Document ID:	    37802.1
Subject:            Snapshots and Triggers
Author:            TOSCROFT
Modified:           22 Apr 96 


SNAPSHOTS AND TRIGGERS
----------------------

Read-Only Snapshots:

    Triggers are not supported on read-only snapshots; we can only allow 
    updates to the snapshots to come from the refresh mechanism. 

    Oracle7 only guarantees that at the completion of the refresh the snapshot 
    data will be equal to that in the master table at the time of refresh. No 
    guarantee is made regarding the way in which these changes will be applied, 
    or the order in which they will be applied. It is possible that this method 
    will be changed between releases. 

    It is not possible, therefore, to predict how triggers will behave during a 
    snapshot refresh so they cannot be officially supported.

    If referential constraints or other business rules are necessary, then they 
    can be enforced at the master site; these will then be reflected at the 
    snapshot site when the refresh completes, providing all related tables are 
    refreshed as one refresh group.

Updatable Snapshots:

    It is possible to define triggers on updatable snapshots. To do this, 
    however, you need to ensure that these triggers are only fired when updates 
    are made by the local application, rather than when a refresh is performed. 
    The mechanism to ensure this is explained in the Oracle7 Server Distributed 
    Systems manual - 'Volume II: Replicated Data'. See the chapter entitled 
    'Advanced Techniques' (ch. 7 in Release 7.2, ch. 8 in Release 7.3).