33. Idempotency Keys

The brief

Idempotency Keys. Any request can time out after the server has already acted. The client cannot tell that from a request that never arrived, and the only safe response to uncertainty is to retry — which, without help, charges the card twice. Work from the scoping questions below. State assumptions for any unspecified load, guarantee or target, then trace your design end to end. Explain one difficult case and a credible alternative; the worked example is a reference, not a required implementation.

• Set the scope: Which retried operation must have only one business effect? Who chooses the request identity and how long is it remembered?
• Define the contract: What if the same identity arrives with a different payload? What should a retry observe while the first request is still running?
• Test the boundaries: What happens after the effect commits but before a response is recorded?

Constraints

Explicit scope and guarantees

Resolve the scoping questions for Idempotency Keys. Separate stated behavior from assumptions, and identify what is outside your design.

Supported operating targets

Declare relevant volume, latency, freshness, quality or cost targets with units. Show calculations or an evaluation plan that can test them; unspecified targets are your assumptions, not hidden pass criteria.

Failure and boundary behavior

Explain how your guarantees hold in a difficult case relevant to this subject. Address: What should a retry observe while the first request is still running? What happens after the effect commits but before a response is recorded?

What to cover

1. 01

   Scope and behavior contract

   Identify users, required behavior and exclusions. Answer: Which retried operation must have only one business effect? Who chooses the request identity and how long is it remembered?

2. 02

   State and interfaces

   Define the information owned by the system and the inputs, outputs and errors at its boundaries. Resolve: What if the same identity arrives with a different payload? What should a retry observe while the first request is still running?

3. 03

   Capacity and operating targets

   Estimate the dominant workload and resource demand with units and explicit assumptions. For a learned system, also state how quality is measured and what data is available.

4. 04

   Architecture and central flow

   Draw or describe the responsibilities needed for Idempotency Keys. Trace a representative request, event or job from its input to a visible result; identify durable state owners.

5. 05

   Failure and boundary walkthrough

   Walk through a difficult case step by step, including detection and recovery. Consider: What happens after the effect commits but before a response is recorded?

6. 06

   Tradeoffs and operations

   Compare a credible alternative using your chosen workload and guarantees. Explain a remaining risk, a signal to watch and when you would change the design.

Worked designs

Explore the architecture and decisions, then build on an example with Coach.

Official design

Idempotency Keys

The caller names the operation, the server stores the first response under that name, and a retry replays it instead of doing the work twice.

Explore the design

Review rubric

AI feedback uses these criteria. Scores are practice feedback.