First, let's take a look at the futures::Sink trait itself:

pub trait Sink {
    type SinkItem;
    type SinkError;

    fn poll_ready(
        &mut self, 
        cx: &mut Context
    ) -> Result<Async<()>, Self::SinkError>;
    fn start_send(
        &mut self, 
        item: Self::SinkItem
    ) -> Result<(), Self::SinkError>;
    fn poll_flush(
        &mut self, 
        cx: &mut Context
    ) -> Result<Async<()>, Self::SinkError>;
    fn poll_close(
        &mut self, 
        cx: &mut Context
    ) -> Result<Async<()>, Self::SinkError>;
}

We are already familiar with the Item and Error concepts from futures and streams, so we will move on to the required functions:

• poll_ready must be invoked with the returning value of Ok(futures::Async::Ready(())) before each attempt at using start_send. If the sink receives an error, the sink will no longer be able to receive items.
• start_send, as stated previously...