Hi, no, not at the MDS level. If we allowed nested MDSes, it would be easy, but we don't.
Because of your especially easy requirements, you can try it at the JDBC driver URL.
If it is possible to define your URLs so they connect to your primary site if available,
but connect to the standby site if not, that might work for you.
There are 2 ways to connect WLS to a Oracle RAC.
1. Use the Oracle RAC service URL which contains the details of all the RAC nodes and the respective IP address and DNS.
2. Connect to the primary cluster as you are currently doing and use a MDS to load-balance/failover between multiple nodes in the primary RAC (if applicable).
In case of a primary RAC nodes failure and switch to DR RAC nodes, use WLST scripts to change the connection URL and restart the application to remove any old connections.
Such DB fail-over tests can be conducted in a test/reference environment to set up the required log monitoring and subsequent steps to measure the timelines.