Idempotency lets repeated attempts represent one logical operation. The server must store and enforce the key with the operation’s outcome.
Before you start
You should understand HTTP, database transactions and the difference between one process and independently failing services. Draw the participants and message direction before choosing a pattern. Include timeout, duplicate delivery and recovery in the model instead of considering only successful requests.
The practical goal is to reason through this situation: A retried order creation reuses the same key after a timeout. Read the walkthrough first, then try the interview exercise before opening its answer. The important part is explaining the decision and its consequences, rather than remembering a definition alone.
Step-by-step walkthrough
Step 1: Identify one logical operation
Create a stable key before the first attempt.
Step 2: Store outcome atomically
Coordinate key ownership and the business operation against concurrent retries.
Step 3: Reject conflicting reuse
Bind key to caller scope and payload meaning with an explicit retention period.
Worked scenario
A retried order creation reuses the same key after a timeout.
A client times out creating order A and retries with the same key. The server returns the recorded result rather than creating order B. If the second attempt changes quantity under that key, reject the conflict instead of silently treating different intent as the same operation.
Common mistake
Generating a fresh key for each retry creates separate operations.
Verify the behavior
Send concurrent identical requests and a conflicting payload; assert one logical creation.
Interview exercise
Handle different payloads with one key.
Answer and reasoning
Reject inconsistent reuse or enforce a documented rule; bind the key to scope, request meaning and a retention period.
Continue learning
Compare the scenario with the Microservices interview questions and test your understanding with the Microservices MCQs. For terminology and implementation details, consult the reference material.