Idempotency for safe retries in HLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does idempotency mean in system design?

Idempotency means that performing the same operation multiple times has the same effect as doing it once. It helps systems handle retries safely without causing duplicate effects.

beginner

Why is idempotency important for safe retries?

Idempotency ensures that if a request is retried due to failure or timeout, it won't cause unintended side effects like duplicate transactions or data corruption.

intermediate

Name a common technique to implement idempotency in APIs.

Using a unique client-generated idempotency key with each request. The server stores the result for that key and returns the same response if the key is reused.

intermediate

How does storing request state help achieve idempotency?

By saving the outcome of a request identified by a unique key, the system can return the saved result on retries instead of processing the request again.

advanced

What is a potential challenge when implementing idempotency?

Managing storage for idempotency keys and their results can grow large. Also, deciding how long to keep these keys to balance memory and correctness is tricky.

What is the main goal of idempotency in system design?

A Balance load across servers

B Make requests faster

C Ensure repeated requests have the same effect as one

D Encrypt data for security

Idempotency ensures that repeating the same request does not change the result beyond the initial application.

Which of these is a common way to implement idempotency in APIs?

A Use a unique idempotency key per request

B Encrypt all requests

C Use random request IDs each time

D Ignore duplicate requests

A unique idempotency key lets the server recognize repeated requests and respond with the same result.

What happens if a system is not idempotent and a request is retried?

A It may cause duplicate actions or errors

B It speeds up processing

C It encrypts the data

D It balances the load

Without idempotency, retries can cause repeated side effects like duplicate transactions.

How long should idempotency keys be stored?

A Never store them

B Forever

C Only a few seconds

D Long enough to cover possible retries

Keys should be stored long enough to handle retries but not indefinitely to save resources.

Which system component usually stores idempotency keys and results?

A Client browser

B Database or cache

C Load balancer

D DNS server

Databases or caches store idempotency keys and their results to support safe retries.

Explain idempotency and why it is critical for safe retries in distributed systems.

Think about what happens when a request is sent multiple times.

Describe how you would design an API to be idempotent for safe retries.

Consider how the server recognizes repeated requests.