Unit 02 of 03
Concurrency Patterns
Distributing work with a fixed number of workers wired to job and result channels, a job being fanned out to many and the results gathered back, each stage's own closing responsibility in stream processing, context cancellation and carrying request-scoped values, and where timeouts and deadlines do and do not prevent a resource leak.
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Lessons
- 01 Worker Pools A fixed number of workers reads from a shared job channel; when the channel closes, all of them stop for the same reason — what they waited for arrived. When five jobs are distributed to eight workers, the minimum count of workers that stopped without ever taking a job is derived from the total jobs taken and the total reporting worker count. Limiting measurement: raising the worker count does not split the existing work, it only leaves more workers idle; in a job channel that is never closed, workers never stop at all. 3 min read
- 02 Fan-Out and Fan-In When a source is fanned out to a fixed number of consumers (fan-out), the results merge back into a single fan-in channel (fan-in); the fan-in side's reason for stopping depends on a wait group closing the fan-in channel once all producers finish. Limiting measurement: the question of who closes the fan-in channel has exactly one answer — no producer can safely close it on its own (of three producers that try, two panic while one succeeds), and if it is never closed, the fan-in side never stops. 2 min read
- 03 Pipelines In a pipeline arrangement, each stage opens its own output channel and closes it itself; five values passing through a generation stage, a squaring stage, and an add-one stage flow through a chain where no stage disturbs the order. Limiting measurement: when the consumer gives up early from the middle of the chain, the stages ahead of it never stop at all — even though every stage's closing responsibility is correctly set up, none of them knows when the consumer at the far end of the chain has given up. The fix is the subject of the next lesson. 2 min read
- 04 The Context Package A single cancellation reaches all four goroutines waiting on ctx.Done() at the same time; all of them stop in the 'cancelled' class and ctx.Err() carries the same fixed reason. What a request-scoped value (context.WithValue) is for, and what it is not for, are measured separately. Limiting measurement: a context does not force anyone to stop, it only notifies — a goroutine that never checks ctx.Done() completes its own work to the end, even though the context has already been cancelled. 2 min read
- 05 Timeouts and Deadlines A timeout is a branch choice in a select statement: if a result is already ready, the result branch is chosen; if the context is already cancelled (representing a timeout), the cancellation branch is chosen — both deterministic, because only one branch in the setup is actually ready. Duration is never measured anywhere, the only thing printed is which branch was chosen. Limiting measurement: a timeout rescues the waiting side but does not stop the awaited work — a work goroutine whose result no one will take anymore is left behind, as a leak, behind the caller who exited its branch. 2 min read