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https://github.com/matrixai/js-async-cancellable

Asynchronous Cancellation (Promises) for JavaScript/TypeScript
https://github.com/matrixai/js-async-cancellable

cancellation promise

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Asynchronous Cancellation (Promises) for JavaScript/TypeScript

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# js-async-cancellable



This library provides the ability to cancel asynchronous tasks. Cancelling

asynchronous tasks was never standardised in JavaScript. This was due to the

myriad complexity of cancellation illustrated by

https://github.com/tc39/proposal-cancellation:



> The following are some architectural observations provided by **Dean Tribble**

> on the

> [es-discuss mailing list](https://mail.mozilla.org/pipermail/es-discuss/2015-March/041887.html):

>

> _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.



This library attempts to address each concern.



1. Cancel requests, not results - cancellation only affects the immediate

   promise and all downstream promises, not upstream promises unless explicitly

   configured via a signal handler

2. Cancellation is heterogenous - cancellation can be customised through a

   signal handler or an `AbortController`, this allows the user to define how

   cancellation should propagate through the application

3. Cancellation should be smart - during the `PromiseCancellable.then` binding,

   both the the fulfilled and rejected handler takes the `signal: AbortSignal`,

   here it is possible to customise the logic of fulfillment and rejection

   depending on the situation, therefore cancellation can be as smart as you

   want it to be

4. Cancellation is a race - this is achieved by using `AbortSignal`, one cannot

   ask if cancellation is finished, it is purely an event driven system

5. Cancellation is "don't care" - `PromiseCancellation.cancel` is purely

   advisory, the default behaviour is that the immediate promise is rejected

   early, however this can be customised, therefore the default intention is

   that the promise can be rejected with the cancellation reason, and means you

   don't care about the result anymore, it does not imply anything stronger than

   this

6. Cancellation requires separation of concerns - by supplying a signal handler

   or abort controller, it is possible to separate any concern

7. Cancellation recovery is transient - during the `PromiseCancellable.then`,

   `PromiseCancellable.catch` and `PromiseCancellable.finally`, the signal is

   passed in, it is possible to change how you cancel depending on what the

   current situation is.



## Installation



```sh

npm install --save @matrixai/async-cancellable

```



## Development



Run `nix develop`, and once you're inside, you can use:



```sh

# install (or reinstall packages from package.json)

npm install

# build the dist

npm run build

# run the repl (this allows you to import from ./src)

npm run tsx

# run the tests

npm run test

# lint the source code

npm run lint

# automatically fix the source

npm run lintfix

```



### Docs Generation



```sh

npm run docs

```



See the docs at: https://matrixai.github.io/js-async-cancellable/



### Publishing



Publishing is handled automatically by the staging pipeline.



Prerelease:



```sh

# npm login

npm version prepatch --preid alpha # premajor/preminor/prepatch

git push --follow-tags

```



Release:



```sh

# npm login

npm version patch # major/minor/patch

git push --follow-tags

```



Manually:



```sh

# npm login

npm version patch # major/minor/patch

npm run build

npm publish --access public

git push

git push --tags

```