---------------------------------------------------------------------
| some process          |                server process             |
------------------------+--------------------------------------------
   [client]             |      [callback]     :        [behaviour]
                        |                     :
 callback:start >-------|---------------------:--> starting the process
                        |                     :           V
                        |                     :           |
                        |       init()  <-----:-----------`
                        |         |           :
                        |         `-----------:------> initial state
  {ok, Pid}  <----------|---------------------:----------,/
                        |                     :
 callback:store  >------|---------------------:--> handles message
 (calls the process)    |    (formats msg)    :           V
                        |                     :           |
                        |    handle_call() <--:-----------`
                        |         |           :
                        |          `----------:--> updates state, sends reply
                        |                     :        V
                        |                     :        |
   gets result <--------|---------------------:--------`
                        |                     :