How does using a balancer differ from using Sink.forEachParallel?

Akka Libraries
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How does using a balancer differ from using Sink.forEachParallel?

This topic is based on a conversation between an anonymous user of Akka and our “Ask AI about Akka” bot in the documentation.

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A balancer and Sink.forEachParallel are both used in Akka Streams to handle concurrent processing, but they serve different purposes and are used in different scenarios.

A balancer, as described in the Akka Streams Cookbook, is a special fan-out operation that tries to route elements to available downstream consumers. It is used to distribute incoming jobs to a pool of workers, which can process the jobs in parallel. The balancer ensures that the workload is evenly distributed among the workers. Here is an example of how to use a balancer:

def balancer[In, Out](worker: Flow[In, Out, Any], workerCount: Int): Flow[In, Out, NotUsed] = {
  import GraphDSL.Implicits._

  Flow.fromGraph(GraphDSL.create() { implicit b =>
    val balancer = b.add(Balance[In](workerCount, waitForAllDownstreams = true))
    val merge = b.add(Merge[Out](workerCount))

    for (_ <- 1 to workerCount) {
      // for each worker, add an edge from the balancer to the worker, then wire
      // it to the merge element
      balancer ~> worker.async ~> merge
    }

    FlowShape(balancer.in, merge.out)
  })
}

val processedJobs: Source[Result, NotUsed] = myJobs.via(balancer(worker, 3))

On the other hand, Sink.forEachParallel is a sink that will consume a stream, invoking a given function for each element in the stream in parallel. It is used when you want to perform some side-effecting operation (like printing to the console or updating a database) for each element in the stream, and you want these operations to be performed in parallel. Note that Sink.forEachParallel does not change the order of the elements.

In summary, a balancer is used when you want to distribute work among a pool of workers, while Sink.forEachParallel is used when you want to perform a side-effecting operation on each element in a stream in parallel.