Invoked once by Broadway during Broadway.start_link/2. see more here https://hexdocs.pm/broadway/Broadway.Producer.html#c:prepare_for_start/2

@broadway_opts [ hibernate_after: 15000, context: :context_not_set, resubscribe_interval: 100, max_seconds: 5, max_restarts: 3, shutdown: 30000, name: MS.Pipeline, producer: [

hibernate_after: 15000,
transformer: nil,
concurrency: 1,
module:
  {MS.BroadwayMongo.Producer, <producer_opts>}

], processors: [default: [hibernate_after: 15000, max_demand: 10, concurrency: 1]], batchers: [

default: [hibernate_after: 15000, batch_timeout: 1000, concurrency: 1, batch_size: 1]

] ]