pekko/akka-docs/src/main/paradox/stream/operators/Source/queue.md

Source.queue

Materialize a BoundedSourceQueue or SourceQueue onto which elements can be pushed for emitting from the source.

@refSource operators

Signature (BoundedSourceQueue)

@apidocSource.queue { scala="#queueT:akka.stream.scaladsl.Source[T,akka.stream.scaladsl.BoundedSourceQueue[T]]" java="#queue(int)" }

Description (BoundedSourceQueue)

The BoundedSourceQueue is an optimized variant of the SourceQueue with OverflowStrategy.dropNew. The BoundedSourceQueue will give immediate, synchronous feedback whether an element was accepted or not and is therefore recommended for situations where overload and dropping elements is expected and needs to be handled quickly.

In contrast, the SourceQueue offers more variety of OverflowStrategies but feedback is only asynchronously provided through a @scala[Future]@java[CompletionStage] value. In cases where elements need to be discarded quickly at times of overload to avoid out-of-memory situations, delivering feedback asynchronously can itself become a problem. This happens if elements come in faster than the feedback can be delivered in which case the feedback mechanism itself is part of the reason that an out-of-memory situation arises.

In summary, prefer BoundedSourceQueue over SourceQueue with OverflowStrategy.dropNew especially in high-load scenarios. Use SourceQueue if you need one of the other OverflowStrategies.

The BoundedSourceQueue contains a buffer that can be used by many producers on different threads. When the buffer is full, the BoundedSourceQueue will not accept more elements. The return value of BoundedSourceQueue.offer() immediately returns a QueueOfferResult (as opposed to an asynchronous value returned by SourceQueue). A synchronous result is important in order to avoid situations where offer acknowledgements are handled slower than the rate of which elements are offered, which will eventually lead to an Out Of Memory error.

Example (BoundedSourceQueue)

Scala

@@snip IntegrationDocSpec.scala { #source-queue-synchronous }

Java

@@snip IntegrationDocTest.java { #source-queue-synchronous }

Signature (SourceQueue)

@apidocSource.queue { scala="#queueT:akka.stream.scaladsl.Source[T,akka.stream.scaladsl.SourceQueueWithComplete[T]]" java="#queue(int,akka.stream.OverflowStrategy)" } @apidocSource.queue { scala="#queueT:akka.stream.scaladsl.Source[T,akka.stream.scaladsl.SourceQueueWithComplete[T]]" java="#queue(int,akka.stream.OverflowStrategy,int)" }

Description (SourceQueue)

Materialize a SourceQueue onto which elements can be pushed for emitting from the source. The queue contains a buffer, if elements are pushed onto the queue faster than the source is consumed the overflow will be handled with a strategy specified by the user. Functionality for tracking when an element has been emitted is available through SourceQueue.offer.

Using Source.queue you can push elements to the queue and they will be emitted to the stream if there is demand from downstream, otherwise they will be buffered until request for demand is received. Elements in the buffer will be discarded if downstream is terminated.

In combination with the queue, the @refthrottle operator can be used to control the processing to a given limit, e.g. 5 elements per 3 seconds.

Example (SourceQueue)

Scala

@@snip IntegrationDocSpec.scala { #source-queue }

Java

@@snip IntegrationDocTest.java { #source-queue }

Reactive Streams semantics

@@@div { .callout }

emits when there is demand and the queue contains elements

completes when downstream completes

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