Cancellation architectural observations

Gray Zhang otakustay at icloud.com
Mon Mar 2 06:32:34 PST 2015


+1 to the ignore term, I’ve opened an issue about it in https://github.com/promises-aplus/cancellation-spec/issues/14

IMO the term cancel(or abort) and ignore are totally different things, the former one means “do not continue, stop it right now” and the “stop” state should be broadcast to everyone who is interested in the work, while the latter means “I don’t care about the result anymore, just play it as you like”, it means the async progress can be continued

In practice both scenario are commonly seen, we may abort a resource fetch in order to save bandwidth and opened connections, or we may in other side just ignore it since continue to complete the fetch can result in a local cache, which speeds up our fetch next time

Back to the ignore scenario, there is still some confusion left for me:

Should ignore be a individual behavior, or a global one. This means should we just be able to “ignore” one callback, or just take a promise to a “ignore” state in which all callbacks are revoked
Should later then calls be aware that the promise is already ignored, or ignorance should be completely transparent to clients (which means all later then are effortless)


Best regards

Gray Zhang



在 2015年3月2日 下午7:26:52, Gundersen Marius (gundersen at gmail.com) 写到:

I think a better term than cancel would be ignore. If the promise has side effects then it is impossible for a cancel method to guarantee that the side effects do not happen. For example a POST request to the server that updates the server state can be cancelled, but it is not possible to know if it happens before or after the server has updated its state. It is therefore dangerous to think of it as cancelling. A better concept is to ignore the results of the promise. Even if the promise has been completed, the results can be ignored. Depending on what job the promise is doing it can decide if it wants to stop the task or not. A simple async task might not need to have a way to be cancelled; the result of the task can just be ignored. But a long running task that is ignored might decide to stop what it is doing to save processing power/battery.

```js
//this promise has a way to stop what it is doing
var p1 = new Promise(function(resolve, reject, ignored){
  var task = setTimeout(resolve.bind(null, "the result"), 1000);
  ignored(() => clearTimeout(task));
});

//ignore the result from the above promise, triggers the ignored listener
p1.ignore();
p1.then(result => "this is never called");

//this promise cannot be cancelled, but the result can be ignored
var p2 = Promise.resolve("the result");

//the callback is not called until next event loop cycle
p2.then(result => "this is never called");

//the result is ignored, the above callback is never run
p2.ignore();
```

Maybe calling ignore on a rejected or resolved promise should throw.

Marius Gundersen

On Mon, Mar 2, 2015 at 8:59 AM, Salvador de la Puente González <salva at unoyunodiez.com> wrote:
I think this did not reach the mailing list to the W3:
https://esdiscuss.org/topic/cancelable-promises#content-9

Actually the result cancellation is different from promise cancellation. First is full of implementation details, thus you are talking about smart cancellation. It is not smart, is the only way to provide control, thus, the cancellationToken.

In the other side you're talking about "don't care" what is actually related with the flow control. It turns out, after a timeout, you are no longer interested in the result so you don't use it because you don't care.

I insist, two concepts: one for cancelling a control flow, another for an specific operation of the implementation.

El 02/03/2015 08:06, "Dean Tribble" <tribble at e-dean.com> escribió:
Another thread here brought up the challenge of supporting cancellation in an async environment. I spent some time on that particular challenge a few years ago, and it turned out to be bigger and more interesting than it appeared on the surface. In the another thread, Ron Buckton pointed at the .Net approach and it's use in JavaScript:
 
AsyncJS (http://github.com/rbuckton/asyncjs) uses a separate abstraction for cancellation based on the .NET CancellationTokenSource/CancellationToken types. You can find more information about this abstraction in the MSDN documentation here: https://msdn.microsoft.com/en-us/library/dd997364(v=vs.110).aspx

It's great that asyncjs already has started using it. I was surprised at how well the cancellationToken approach worked in both small applications and when extended to a very large async system. I'll summarize some of the architectural observations, especially from extending it to async:

Cancel requests, not results
Promises are like object references for async; any particular promise might be returned or passed to more than one client. Usually, programmers would be surprised if a returned or passed in reference just got ripped out from under them by another client. this is especially obvious when considering a library that gets a promise passed into it. Using "cancel" on the promise is like having delete on object references; it's dangerous to use, and unreliable to have used by others.

Cancellation is heterogeneous
It can be misleading to think about canceling a single activity. In most systems, when cancellation happens, many unrelated tasks may need to be cancelled for the same reason. For example, if a user hits a stop button on a large incremental query after they see the first few results, what should happen?
the async fetch of more query results should be terminated and the connection closed
background computation to process the remote results into renderable form should be stopped
rendering of not-yet rendered content should be stopped. this might include retrieval of secondary content for the items no longer of interest (e.g., album covers for the songs found by a complicated content search)
the animation of "loading more" should be stopped, and should be replaced with "user cancelled"
etc.
Some of these are different levels of abstraction, and for any non-trivial application, there isn't a single piece of code that can know to terminate all these activities. This kind of system also requires that cancellation support is consistent across many very different types of components. But if each activity takes a cancellationToken, in the above example, they just get passed the one that would be cancelled if the user hits stop and the right thing happens.

Cancellation should be smart
Libraries can and should be smart about how they cancel. In the case of an async query, once the result of a query from the server has come back, it may make sense to finish parsing and caching it rather than just reflexively discarding it. In the case of a brokerage system, for example, the round trip to the servers to get recent data is the expensive part. Once that's been kicked off and a result is coming back, having it available in a local cache in case the user asks again is efficient. If the application spawned another worker, it may be more efficient to let the worker complete (so that you can reuse it) rather than abruptly terminate it (requiring discarding of the running worker and cached state).

Cancellation is a race
In an async system, new activities may be getting continuously scheduled by asks that are themselves scheduled but not currently running. The act of cancelling needs to run in this environment. When cancel starts, you can think of it as a signal racing out to catch up with all the computations launched to achieve the now-cancelled objective. Some of those may choose to complete (see the caching example above). Some may potentially keep launching more work before that work itself gets signaled (yeah it's a bug but people write buggy code). In an async system, cancellation is not prompt. Thus, it's infeasible to ask "has cancellation finished?" because that's not a well defined state. Indeed, there can be code scheduled that should and does not get cancelled (e.g., the result processor for a pub/sub system), but that schedules work that will be cancelled (parse the publication of an update to the now-cancelled query). 

Cancellation is "don't care"
Because smart cancellation sometimes doesn't stop anything and in an async environment, cancellation is racing with progress, it is at most "best efforts". When a set of computations are cancelled, the party canceling the activities is saying "I no longer care whether this completes". That is importantly different from saying "I want to prevent this from completing". The former is broadly usable resource reduction. The latter is only usefully achieved in systems with expensive engineering around atomicity and transactions. It was amazing how much simpler cancellation logic becomes when it's "don't care".
 
Cancellation requires separation of concerns
In the pattern where more than one thing gets cancelled, the source of the cancellation is rarely one of the things to be cancelled. It would be a surprise if a library called for a cancellable activity (load this image) cancelled an unrelated server query just because they cared about the same cancellation event. I find it interesting that the separation between cancellation token and cancellation source mirrors that separation between a promise and it's resolver.

Cancellation recovery is transient
As a task progresses, the cleanup action may change. In the example above, if the data table requests more results upon scrolling, it's cancellation behavior when there's an outstanding query for more data is likely to be quite different than when it's got everything it needs displayed for the current page. That's the reason why the "register" method returns a capability to unregister the action.


I don't want to derail the other threads on the topic, but thought it useful to start articulating some of the architectural background for a consistent async cancellation architecture.

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