Message delivery guarantees in HLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is At-most-once message delivery guarantee?

At-most-once means a message is delivered zero or one time. It may be lost but never duplicated.

beginner

Explain At-least-once message delivery guarantee.

At-least-once means a message is delivered one or more times. It may be duplicated but never lost.

intermediate

What does Exactly-once delivery guarantee ensure?

Exactly-once means each message is delivered once and only once, no loss and no duplicates.

intermediate

Why is Exactly-once delivery hard to achieve in distributed systems?

Because it requires coordination to avoid duplicates and loss despite failures, network delays, and retries.

beginner

Give a real-life example of At-least-once delivery.

Like a postal service that may deliver the same letter twice if unsure it was received, but never loses it.

Which delivery guarantee may cause duplicate messages?

A At-least-once

B Exactly-once

C At-most-once

D None of the above

At-least-once delivery can cause duplicates because messages may be retried.

Which guarantee ensures no message loss but may deliver duplicates?

A At-most-once

B Best-effort

C Exactly-once

D At-least-once

At-least-once guarantees no loss but may deliver duplicates.

Which guarantee might lose messages but never duplicates?

A Exactly-once

B At-least-once

C At-most-once

D Reliable delivery

At-most-once may lose messages but never duplicates.

Exactly-once delivery is:

A Hard but achievable with coordination

B Easy to implement

C Impossible in practice

D Same as at-least-once

Exactly-once is hard but achievable with proper coordination and idempotency.

Which guarantee is best for critical financial transactions?

A At-least-once

B Exactly-once

C At-most-once

D No guarantee

Exactly-once ensures no duplicates or loss, critical for financial transactions.

Describe the differences between at-most-once, at-least-once, and exactly-once message delivery guarantees.

Think about message loss and duplication.

Explain why exactly-once delivery is challenging in distributed systems and how it can be approached.

Consider network issues and system failures.