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Re: Cancellation architectural observations

From: Salvador de la Puente González <salva@unoyunodiez.com>
Date: Mon, 2 Mar 2015 08:59:14 +0100
Message-ID: <CA+tDh5UrOnsueK=e_10XROhXZr0_99yF6nG2KH_P0e10D81+Gw@mail.gmail.com>
To: Dean Tribble <tribble@e-dean.com>
Cc: es-discuss@mozilla.org, public-script-coord@w3.org
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@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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Received on Monday, 2 March 2015 07:59:43 UTC

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