Why must every cache entry have a TTL even if you invalidate on write?
PICTURE THIS: HOW TO EXPLAIN IT
Simple meaning
Invalidation messages get lost, deploys race, and some keys are never written again.
WHY — Redis instead of guessing?
Why interviewers care about Redis:
on Redis.
the situation, the default choice, and one exception - that reads as experience.
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:
- 1Invalidation messages get lost,
deploys race, and some keys are never written again.
- 2TTL is the safety
net that bounds staleness.
- 3Product interviewers like the
phrase: TTL for correctness envelope, explicit delete for freshness.
- 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
Invalidation messages get lost, deploys race, and some keys are never written again. TTL is the safety net that bounds staleness.