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

Source.combine

Combine several sources, using a given strategy such as merge or concat, into one source.

@refSource operators

@@@div { .group-scala }

Signature

@@signature Source.scala { #combine }

@@@

Description

Provides a way to create a "fan-in" of multiple sources without having to use the more advanced @ref:GraphDSL.

The way the elements from the sources are combined is pluggable through the strategy parameter which accepts a function @scala[Int => Graph[FanInShape]]@java[Integer -> Graph<FanInShape>] where the integer parameter specifies the number of sources that the graph must accept. This makes it possible to use combine with the built-in Concat and Merge by @scala[expanding their apply methods to functions]@java[using a method reference to their create methods], but also to use an arbitrary strategy.

Combine is most useful when you have more sources than 2 or want to use a custom operator, as there are more concise operators for 2-source @ref:concat and @ref:merge

Some of the built-in operators that can be used as strategy are:

• @apidoc[akka.stream.*.Merge]
• @apidoc[akka.stream.(javadsl|scaladsl).Concat]
• @apidoc[MergePrioritized]
• @apidoc[MergeLatest]
• @apidoc[ZipN]
• @apidoc[ZipWithN]

Examples

In this example we Merge three different sources of integers. The three sources will immediately start contributing elements to the combined source. The individual elements from each source will be in order but the order compared to elements from other sources is not deterministic:

Scala

@@snip Combine.scala { #imports #source-combine-merge }

Java

@@snip Combine.java { #imports #source-combine-merge }

If we instead use Concat the first source will get to emit elements until it completes, then the second source until that completes and so on until all the sources has completed.

Scala

@@snip Combine.scala { #source-combine-concat }

Java

@@snip Combine.java { #source-combine-concat }

Reactive Streams semantics

@@@div { .callout }

emits when there is demand, but depending on the strategy

completes depends on the strategy

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