Revision and Interview Questions

Cheat sheet

Pub/Sub: SUBSCRIBE/PSUBSCRIBE/PUBLISH; live broadcast, nothing stored, at-most-once; PUBLISH returns receiver count; slow subscribers disconnected by output buffer limits; reconnect gaps; classic PUBLISH broadcasts to all cluster nodes; sharded Pub/Sub (SSUBSCRIBE/SPUBLISH, Redis 7) scales by hash slot; RESP3 allows other commands on a subscribed connection. Keyspace notifications: notify-keyspace-events, keyspace and keyevent channels, expired events fire on actual deletion and can be lost. Streams: append-only log in one key; IDs ms-seq; XADD, XRANGE/XREVRANGE, XREAD with BLOCK and $; trimming with MAXLEN/MINID and ~; XDEL. Consumer groups: XGROUP CREATE ($, 0, MKSTREAM), XREADGROUP ... >, PEL, XACK, XPENDING, ID 0 to recover own pending, XAUTOCLAIM/XCLAIM, delivery counts for dead-lettering, XGROUP DELCONSUMER; at-least-once, so idempotent consumers. Patterns: job queues (Streams or BLMOVE lists), outbox to stream, partition streams by key for scale. Ops: AOF/RDB, async replication and WAIT, noeviction, XINFO GROUPS lag, memory sizing.

Common interview questions

Answer each with commands and a trade-off.

1. Compare Redis Pub/Sub, lists and Streams as messaging tools.
2. What happens to Pub/Sub messages when a subscriber is offline or slow?
3. Why does sharded Pub/Sub scale better in Redis Cluster?
4. Explain stream IDs and how XREAD with BLOCK and $ works.
5. How do consumer groups distribute work and track progress?
6. What is the Pending Entries List, and how do you recover from a crashed consumer?
7. How would you handle poison messages in a Redis stream?
8. How do you bound memory use of a stream?
9. What durability guarantees does Redis give for stream entries?
10. When would you choose Kafka or RabbitMQ instead of Redis Streams?

Always state the delivery guarantee

Interviewers check whether you know that Pub/Sub is at-most-once and consumer groups are at-least-once. Saying which one you rely on, and how you handle duplicates or losses, is what makes an answer strong.

त्वरित जाँच: Which command acknowledges that a consumer in a group has processed a stream entry?

• XADD
• XTRIM
• PUBLISH
• XACK