifies a representation version; a precondition limits when a method proceeds; optimistic concurrency checks for conflicting changes rather than holding a lock across client work.
Key facts
Safe methods
- Detail
- GET and HEAD request retrieval without an intended state change.
PUT and DELETE
- Detail
- Idempotent under their HTTP semantics; not safe reads.
POST and PATCH
- Detail
- Have no method-level idempotency guarantee.
Repeated response
- Detail
- Can differ while the intended effect remains idempotent.
If-Match comparison
- Detail
- Uses strong comparison; weak tags do not match.
Existence-only condition
- Detail
- If-Match wildcard requires a current representation.
Cached representation
- Detail
- Send its ETag in If-None-Match for revalidation.
Validator construction
- Detail
- Generation algorithm and quoted field syntax are distinct.
- Why should a client avoid assuming that every ETag encodes one mandatory hash algorithm?
- When transmitting an entity tag, how is its opaque value delimited?
- Which alternative forms can construct an If-Match field without mixing them?
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