What is lock contention, and how do you reduce it in a hot counter path?
PICTURE THIS: HOW TO EXPLAIN IT
Simple meaning
Many threads blocking on the same monitor or striped lock serialize work and burn CPU on context switches.
WHY — Concurrency instead of guessing?
Why interviewers care about Concurrency:
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:
- 1Many threads blocking on
the same monitor or striped lock serialize work and burn CPU on context switches.
- 2Split the counter with
LongAdder, shard keys, or move aggregation out of the request path into Kafka.
- 3Profiling with a lock
profiler beats guessing synchronized on a global cache.
- 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
Many threads blocking on the same monitor or striped lock serialize work and burn CPU on context switches. Split the counter with LongAdder, shard keys, or move aggregation out of the request path into Kafka.