Buf represents a fixed, immutable byte buffer with efficient positional access. Buffers may be sliced and concatenated, and thus be used to implement bytestreams.

A ByteReader provides a stateful API to extract bytes from an underlying buffer, which in most cases is a Buf. This conveniently allows codec implementations to decode frames, specifically when they need to decode and interpret the bytes as a numeric value.

ByteWriters allow efficient encoding to a buffer. Concatenating Bufs together is a common pattern for codecs (especially on the encode path), but using the concat method on Buf results in a suboptimal representation. In many cases, a ByteWriter not only allows for a more optimal representation (i.e., sequential accessible out regions), but also allows for writes to avoid allocations. What this means in practice is that builders are stateful. Assume that the builder implementations are ""not"" threadsafe unless otherwise noted.

Provides the Reader API for an InputStream.

The given InputStream will be closed when Reader.read reaches the EOF or a call to discard or close.

A synchronous in-memory pipe that connects Reader and Writer in the sense that a reader's input is the output of a writer.

A pipe is structured as a smash of both interfaces, a Reader and a Writer such that can be passed directly to a consumer or a producer.

def consumer(r: Reader[Buf]): Future[Unit] = ???
def producer(w: Writer[Buf]): Future[Unit] = ???

val p = new Pipe[Buf]

consumer(p)
producer(p)

Reads and writes on the pipe are matched one to one and only one outstanding read or write is permitted in the current implementation (multiple pending writes or reads resolve into IllegalStateException while leaving the pipe healthy). That is, the write (its returned com.twitter.util.Future) is resolved when the read consumes the written data.

Here is, for example, a very typical write-loop that writes into a pipe-backed Writer:

def writeLoop(w: Writer[Buf], data: List[Buf]): Future[Unit] = data match {
  case h :: t => p.write(h).before(writeLoop(w, t))
  case Nil => w.close()
}

Reading from a pipe-backed Reader is no different from working with any other reader:

def readLoop(r: Reader[Buf], process: Buf => Future[Unit]): Future[Unit] = r.read().flatMap {
  case Some(chunk) => process(chunk).before(readLoop(r, process))
  case None => Future.Done
}

Thread Safety

It is safe to call read, write, fail, discard, and close concurrently. The individual calls are synchronized on the given Pipe.

Closing or Failing Pipes

Besides expecting a write or a read, a pipe can be closed or failed. A writer can do both close and fail the pipe, while reader can only fail the pipe via discard.

The following behavior is expected with regards to reading from or writing into a closed or a failed pipe:

• Writing into a closed pipe yields IllegalStateException
• Reading from a closed pipe yields EOF (com.twitter.util.Future.None)
• Reading from or writing into a failed pipe yields a failure it was failed with

It's also worth discussing how pipes are being closed. As closure is always initiated by a producer (writer), there is a machinery allowing it to be notified when said closure is observed by a consumer (reader).

The following rules should help reasoning about closure signals in pipes:

- Closing a pipe with a pending read resolves said read into EOF and returns a com.twitter.util.Future.Unit - Closing a pipe with a pending write by default fails said write with IllegalStateException and returns a future that will be satisfied when a consumer observes the closure (EOF) via read. If a timer is provided, the pipe will wait until the provided deadline for a successful read before failing the write. - Closing an idle pipe returns a future that will be satisfied when a consumer observes the closure (EOF) via read when a timer is provided, otherwise the pipe will be closed immedidately.

A proxy ByteReader that forwards all calls to another ByteReader. This is useful if you want to wrap-but-modify an existing ByteReader.

A proxy ByteWriter that forwards all calls to another ByteWriter. This is useful if you want to wrap-but-modify an existing ByteWriter.

A reader exposes a pull-based API to model a potentially infinite stream of arbitrary elements.

Given the pull-based API, the consumer is responsible for driving the computation. A very typical code pattern to consume a reader is to use a read-loop:

def consume[A](r: Reader[A])(process: A => Future[Unit]): Future[Unit] =
  r.read().flatMap {
    case Some(a) => process(a).before(consume(r)(process))
    case None => Future.Done // reached the end of the stream; no need to discard
  }

One way to reason about the read-loop idiom is to view it as a subscription to a publisher (reader) in the Pub/Sub terminology. Perhaps the major difference between readers and traditional publishers, is readers only allow one subscriber (read-loop). It's generally safer to assume the reader is fully consumed (stream is exhausted) once its read-loop is run.

Error Handling

Given the read-loop above, its returned com.twitter.util.Future could be used to observe both successful and unsuccessful outcomes.

consume(stream)(processor).respond {
  case Return(()) => println("Consumed an entire stream successfully.")
  case Throw(e) => println(s"Encountered an error while consuming a stream: $e")
}

Test mixin that creates a temporary thread-local folder for a block of code to execute in. The folder is recursively deleted after the test.

Note that multiple uses of TempFolder cannot be nested, because the temporary directory is effectively a thread-local global.

A writer represents a sink for a stream of arbitrary elements. While Reader provides an API to consume streams, Writer provides an API to produce streams.

Similar to Readers, a very typical way to work with Writers is to model a stream producer as a write-loop:

def produce[A](w: Writer[A])(generate: () => Option[A]): Future[Unit] =
  generate match {
    case Some(a) => w.write(a).before(produce(w)(generate))
    case None => w.close() // signal EOF and quit producing
  }

Closures and Failures

Writers are com.twitter.util.Closable and the producer MUST close the stream when it finishes or fail the stream when it encounters a failure and can no longer continue. Streams backed by network connections are particularly prone to resource leaks when they aren't cleaned up properly.

Observing stream failures in the producer could be done via a com.twitter.util.Future returned form a write-loop:

produce(writer)(generator).respond {
  case Return(()) => println("Produced a stream successfully.")
  case Throw(e) => println(s"Could not produce a stream because of a failure: $e")
}

A Java adaptation of the com.twitter.io.Buf companion object.