amp/docs/cancellation

layout: docs title: Cancellation permalink: /cancellation/

Amp provides a Cancellation primitive to allow the cancellation of operations.

request("...", new Amp\TimeoutCancellation(30));

$deferredCancellation = new Amp\DeferredCancellation();
Loop::onSignal(SIG_INT, function () use ($deferredCancellation) {
    $deferredCancellation->cancel();
});

request("...", $deferredCancellation->getCancellation());

Every operation that supports cancellation accepts an instance of Cancellation as (optional) argument. $cancellation->throwIfRequested() can be used to fail the operation with a CancelledException. As $cancellation is often an optional parameter and might be null, these calls need to be guarded with a if ($cancellation) or similar check. Instead of doing so, it's often easier to simply set the token to $cancellation ??= new NullCancellationToken at the beginning of the method.

While throwIfRequested() works well, some operations might want to subscribe with a callback instead. They can do so using Cancellation::subscribe() to subscribe any cancellation requests that might happen.

If the operation consists of any sub-operations that support cancellation, it passes that same Cancellation instance down to these sub-operations.

The original caller creates a Cancellation by creating an instance of DeferredCancellation and passing $deferred->getCancellation() to the operation as shown in the above example, or using one of the other implementations of Cancellation, such as TimeoutCancellation. Only the original caller has access to the DeferredCancellation and can cancel the operation using DeferredCancellation::cancel(), similar to the way it works with DeferredFuture and Future.

{:.note}

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.