flatMapConcat

Transform each input element into a Source whose elements are then flattened into the output stream through concatenation.

Nesting and flattening operators

Signature

Flow.flatMapConcatFlow.flatMapConcat

Description

Transform each input element into a Source whose elements are then flattened into the output stream through concatenation. This means each source is fully consumed before consumption of the next source starts.

See also: flatMapMerge, mapConcat

Example

In the following example flatMapConcat is used to create a Source for each incoming customerId. This could be, for example, a calculation or a query to a database. Each customer is then passed to lookupCustomerEvents which returns a Source. All the events for a customer are delivered before moving to the next customer.

Scala

copysourceval source: Source[String, NotUsed] = Source(List("customer-1", "customer-2"))

// e.g. could b a query to a database
def lookupCustomerEvents(customerId: String): Source[String, NotUsed] = {
  Source(List(s"$customerId-event-1", s"$customerId-event-2"))
}

source.flatMapConcat(customerId => lookupCustomerEvents(customerId)).runForeach(println)

// prints - events from each customer consecutively
// customer-1-event-1
// customer-1-event-2
// customer-2-event-1
// customer-2-event-2

Java

copysource// e.g. could be a query to a database
private Source<String, NotUsed> lookupCustomerEvents(String customerId) {
  return Source.from(Arrays.asList(customerId + "-event-1", customerId + "-event-2"));
}
  Source.from(Arrays.asList("customer-1", "customer-2"))
      .flatMapConcat(this::lookupCustomerEvents)
      .runForeach(System.out::println, system);
  // prints - events from each customer consecutively
  // customer-1-event-1
  // customer-1-event-2
  // customer-2-event-1
  // customer-2-event-2

Reactive Streams semantics