Coordination Patterns

Two things: weights, so one unit can consume several units of capacity rather than exactly one, and a context-aware Acquire that returns an error when the caller's context is cancelled. A channel semaphore counts identical, uncancellable permits.

They run on after the caller has returned, so anything they touch is touched too late: a shared destination file gets a second writer, caller-owned memory gets written after the function returned, and their sockets and buffers stay held. Give each attempt private state and cancel the shared context on return.

A channel send copies only the slice header, so both stages point at the same backing array. Overwriting it for the next frame corrupts the frame already in flight, so treat a send as handing over ownership of those bytes.

Judge it by who bears the cost. In a binary the dependency stops with you, and errgroup beats twenty lines of join logic you must own. In a library it joins every importer's module graph, so the bar is higher.

Treat the ceiling as a budget to allocate, not a constant to hard-code. Target below it, divide it explicitly between callers and replicas, decide whether overflow waits or is shed, and coalesce duplicate in-flight calls because each one is billable.