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#201 in Asynchronous

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The feature-rich, portable async channel library > crates.io > docs.rs

Why use Postage?

  • Includes a rich set of channels. | barrier | broadcast | dispatch | mpsc | oneshot | watch
  • Works with any executor.
    • Currently regressions are written for tokio and async-std.
    • With the futures-traits feature, channels implement the futures Sink/Stream traits.
  • Thoroughly tested.
    • Channels have full unit test coverage, and integration test coverage with multiple async executors.
  • Includes built-in Sink and Stream combinators.
    • Sinks can be chained and filtered.
    • Streams can be chained, filtered, mapped, and merged.
    • Sinks and streams can log their values, for easy app debugging.



Barrier channels can be used to synchronize events, but do not transmit any data. When the sender is dropped (or tx.send(()) is called), the receiver is awoken. This can be used to asynchronously coordinate actions between tasks.


The broadcast channel provides reliable broadcast delivery between multiple senders and multiple receivers. The channel has a fixed capacity, and senders are suspended if the buffer is filled.

When a receiver is cloned, both receivers will be sent the same series of messages.

Senders also provide a subscribe() method which creates a receiver that will observe all messages sent after the call to subscribe.


The dispatch channel provides multi-sender, multi-receiver message dispatch. A message will be observed by at most one reciever. The channel has a fixed capacity, and senders are suspended if the buffer is filled.

Receivers can be created with rx.clone(), or tx.subscribe().


Postage includes a fixed-capacity multi-producer, single-consumer channel. The producer can be cloned, and the sender task is suspended if the channel becomes full.


Oneshot channels transmit a single value between a sender and a reciever. Neither can be cloned. If the sender drops, the receiver recieves a None value.


Watch channels can be used to asynchronously transmit state. When receivers are created, they immediately recieve an initial value. They will also recieve new values, but are not guaranteed to recieve every value.

Values transmitted over watch channels must implement Default. A simple way to achieve this is to transmit Option<T>.


Benchmarks of postage channels, and comparable async-std/tokio channels.

  • send/recv measures the total time to send and receive an item.
  • send full measures the time to send an item and get a Poll::Pending value on a full channel.
  • recv empty measures the time to get a Poll::Pending value on an empty channel.

All benchmarks were taken with criterion and are in the benches directory.

Package Channel send/recv send full recv empty
broadcast postage 114ns 7ns 8ns
broadcast tokio 98ns (-14%) 54ns 37ns
dispatch postage 80ns 26ns 25ns
dispatch async_std 41ns (-48%) 10ns 11ns
mpsc postage 83ns 27ns 30ns
mpsc tokio 85ns (+1%) 2ns 35ns
watch postage 96ns - 7ns
watch tokio 73ns (-23%) - 75ns


~37K SLoC