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doc: Add pipeToSelf in interaction-patterns.md, #27877 (#27912)

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Patrik Nordwall 2019-10-08 15:07:55 +02:00 • committed by Johan Andrén
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@ -216,6 +216,40 @@ Java
* There can only be a single response to one `ask` (see @ref:[per session child Actor](#per-session-child-actor))
* When `ask` times out, the receiving actor does not know and may still process it to completion, or even start processing it after the fact
## Send Future result to self
When using an API that returns a @scala[`Future`]@java[`CompletionStage`] from an actor it's common that you would
like to use the value of the in the actor when the @scala[`Future`]@java[`CompletionStage`] is completed. For
this purpose the `ActorContext` provides a `pipeToSelf` method.
**Example:**
![pipe-to-self.png](./images/pipe-to-self.png)
An actor, `CustomerRepository`, is invoking a method on `CustomerDataAccess` that returns a @scala[`Future`]@java[`CompletionStage`].
Scala
: @@snip [InteractionPatternsSpec.scala](/akka-actor-typed-tests/src/test/scala/docs/akka/typed/InteractionPatternsSpec.scala) { #pipeToSelf }
Java
: @@snip [InteractionPatternsTest.java](/akka-actor-typed-tests/src/test/java/jdocs/akka/typed/InteractionPatternsTest.java) { #pipeToSelf }
It could be tempting to just use @scala[`onComplete on the Future`]@java[`a callback on the CompletionStage`], but
that introduces the risk of accessing internal state of the actor that is not thread-safe from an external thread.
For example, the `numberOfPendingOperations` counter in above example can't be accessed from such callback.
Therefore it is better to map the result to a message and perform further processing when receiving that message.
**Useful when:**
* Accessing APIs that are returning @scala[`Future`]@java[`CompletionStage`] from an actor, such as a database or
an external service
* The actor needs to continue processing when the @scala[`Future`]@java[`CompletionStage`] has completed
* Keep context from the original request and use that when the @scala[`Future`]@java[`CompletionStage`] has completed,
for example an `replyTo` actor reference
**Problems:**
* Boilerplate of adding wrapper messages for the results
## Per session child Actor