Grokking Concurrency in Go
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The Concurrent Map

A team added visit counting to a web service. Each request handler ran in its own goroutine and increased a counter in a shared map[string]int. The change passed its tests, which sent one request at a time. Under real traffic, the service stopped within minutes. The program below reproduces the failure, and it ends with the same error the log showed:

Output (trimmed):

fatal error: concurrent map writes

goroutine 8 [running]:
internal/runtime/maps.fatal(...)
main.main.func1()

The map was never safe for concurrent writes

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