INITIAL - INITIAL indicates the associated notification is that of the current state and a fully-specified Resource is provided.
INITIAL_SYNC_COMPLETE - INITIAL_SYNC_COMPLETE indicates all existing-state has been streamed to the client. This status will be sent in an otherwise-empty message and no subsequent INITIAL messages should be expected.
UPDATED - UPDATED indicates the associated notification carries modification to the last-streamed state. This indicates the contained Resource may be a partial diff, though, it may contain a fully-specified Resource.
DELETED - DETLETED indicates the associated notification carries a deletion. The Resource's key will always be set in this case, but no other fields should be expected.