Invoked when the server is started.

start_link/3 (or start/3) will block until this callback returns. args is the argument term (second argument) passed to start_link/3 (or start/3).

In case of successful start, this callback must return a tuple where the first element is the stage type, which is one of:

• :producer
• :consumer
• :producer_consumer (if the stage is acting as both)

For example:

def init(args) do
  {:producer, some_state}
end

The returned tuple may also contain 3 or 4 elements. The third element may be the :hibernate atom or a set of options defined below.

Returning :ignore will cause start_link/3 to return :ignore and the process will exit normally without entering the loop or calling terminate/2.

Returning {:stop, reason} will cause start_link/3 to return {:error, reason} and the process to exit with reason reason without entering the loop or calling terminate/2.

Options

This callback may return options. Some options are specific to the chosen stage type while others are shared across all types.

:producer options

• :demand - when :forward, the demand is always forwarded to the c:handle_demand/2 callback. When :accumulate, demand is accumulated until its mode is set to :forward via demand/2. This is useful as a synchronization mechanism, where the demand is accumulated until all consumers are subscribed. Defaults to :forward.

:producer and :producer_consumer options

• :buffer_size - the size of the buffer to store events without demand. Can be :infinity to signal no limit on the buffer size. Check the "Buffer events" section of the module documentation. Defaults to 10_000 for :producer, :infinity for :producer_consumer.

• :buffer_keep - returns whether the :first or :last entries should be kept on the buffer in case the buffer size is exceeded. Defaults to :last.

• :dispatcher - the dispatcher responsible for handling demands. Defaults to GenStage.DemandDispatch. May be either an atom representing a dispatcher module or a two-element tuple with the dispatcher module and the dispatcher options.

:consumer and :producer_consumer options

• :subscribe_to - a list of producers to subscribe to. Each element represents either the producer module or a tuple with the producer module and the subscription options (as defined in sync_subscribe/2).

Callback implementation for GenStage.init/1.

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