Flow allows developers to express computations on collections, similar to the Enum and Stream modules, although computations will be executed in parallel using multiple GenStages.

Here is a quick example on how to count words in a document in parallel with Flow:

File.stream!("path/to/some/file")
|> Flow.from_enumerable()
|> Flow.flat_map(&String.split(&1, " "))
|> Flow.partition()
|> Flow.reduce(fn -> %{} end, fn word, acc ->
  Map.update(acc, word, 1, & &1 + 1)
end)
|> Enum.to_list()

See documentation for Flow or José Valim's keynote at ElixirConf 2016 introducing the main concepts behind GenStage and Flow.

Flow requires Elixir v1.7 and Erlang/OTP 22+. Add :flow to your list of dependencies in mix.exs:

def deps do
  [{:flow, "~> 1.0"}]
end

Flow pipelines starts several processes linked to the current process. This means that, if there is an error in your Flow, it will shut down the Livebook runtime. You can avoid this in your notebooks in two different ways:

1. Use Flow.stream(flow, link: false) to explicitly convert a Flow to a non-linked stream. You can them invoke Enum and Stream functions regularly:

   Flow.from_enumerable([1, 2, 3])
   |> Flow.map(& &1 * 2)
   |> Flow.stream(link: false)
   |> Enum.to_list()

2. By trapping exits once before the Flow computation starts:

   Process.flag(:trap_exit, true)

Copyright 2017 Plataformatec
Copyright 2020 Dashbit

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

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