What is a hot key in Redis, and how would you shard a viral product page cache?
PICTURE THIS: HASH MAP
Simple meaning
One key taking a huge share of QPS saturates a single Redis thread and CPU.
WHY — Redis instead of guessing?
Why interviewers care about Redis:
who only read docs from people who shipped.
and tied to Backend work.
Name the idea, why it exists, then one short example.
End with when you use it and one common pitfall.
STEPS — What happens step by step?
Before you speak the answer, walk the interviewer through these steps:
- 1One key taking a
huge share of QPS saturates a single Redis thread and CPU.
- 2You shard by hashing
into N sibling keys, cache locally in the app with a tiny TTL, or use a read replica.
- 3Also avoid a single
global lock key for the whole catalog.
- 4Give an example
One tiny concrete case you can say aloud.
- 5Common mistake
What juniors usually get wrong.
- 6Close
When you pick this over the alternative.
EXAMPLE — See it in action
Here's a short line you can speak, broken into clear beats:
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
One key taking a huge share of QPS saturates a single Redis thread and CPU. You shard by hashing into N sibling keys, cache locally in the app with a tiny TTL, or use a read replica.