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41 lines
2.1 KiB
Markdown
41 lines
2.1 KiB
Markdown
---
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layout: "docs"
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title: "Cancellation"
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permalink: "/cancellation/"
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---
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Amp provides a `Cancellation` primitive to allow the cancellation of operations.
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```php
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request("...", new Amp\TimeoutCancellation(30));
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```
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```php
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$deferredCancellation = new Amp\DeferredCancellation();
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Loop::onSignal(SIG_INT, function () use ($deferredCancellation) {
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$deferredCancellation->cancel();
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});
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request("...", $deferredCancellation->getCancellation());
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```
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Every operation that supports cancellation accepts an instance of `Cancellation` as (optional)
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argument. `$cancellation->throwIfRequested()` can be used to fail the operation with a `CancelledException`.
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As `$cancellation` is often an optional parameter and might be `null`, these calls need to be guarded with
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a `if ($cancellation)` or similar check. Instead of doing so, it's often easier to simply set the token
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to `$cancellation ??= new NullCancellationToken` at the beginning of the method.
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While `throwIfRequested()` works well, some operations might want to subscribe with a callback instead. They can do so
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using `Cancellation::subscribe()` to subscribe any cancellation requests that might happen.
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If the operation consists of any sub-operations that support cancellation, it passes that same `Cancellation`
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instance down to these sub-operations.
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The original caller creates a `Cancellation` by creating an instance of `DeferredCancellation` and
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passing `$deferred->getCancellation()` to the operation as shown in the above example, or using one of the other
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implementations of `Cancellation`, such as `TimeoutCancellation`. Only the original caller has access to
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the `DeferredCancellation` and can cancel the operation using `DeferredCancellation::cancel()`, similar to the way it
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works with `DeferredFuture` and `Future`.
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{:.note}
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> Cancellations are advisory only. A DNS resolver might ignore cancellation requests after the query has been sent as the response has to be processed anyway and can still be cached. An HTTP client might continue a nearly finished HTTP request to reuse the connection, but might abort a chunked encoding response as it cannot know whether continuing is actually cheaper than aborting.
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