Module tokio::sync::mpsc

A multi-producer, single-consumer queue for sending values across asynchronous tasks.

Similar to std, channel creation provides Receiver and Sender handles. Receiver implements Stream and allows a task to read values out of the channel. If there is no message to read, the current task will be notified when a new value is sent. If the channel is at capacity, the send is rejected and the task will be notified when additional capacity is available. In other words, the channel provides backpressure.

This module provides two variants of the channel: bounded and unbounded. The bounded variant has a limit on the number of messages that the channel can store, and if this limit is reached, trying to send another message will wait until a message is received from the channel. An unbounded channel has an infinite capacity, so the send method never does any kind of sleeping. This makes the UnboundedSender usable from both synchronous and asynchronous code.

Disconnection

When all Sender handles have been dropped, it is no longer possible to send values into the channel. This is considered the termination event of the stream. As such, Receiver::poll returns Ok(Ready(None)).

If the Receiver handle is dropped, then messages can no longer be read out of the channel. In this case, all further attempts to send will result in an error.

Clean Shutdown

When the Receiver is dropped, it is possible for unprocessed messages to remain in the channel. Instead, it is usually desirable to perform a “clean” shutdown. To do this, the receiver first calls close, which will prevent any further messages to be sent into the channel. Then, the receiver consumes the channel to completion, at which point the receiver can be dropped.

Communicating between sync and async code

When you want to communicate between synchronous and asynchronous code, there are two situations to consider:

Bounded channel: If you need a bounded channel, you should use a bounded Tokio mpsc channel for both directions of communication. To call the async send or recv methods in sync code, you will need to use Handle::block_on, which allow you to execute an async method in synchronous code. This is necessary because a bounded channel may need to wait for additional capacity to become available.

Unbounded channel: You should use the kind of channel that matches where the receiver is. So for sending a message from async to sync, you should use the standard library unbounded channel or crossbeam. Similarly, for sending a message from sync to async, you should use an unbounded Tokio mpsc channel.

Modules

error

Channel error types

Structs

Receiver	Receive values from the associated Sender.
Sender	Send values to the associated Receiver.
UnboundedReceiver	Receive values from the associated UnboundedSender.
UnboundedSender	Send values to the associated UnboundedReceiver.

Functions

channel	Creates a bounded mpsc channel for communicating between asynchronous tasks, returning the sender/receiver halves.
unbounded_channel	Creates an unbounded mpsc channel for communicating between asynchronous tasks.