Module tokio::sync::mpsc

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

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 will always complete immediately. This makes the UnboundedSender usable from both synchronous and asynchronous code.

Similar to the mpsc channels provided by std, the channel constructor functions provide separate send and receive handles, Sender and Receiver for the bounded channel, UnboundedSender and UnboundedReceiver for the unbounded channel. If there is no message to read, the current task will be notified when a new value is sent. Sender and UnboundedSender allow sending values into the channel. If the bounded 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 channel is also suitable for the single-producer single-consumer use-case. (Unless you only need to send one message, in which case you should use the oneshot channel.)

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. Additionally, all unread messages will be drained from the channel and dropped.

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. Instead of calling the async send or recv methods, in synchronous code you will need to use the blocking_send or blocking_recv methods.

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.

Please be aware that the above remarks were written with the mpsc channel in mind, but they can also be generalized to other kinds of channels. In general, any channel method that isn’t marked async can be called anywhere, including outside of the runtime. For example, sending a message on a oneshot channel from outside the runtime is perfectly fine.

Multiple runtimes

The mpsc channel does not care about which runtime you use it in, and can be used to send messages from one runtime to another. It can also be used in non-Tokio runtimes.

There is one exception to the above: the send_timeout must be used from within a Tokio runtime, however it is still not tied to one specific Tokio runtime, and the sender may be moved from one Tokio runtime to another.

Allocation behavior

The implementation details described in this section may change in future Tokio releases.

The mpsc channel stores elements in blocks. Blocks are organized in a linked list. Sending pushes new elements onto the block at the front of the list, and receiving pops them off the one at the back. A block can hold 32 messages on a 64-bit target and 16 messages on a 32-bit target. This number is independent of channel and message size. Each block also stores 4 pointer-sized values for bookkeeping (so on a 64-bit machine, each message has 1 byte of overhead).

When all values in a block have been received, it becomes empty. It will then be freed, unless the channel’s first block (where newly-sent elements are being stored) has no next block. In that case, the empty block is reused as the next block.

Modules

• error
  Channel error types.

Structs

• OwnedPermit
  Owned permit to send one value into the channel.
• Permit
  Permits to send one value into the channel.
• PermitIterator
  An Iterator of Permit that can be used to hold n slots in the channel.
• Receiver
  Receives values from the associated Sender.
• Sender
  Sends values to the associated Receiver.
• UnboundedReceiver
  Receive values from the associated UnboundedSender.
• UnboundedSender
  Send values to the associated UnboundedReceiver.
• WeakSender
  A sender that does not prevent the channel from being closed.
• WeakUnboundedSender
  An unbounded sender that does not prevent the channel from being closed.

Functions

• channel
  Creates a bounded mpsc channel for communicating between asynchronous tasks with backpressure.
• unbounded_channel
  Creates an unbounded mpsc channel for communicating between asynchronous tasks without backpressure.