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Fan-out, fan-in

Category: Communication · Status: stub · Lessons: chapter 05, Message passing (planned)

One line: Spreading the items of one channel across several workers, then merging their results back into one channel.

How it connects

flowchart LR
  n_channel["Channel"]
  n_fan_out_fan_in["Fan-out, fan-in"]
  n_fan_out_fan_in -->|uses| n_channel
  classDef center stroke-width:3px
  class n_fan_out_fan_in center
  classDef outside stroke-dasharray: 4 3
  class n_channel outside

In each language
Rust fan-in comes free, since cloned Senders all feed one mpsc ↗ receiver; fan-out needs that receiver shared, which the Rust Book does with Arc<Mutex<T>> ↗
Go several goroutines reading one channel (fan-out) and a merge copying many channels onto one (fan-in), both named in the Go blog's pipelines post ↗
Java ExecutorCompletionService ↗ puts each task on a queue as it completes, for take to collect
Python asyncio.gather ↗ collects results in the order of its arguments, not the order they finish in
JavaScript Promise.all ↗ collects fulfillment values in the order of the promises passed, regardless of completion order
Kotlin the guide's fan-out ↗ (several coroutines receive from one channel) and fan-in ↗ (several coroutines send to one)
Erlang and Elixir Task.async_stream ↗ runs a function over a collection with up to max_concurrency tasks and returns results in input order unless ordered: false

Where to read more