Control demand yourself

Most pipelines run well on automatic demand. Two tools help when you need direct control: manual demand for consumers that set their own pace, and accumulated demand for starting a pipeline in one go.

Manual demand

Some consumers must control the flow themselves — for example, because they forward events to another process. Set the subscription’s demand mode to Manual. The consumer then receives events only after you call sluice.ask:

let assert Ok(subscription) =
  sluice.subscription(to: counter.data)
  |> sluice.demand_mode(sluice.Manual)
  |> sluice.subscribe(consumer: printer.data)

// The consumer receives at most 10 events.
sluice.ask(subscription:, count: 10)

The on_events callback also receives the subscription, so a sink can ask for its next batch from inside the callback and set its own pace.

Accumulate demand at the start

A source with accumulate_demand holds incoming asks and produces nothing. Calling sluice.forward_demand(outlet:) releases the held asks. Use this to wire up a whole pipeline before any events start to move.