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Add support for cancelling async requests in low-level REST client (#45379)

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The low-level REST client exposes a `performRequestAsync` method that
allows to send async requests, but today it does not expose the ability
to cancel such requests. That is something that the underlying apache
async http client supports, and it makes sense for us to expose.

This commit adds a return value to the `performRequestAsync` method,
which is backwards compatible. A `Cancellable` object gets returned,
which exposes a `cancel` public method. When calling `cancel`, the
on-going request associated with the returned `Cancellable` instance
will be cancelled by calling its `abort` method. This works throughout
multiple retries, though some special care was needed for the case where
`cancel` is called between different attempts (when one attempt has
failed and the consecutive one has not been sent yet).

Note that cancelling a request on the client side does not automatically 
translate to cancelling the server side execution of it. That needs to be 
specifically implemented, which is on the work for the search API (see #43332).

Relates to #44802
This commit is contained in:
Luca Cavanna 2019-08-13 16:48:06 +02:00 • committed by GitHub
parent e0c9234ba9
commit 4baab594aa
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8 changed files with 366 additions and 86 deletions
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18
docs/java-rest/low-level/usage.asciidoc
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@ -224,7 +224,7 @@ Once the `RestClient` has been created, requests can be sent by calling either
will block the calling thread and return the `Response` when the request is
successful or throw an exception if it fails. `performRequestAsync` is
asynchronous and accepts a `ResponseListener` argument that it calls with a
`Response` when the request is successful or with an `Exception` if it4 fails.
`Response` when the request is successful or with an `Exception` if it fails.
This is synchronous:
@ -329,6 +329,22 @@ include-tagged::{doc-tests}/RestClientDocumentation.java[rest-client-async-examp
<2> Handle the returned exception, due to communication error or a response
with status code that indicates an error
==== Cancelling asynchronous requests
The `performRequestAsync` method returns a `Cancellable` that exposes a single
public method called `cancel`. Such method can be called to cancel the on-going
request. Cancelling a request will result in aborting the http request through
the underlying http client. On the server side, this does not automatically
translate to the execution of that request being cancelled, which needs to be
specifically implemented in the API itself.
["source","java",subs="attributes,callouts,macros"]
--------------------------------------------------
include-tagged::{doc-tests}/RestClientDocumentation.java[rest-client-async-cancel]
--------------------------------------------------
<1> Process the returned response, in case it was ready before the request got cancelled
<2> Handle the returned exception, which will most likely be a `CancellationException` as the request got cancelled
[[java-rest-low-usage-responses]]
=== Reading responses