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It is often convenient to have the ability to react to state changes. All callbacks are subject to Orleans' turn-based guarantees; see also the section on Concurrency Guarantees.

Track confirmed state

To be notified of any changes to the confirmed state, JournaledGrain<TGrainState,TEventBase> subclasses can override this method:

protected override void OnStateChanged()
{
   // read state and/or event log and take appropriate action
}

OnStateChanged is called whenever the confirmed state is updated, i.e. the version number increases. This can happen when

  1. A newer version of the state was loaded from storage.
  2. An event that was raised by this instance has been successfully written to storage.
  3. A notification message was received from some other instance.

Note that since all grains initially have version zero, until the initial load from storage completes, this means that OnStateChanged() is called whenever the initial load completes with a version larger than zero.

Track tentative state

To be notified of any changes to the tentative state, JournaledGrain subclasses can override this method:

protected override void OnTentativeStateChanged()
{
   // read state and/or events and take appropriate action
}

OnTentativeStateChanged() is called whenever the tentative state changes, i.e. if the combined sequence (ConfirmedEvents + UnconfirmedEvents) changes. In particular, a callback to OnTentativeStateChanged() always happens during RaiseEvent.