Interview Q&A / System Design

1. What is a dead letter queue and when do you use one?

   A dead letter queue holds messages that failed processing after the allowed retries. It stops one bad message from blocking the queue forever and keeps the message for inspection and later reprocessing.

   BeginnerQuick answer1 min read
2. What is a single point of failure and how do you remove it?

   A single point of failure is one component whose failure takes the whole system down. You remove it with redundancy: run several copies, add failover, and spread them across machines and zones.

   BeginnerQuick answer1 min read
3. What is the difference between SLI, SLO and SLA?

   An SLI is a measured number like request success rate. An SLO is the internal target for that number, such as 99.9%. An SLA is the contract with customers that promises a level and usually names a penalty if missed.

   BeginnerQuick answer1 min read
4. What does at-least-once delivery mean, and can you get exactly-once?

   At-least-once means a message is never lost but may be delivered more than once. True exactly-once delivery is not practical across a network, so systems use at-least-once plus idempotent consumers to get exactly-once effects.

   IntermediateQuick answer1 min read
5. What is backpressure and how do you handle a queue that keeps growing?

   Backpressure means slowing or limiting producers when consumers cannot keep up. For a growing queue, add consumers, make processing faster, reject or throttle new work, and set limits so the queue cannot grow without bound.

   IntermediateQuick answer1 min read
6. Why should retries use exponential backoff with jitter?

   Backoff spaces out retries (1s, 2s, 4s...) so a struggling service gets room to recover. Jitter adds randomness so thousands of clients do not all retry at the same instant and cause a retry storm.

   IntermediateQuick answer1 min read