Replication :: Initial Sync Of CDC Subscriber

I'm trying to work out how to synchronize a source and target database in an Oracle CDC implementation. We're in the architecture design stage of a near-real-time Operational Data Store style solution. Both the ODS and our pilot source system will be Oracle 10g. Our plan is to use Oracle Asynch Hotlog Change Data Capture to capture change data in near-real-time so that it can be applied to the ODS.

I understand the CDC apply process once the ODS and Source System are synchronized: DBMS_CDC_SUBSCRIBE.EXTEND_WINDOW to release the next window of change data, select from the publish views, then DBMS_CDC_SUBSCRIBE.PURGE_WINDOW to register the change data is no longer required.

But how do we do the initial synchronization if the source system is live and contains data and the ODS is new (and empty)? The easiest way would be to somehow flag EVERY row as change data. eg. Truncate every table and import. This would not be so good for existing CDC subscribers.

A more logical way would be to:

- Take a hot backup of the live prod database
- Activate CDC (DBMS_CDC_SUBSCRIBE.ACTIVATE_SUBSCRIPTION) on the source system to start tracking changes
- Manually build the ODS from the snapshot
- Start applying changes from CDC

if the source system is live, how can we GUARANTEE that the first 2 steps (snapshot and ACTIVATE_SUBSCRIPTION) are performed at EXACTLY the same time (ie. same SCN)?