GenStage is an Elixir behaviour that was in the making for a while. The plan was to include it in the Elixir standard library but it grew sufficiently enough to justify its own :gen_stage library.
Elixir lets you lazily process collections with the Stream module, and we previously saw how we can process a collection in parallel by spawning tasks for each element in the collection. However, if the process through which we are passing our collection has many steps, the usage of Task.async_stream isn't the best solution because, for each step, it will wait for all tasks to return or time out, before calling the next wave of Task.async_stream.
What we want is something that can spawn the needed processes to consume the collection in ...
