Trait tokio_util::codec::Decoder

source ·
pub trait Decoder {
    type Item;
    type Error: From<Error>;

    // Required method
    fn decode(
        &mut self,
        src: &mut BytesMut
    ) -> Result<Option<Self::Item>, Self::Error>;

    // Provided methods
    fn decode_eof(
        &mut self,
        buf: &mut BytesMut
    ) -> Result<Option<Self::Item>, Self::Error> { ... }
    fn framed<T: AsyncRead + AsyncWrite + Sized>(self, io: T) -> Framed<T, Self>
       where Self: Sized { ... }
Available on crate feature codec only.
Expand description

Decoding of frames via buffers.

This trait is used when constructing an instance of Framed or FramedRead. An implementation of Decoder takes a byte stream that has already been buffered in src and decodes the data into a stream of Self::Item frames.

Implementations are able to track state on self, which enables implementing stateful streaming parsers. In many cases, though, this type will simply be a unit struct (e.g. struct HttpDecoder).

For some underlying data-sources, namely files and FIFOs, it’s possible to temporarily read 0 bytes by reaching EOF.

In these cases decode_eof will be called until it signals fulfillment of all closing frames by returning Ok(None). After that, repeated attempts to read from the Framed or FramedRead will not invoke decode or decode_eof again, until data can be read during a retry.

It is up to the Decoder to keep track of a restart after an EOF, and to decide how to handle such an event by, for example, allowing frames to cross EOF boundaries, re-emitting opening frames, or resetting the entire internal state.

Required Associated Types§


type Item

The type of decoded frames.


type Error: From<Error>

The type of unrecoverable frame decoding errors.

If an individual message is ill-formed but can be ignored without interfering with the processing of future messages, it may be more useful to report the failure as an Item.

From<io::Error> is required in the interest of making Error suitable for returning directly from a FramedRead, and to enable the default implementation of decode_eof to yield an io::Error when the decoder fails to consume all available data.

Note that implementors of this trait can simply indicate type Error = io::Error to use I/O errors as this type.

Required Methods§


fn decode( &mut self, src: &mut BytesMut ) -> Result<Option<Self::Item>, Self::Error>

Attempts to decode a frame from the provided buffer of bytes.

This method is called by FramedRead whenever bytes are ready to be parsed. The provided buffer of bytes is what’s been read so far, and this instance of Decode can determine whether an entire frame is in the buffer and is ready to be returned.

If an entire frame is available, then this instance will remove those bytes from the buffer provided and return them as a decoded frame. Note that removing bytes from the provided buffer doesn’t always necessarily copy the bytes, so this should be an efficient operation in most circumstances.

If the bytes look valid, but a frame isn’t fully available yet, then Ok(None) is returned. This indicates to the Framed instance that it needs to read some more bytes before calling this method again.

Note that the bytes provided may be empty. If a previous call to decode consumed all the bytes in the buffer then decode will be called again until it returns Ok(None), indicating that more bytes need to be read.

Finally, if the bytes in the buffer are malformed then an error is returned indicating why. This informs Framed that the stream is now corrupt and should be terminated.

Buffer management

Before returning from the function, implementations should ensure that the buffer has appropriate capacity in anticipation of future calls to decode. Failing to do so leads to inefficiency.

For example, if frames have a fixed length, or if the length of the current frame is known from a header, a possible buffer management strategy is:

impl Decoder for MyCodec {
    // ...

    fn decode(&mut self, src: &mut BytesMut) -> Result<Option<Self::Item>, Self::Error> {
        // ...

        // Reserve enough to complete decoding of the current frame.
        let current_frame_len: usize = 1000; // Example.
        // And to start decoding the next frame.
        let next_frame_header_len: usize = 10; // Example.
        src.reserve(current_frame_len + next_frame_header_len);

        return Ok(None);

An optimal buffer management strategy minimizes reallocations and over-allocations.

Provided Methods§


fn decode_eof( &mut self, buf: &mut BytesMut ) -> Result<Option<Self::Item>, Self::Error>

A default method available to be called when there are no more bytes available to be read from the underlying I/O.

This method defaults to calling decode and returns an error if Ok(None) is returned while there is unconsumed data in buf. Typically this doesn’t need to be implemented unless the framing protocol differs near the end of the stream, or if you need to construct frames across eof boundaries on sources that can be resumed.

Note that the buf argument may be empty. If a previous call to decode_eof consumed all the bytes in the buffer, decode_eof will be called again until it returns None, indicating that there are no more frames to yield. This behavior enables returning finalization frames that may not be based on inbound data.

Once None has been returned, decode_eof won’t be called again until an attempt to resume the stream has been made, where the underlying stream actually returned more data.


fn framed<T: AsyncRead + AsyncWrite + Sized>(self, io: T) -> Framed<T, Self>where Self: Sized,

Provides a Stream and Sink interface for reading and writing to this Io object, using Decode and Encode to read and write the raw data.

Raw I/O objects work with byte sequences, but higher-level code usually wants to batch these into meaningful chunks, called “frames”. This method layers framing on top of an I/O object, by using the Codec traits to handle encoding and decoding of messages frames. Note that the incoming and outgoing frame types may be distinct.

This function returns a single object that is both Stream and Sink; grouping this into a single object is often useful for layering things like gzip or TLS, which require both read and write access to the underlying object.

If you want to work more directly with the streams and sink, consider calling split on the Framed returned by this method, which will break them into separate objects, allowing them to interact more easily.