A counter or in-memory cache sometimes returns inconsistent results under load. How would you investigate a race condition?

Race condition in shared state

Investigate and fix a race condition caused by shared mutable state in a .NET service.

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A race condition happens when the result depends on timing between concurrent operations. If multiple requests read and write the same in-memory state without synchronization, the updates can interleave incorrectly. I would first identify the shared state and the operations that are supposed to be atomic. Then I would reproduce the issue with concurrent tests or load and fix it with a thread-safe structure, synchronization, or by removing the shared mutable state.

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Shared mutable state, atomicity, thread safety, reproduction, synchronization, and production-safe fixes.

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Race conditions are hard because they often disappear during debugging and only show up under concurrency. A useful answer combines code reasoning with stress reproduction and safer design.
A race condition happens when the result depends on timing between concurrent operations. If multiple requests read and write the same in-memory state without synchronization, the updates can interleave incorrectly. I would first identify the shared state and the operations that are supposed to be atomic. Then I would reproduce the issue with concurrent tests or load and fix it with a thread-safe structure, synchronization, or by removing the shared mutable state.
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For example, incrementing an integer with counter++ is not atomic; it reads, increments, and writes. Under concurrent requests, two threads can read the same value and overwrite each other. I might use , a lock, or redesign the flow so the database owns the counter. The trade-off is that locking can reduce throughput, while pushing state to a database adds latency but may give stronger consistency and durability.

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Testing only single-threaded code and declaring it safe.
Locking locally when correctness must hold across multiple servers.
Using ConcurrentDictionary but still performing non-atomic read-modify-write logic.
Fixing symptoms without identifying the real shared state owner.
Ignoring idempotency and database constraints.

A counter or in-memory cache sometimes returns inconsistent results under load. How would you investigate a race condition?

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