b47fcb3e86 | ||
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.github/workflows | ||
chronos | ||
tests | ||
.appveyor.yml | ||
.gitignore | ||
.travis.yml | ||
LICENSE-APACHEv2 | ||
LICENSE-MIT | ||
README.md | ||
chronos.nim | ||
chronos.nimble |
README.md
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()
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.