Chronos - An efficient library for asynchronous programming

Introduction

Chronos is an asyncdispatch fork with a unified callback type, FIFO processing order for Future callbacks and many other changes that diverged from upstream's philosophy.

Installation

You can use Nim's official package manager Nimble to install Chronos:

$ nimble install https://github.com/status-im/nim-chronos.git

Documentation

Concepts

Chronos implements the async/await paradigm in a self-contained library, using macros, with no specific helpers from the compiler.

Our event loop is called a "dispatcher" and a single instance per thread is created, as soon as one is needed.

To trigger a dispatcher's processing step, we need to call poll() - either directly or through a wrapper like runForever() or waitFor(). This step handles any file descriptors, timers and callbacks that are ready to be processed.

Future objects encapsulate the result of an async procedure, upon successful completion, and a list of callbacks to be scheduled after any type of completion - be that success, failure or cancellation.

(These explicit callbacks are rarely used outside Chronos, being replaced by implicit ones generated by async procedure execution and await chaining.)

Async procedures (those using the {.async.} pragma) return Future objects.

Inside an async procedure, you can await the future returned by another async procedure. At this point, control will be handled to the event loop until that future is completed.

Future completion is tested with Future.finished() and is defined as success, failure or cancellation. This means that a future is either pending or completed.

To differentiate between completion states, we have Future.failed() and Future.cancelled().

Dispatcher

You can run the "dispatcher" event loop forever, with runForever() which is defined as:

proc runForever*() =
  while true:
    poll()

You can also run it until a certain future is completed, with waitFor() which will also call Future.read() on it:

proc p(): Future[int] {.async.} =
  await sleepAsync(100.milliseconds)
  return 1

echo waitFor p() # prints "1"

waitFor() is defined like this:

proc waitFor*[T](fut: Future[T]): T =
  while not(fut.finished()):
    poll()
  return fut.read()

Async procedures and methods

The {.async.} pragma will transform a procedure (or a method) returning a specialised Future type into a closure iterator. If there is no return type specified, a Future[void] is returned.

proc p() {.async.} =
  await sleepAsync(100.milliseconds)

echo p().type # prints "Future[system.void]"

Whenever await is encountered inside an async procedure, control is passed back to the dispatcher for as many steps as it's necessary for the awaited future to complete successfully, fail or be cancelled. await calls the equivalent of Future.read() on the completed future and returns the encapsulated value.

proc p1() {.async.} =
  await sleepAsync(1.seconds)

proc p2() {.async.} =
  await sleepAsync(1.seconds)

proc p3() {.async.} =
  let
    fut1 = p1()
    fut2 = p2()
  # Just by executing the async procs, both resulting futures entered the
  # dispatcher's queue and their "clocks" started ticking.
  await fut1
  await fut2
  # Only one second passed while awaiting them both, not two.

waitFor p3()

Not as intuitive, but the same thing happens if we create those futures on the same line as await, due to the Nim compiler's use of hidden temporary variables:

proc p4() {.async.} =
  await p1()
  await p2()
  # Also takes a single second for both futures sleeping concurrently.

waitFor p4()

Don't let await's behaviour of giving back control to the dispatcher surprise you. If an async procedure modifies global state, and you can't predict when it will start executing, the only way to avoid that state changing underneath your feet, in a certain section, is to not use await in it.

Error handling

Exceptions inheriting from CatchableError are caught by hidden try blocks and placed in the Future.error field, changing the future's status to Failed.

When a future is awaited, that exception is re-raised, only to be caught again by a hidden try block in the calling async procedure. That's how these exceptions move up the async chain.

A failed future's callbacks will still be scheduled, but it's not possible to resume execution from the point an exception was raised.

proc p1() {.async.} =
  await sleepAsync(1.seconds)
  raise newException(ValueError, "ValueError inherits from CatchableError")

proc p2() {.async.} =
  await sleepAsync(1.seconds)

proc p3() {.async.} =
  let
    fut1 = p1()
    fut2 = p2()
  await fut1
  echo "unreachable code here"
  await fut2

# `waitFor()` would call `Future.read()` unconditionally, which would raise the
# exception in `Future.error`.
let fut3 = p3()
while not(fut3.finished()):
  poll()

echo "fut3.state = ", fut3.state # "Failed"
if fut3.failed():
  echo "p3() failed: ", fut3.error.name, ": ", fut3.error.msg
  # prints "p3() failed: ValueError: ValueError inherits from CatchableError"

You can put the await in a try block, to deal with that exception sooner:

proc p3() {.async.} =
  let
    fut1 = p1()
    fut2 = p2()
  try:
    await fut1
  except:
    echo "p1() failed: ", fut1.error.name, ": ", fut1.error.msg
  echo "reachable code here"
  await fut2

Exceptions inheriting from Defect are treated differently, being raised directly. Don't try to catch them coming out of poll(), because this would leave behind some zombie futures.

TODO

• Pipe/Subprocess Transports.
• Multithreading Stream/Datagram servers

Contributing

When submitting pull requests, please add test cases for any new features or fixes and make sure nimble test is still able to execute the entire test suite successfully.

License

Licensed and distributed under either of

• MIT license: LICENSE-MIT or http://opensource.org/licenses/MIT

or

• Apache License, Version 2.0, (LICENSE-APACHEv2 or http://www.apache.org/licenses/LICENSE-2.0)

at your option. These files may not be copied, modified, or distributed except according to those terms.