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Event handler concurrency #184

slashdotdash opened this issue Jul 30, 2018 · 3 comments

Event handler concurrency #184

slashdotdash opened this issue Jul 30, 2018 · 3 comments


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@slashdotdash slashdotdash commented Jul 30, 2018

Allow event handlers to specify how many processes are started to allow handling events concurrently.

The behaviour is similar to Greg's Event Store which uses the competing consumer pattern. Increasing the concurrency means the event ordering guarantee is lost since events will be handled out-of-order. To alleviate this issue an optional partitioning function may be provided to ensure events are processed in order where necessary (e.g. per aggregate identity).

defmodule ExampleHandler do
  use Commanded.Event.Handler, 
    name: "ExampleHandler", 
    concurrency: 10, 
    partition_by: fn _event, metadata -> metadata.stream_id end

  def handle(_event, _metadata) do

This feature depends upon Subscription concurrency (#124) to be implemented in the Elixir Event Store. Greg's Event Store already has support for concurrency.

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@Aleksion Aleksion commented Aug 12, 2018

This would be great - especially in situations where concurrency doesn't matter.

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@Freyskeyd Freyskeyd commented Jan 28, 2019

The concurrency parameter can be obscure.

Does it represent the maximum of events handled at the same time?
Does it represent the maximum event handler count for this kind of event handler?
Does it represent the number of event handler started by default?

The partition_by function is called to dispatch the event to the right event handler instance ? What's the default behaviour?

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@slashdotdash slashdotdash commented Jan 28, 2019

  • concurrency - How many event handler processes are started.
  • partition_by - How to distribute events to running event handler processes. By default events will be distributed using a round-robbin strategy for even load.

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