EventStore.start_link

You're seeing just the callback start_link, go back to EventStore module for more information.

start_link(opts)

start_link(opts :: Keyword.t()) ::
  {:ok, pid()} | {:error, {:already_started, pid()}} | {:error, term()}

Starts any connection pooling or supervision and return {:ok, pid} or just :ok if nothing needs to be done.

Returns {:error, {:already_started, pid}} if the event store is already started or {:error, term} in case anything else goes wrong.