You're seeing just the callback code_change, go back to SMPPEX.Session module for more information.
Link to this callback

code_change(old_vsn, state, extra)

View Source


code_change(old_vsn :: term() | {:down, term()}, state(), extra :: term()) ::
  {:ok, state()} | {:error, reason()}

Invoked to change the state of the session when a different version of a module is loaded (hot code swapping) and the state’s term structure should be changed. The method has the same semantics as the original GenServer.code_change/3 callback.