What is request coalescing and when should you use it?

Request coalescing combines multiple identical concurrent requests into one, sharing the result - prevents thundering herd on cache miss. Implementation: for same key/request, first caller proceeds, others wait and receive same result. Use cases: cache population (single DB query on miss), API calls, expensive computations. Libraries: SingleFlight (Go), singleflight pattern. Also: request collapsing (batch multiple different requests). Caution: error handling (propagate to all waiters), timeout handling, lock contention. Essential for high-concurrency cache systems.