SupervisionTrees

This page explains the supervision trees for each of the nodewatch applications.

DxKit

This is the most complex supervision tree in the system, consisting of several sub-trees and using a variety of supervision models. The overall structure looks like this:

                                  x-------------------------x
               x------------------| dxkit_sup (one_for_one) |-----------------x  
               |                  x-------------------------x                 |
               |                        |                                     |
               |                        |                                     |
  x------------x----------x    x--------x------------x    x-------------------x------------------x
  | dxkit_event_subsystem |    | dxkit_net_subsystem |    | dxkit_monitor_subscription_subsystem |
  |     <<supervisor>>    |    |   <<supervisor>>    |    |            <<supervisor>>            |
  x-----------------------x    x---------------------x    x--------------------------------------x

The event subsystem comprises a locally registered gen_event manager and a simple_one_for_one supervisor which acts as a bridge between the event manager and the gen_event handler (callback modules), by wrapping the handler(s) in a gen_server. This handler bridge catches abnormal exits in the event handlers and stops, which in turn triggers the event_bridge to restart the handler. The branch supervisor uses rest_for_one so that the event bridge will restart independently, but a crash in the event handler will force the bridge to also restart - a necessary step if we want to re-register the gen_event handlers with the new manager instance.

                                 x-----------------------x
                                 | dxkit_event_subsystem |
                     x-----------|     <<supervisor>>    |------------x
                     |           x-----------------------x            |
                     |                                                |
         x-----------x---------x                           x----------x---------x
         | dxkit_event_handler |                           | dxkit_event_bridge |
         |  <<gen_event/mgr>>  |                           |   <<supervisor>>   | 
         x---------------------x                           x----------x---------x
                                                                      v
                                                                      v
                                                                      v
                                                           x----------V-----------x
                                                           | event handler bridge |
                                                           |    <<gen_server>>    |
                                                           x----------------------x

TBC.