Trait futures::sink::Sink [] [src]

pub trait Sink {
    type SinkItem;
    type SinkError;
    fn start_send(&mut self,
                  item: Self::SinkItem)
                  -> StartSend<Self::SinkItem, Self::SinkError>; fn poll_complete(&mut self) -> Poll<(), Self::SinkError>; fn close(&mut self) -> Poll<(), Self::SinkError> { ... } fn wait(self) -> Wait<Self> where Self: Sized { ... } fn with<U, F, Fut>(self, f: F) -> With<Self, U, F, Fut> where F: FnMut(U) -> Fut,
              Fut: IntoFuture<Item=Self::SinkItem>,
              Fut::Error: From<Self::SinkError>,
              Self: Sized
{ ... } fn sink_map_err<F, E>(self, f: F) -> SinkMapErr<Self, F> where F: FnOnce(Self::SinkError) -> E, Self: Sized { ... } fn sink_from_err<E: From<Self::SinkError>>(self) -> SinkFromErr<Self, E> where Self: Sized { ... } fn buffer(self, amt: usize) -> Buffer<Self> where Self: Sized { ... } fn flush(self) -> Flush<Self> where Self: Sized { ... } fn send(self, item: Self::SinkItem) -> Send<Self> where Self: Sized { ... } fn send_all<S>(self, stream: S) -> SendAll<Self, S> where S: Stream<Item=Self::SinkItem>,
              Self::SinkError: From<S::Error>,
              Self: Sized
{ ... } }

A Sink is a value into which other values can be sent, asynchronously.

Basic examples of sinks include the sending side of:

  • Channels
  • Sockets
  • Pipes

In addition to such "primitive" sinks, it's typical to layer additional functionality, such as buffering, on top of an existing sink.

Sending to a sink is "asynchronous" in the sense that the value may not be sent in its entirety immediately. Instead, values are sent in a two-phase way: first by initiating a send, and then by polling for completion. This two-phase setup is analogous to buffered writing in synchronous code, where writes often succeed immediately, but internally are buffered and are actually written only upon flushing.

In addition, the Sink may be full, in which case it is not even possible to start the sending process.

As with Future and Stream, the Sink trait is built from a few core required methods, and a host of default methods for working in a higher-level way. The Sink::send_all combinator is of particular importance: you can use it to send an entire stream to a sink, which is the simplest way to ultimately consume a sink.

You can find more information/tutorials about streams online at

Associated Types

The type of value that the sink accepts.

The type of value produced by the sink when an error occurs.

Required Methods

Begin the process of sending a value to the sink.

As the name suggests, this method only begins the process of sending the item. If the sink employs buffering, the item isn't fully processed until the buffer is fully flushed. Since sinks are designed to work with asynchronous I/O, the process of actually writing out the data to an underlying object takes place asynchronously. You must use poll_complete in order to drive completion of a send. In particular, start_send does not begin the flushing process

Return value

This method returns AsyncSink::Ready if the sink was able to start sending item. In that case, you must ensure that you call poll_complete to process the sent item to completion. Note, however, that several calls to start_send can be made prior to calling poll_complete, which will work on completing all pending items.

The method returns AsyncSink::NotReady if the sink was unable to begin sending, usually due to being full. The sink must have attempted to complete processing any outstanding requests (equivalent to poll_complete) before yielding this result. The current task will be automatically scheduled for notification when the sink may be ready to receive new values.


If the sink encounters an error other than being temporarily full, it uses the Err variant to signal that error. In most cases, such errors mean that the sink will permanently be unable to receive items.


This method may panic in a few situations, depending on the specific sink:

  • It is called outside of the context of a task.
  • A previous call to start_send or poll_complete yielded an error.

Flush all output from this sink, if necessary.

Some sinks may buffer intermediate data as an optimization to improve throughput. In other words, if a sink has a corresponding receiver then a successful start_send above may not guarantee that the value is actually ready to be received by the receiver. This function is intended to be used to ensure that values do indeed make their way to the receiver.

This function will attempt to process any pending requests on behalf of the sink and drive it to completion.

Return value

Returns Ok(Async::Ready(())) when no buffered items remain. If this value is returned then it is guaranteed that all previous values sent via start_send will be guaranteed to be available to a listening receiver.

Returns Ok(Async::NotReady) if there is more work left to do, in which case the current task is scheduled to wake up when more progress may be possible.


Returns Err if the sink encounters an error while processing one of its pending requests. Due to the buffered nature of requests, it is not generally possible to correlate the error with a particular request. As with start_send, these errors are generally "fatal" for continued use of the sink.


This method may panic in a few situations, depending on the specific sink:

  • It is called outside of the context of a task.
  • A previous call to start_send or poll_complete yielded an error.

Compatibility nodes

The name of this method may be slightly misleading as the original intention was to have this method be more general than just flushing requests. Over time though it was decided to trim back the ambitions of this method to what it's always done, just flushing.

In the 0.2 release series of futures this method will be renamed to poll_flush. For 0.1, however, the breaking change is not happening yet.

Provided Methods

A method to indicate that no more values will ever be pushed into this sink.

This method is used to indicate that a sink will no longer even be given another value by the caller. That is, the start_send method above will be called no longer (nor poll_complete). This method is intended to model "graceful shutdown" in various protocols where the intent to shut down is followed by a little more blocking work.

Callers of this function should work it it in a similar fashion to poll_complete. Once called it may return NotReady which indicates that more external work needs to happen to make progress. The current task will be scheduled to receive a notification in such an event, however.

Note that this function will imply poll_complete above. That is, if a sink has buffered data, then it'll be flushed out during a close operation. It is not necessary to have poll_complete return Ready before a close is called. Once a close is called, though, poll_complete cannot be called.

Return value

This function, like poll_complete, returns a Poll. The value is Ready once the close operation has completed. At that point it should be safe to drop the sink and deallocate associated resources.

If the value returned is NotReady then the sink is not yet closed and work needs to be done to close it. The work has been scheduled and the current task will recieve a notification when it's next ready to call this method again.

Finally, this function may also return an error.


This function will return an Err if any operation along the way during the close operation fails. An error typically is fatal for a sink and is unable to be recovered from, but in specific situations this may not always be true.

Note that it's also typically an error to call start_send or poll_complete after the close function is called. This method will initiate a close, and continuing to send values after that (or attempt to flush) may result in strange behavior, panics, errors, etc. Once this method is called, it must be the only method called on this Sink.


This method may panic or cause panics if:

  • It is called outside the context of a future's task
  • It is called and then start_send or poll_complete is called

Compatibility notes

Note that this function is currently by default a provided function, defaulted to calling poll_complete above. This function was added in the 0.1 series of the crate as a backwards-compatible addition. It is intended that in the 0.2 series the method will no longer be a default method.

It is highly recommended to consider this method a required method and to implement it whenever you implement Sink locally. It is especially crucial to be sure to close inner sinks, if applicable.

Creates a new object which will produce a synchronous sink.

The sink returned does not implement the Sink trait, and instead only has two methods: send and flush. These two methods correspond to start_send and poll_complete above except are executed in a blocking fashion.

Composes a function in front of the sink.

This adapter produces a new sink that passes each value through the given function f before sending it to self.

To process each value, f produces a future, which is then polled to completion before passing its result down to the underlying sink. If the future produces an error, that error is returned by the new sink.

Note that this function consumes the given sink, returning a wrapped version, much like Iterator::map.

Transforms the error returned by the sink.

Map this sink's error to any error implementing From for this sink's Error, returning a new sink.

If wanting to map errors of a Sink + Stream, use .sink_from_err().from_err().

Adds a fixed-size buffer to the current sink.

The resulting sink will buffer up to amt items when the underlying sink is unwilling to accept additional items. Calling poll_complete on the buffered sink will attempt to both empty the buffer and complete processing on the underlying sink.

Note that this function consumes the given sink, returning a wrapped version, much like Iterator::map.

This method is only available when the use_std feature of this library is activated, and it is activated by default.

A future that completes when the sink has finished processing all pending requests.

The sink itself is returned after flushing is complete; this adapter is intended to be used when you want to stop sending to the sink until all current requests are processed.

A future that completes after the given item has been fully processed into the sink, including flushing.

Note that, because of the flushing requirement, it is usually better to batch together items to send via send_all, rather than flushing between each item.

On completion, the sink is returned.

A future that completes after the given stream has been fully processed into the sink, including flushing.

This future will drive the stream to keep producing items until it is exhausted, sending each item to the sink. It will complete once both the stream is exhausted, and the sink has fully processed and flushed all of the items sent to it.

Doing sink.send_all(stream) is roughly equivalent to stream.forward(sink).

On completion, the pair (sink, source) is returned.