Weblogic: Universal vs Source Address Persistence

Persistence only applies after the initial connection. In other words, the first time a client connects to the virtual server, their connection will be load balanced to an available pool member. That could be either the primary or the secondary server, depending on your other pool configuration settings. Upon connecting again, that same client would be directed to the same pool member they originally load balanced to, based on their IP address if using source address affinity persistence. But for a new client (meaning one whose IP address does not match an existing source address persistence record), load balancing is done first. If both the primary and the secondary are available, either could be chosen.

If the goal is to use only the primary server except if it is down, then I suggest using Priority Group Activation on the pool. Assuming there are only two pool members - the primary and the secondary - set the minimum number of available members to 1, and give the primary member a higher priority group number than the secondary (perhaps 10 for the primary and 5 for the secondary). Make sure you apply an appropriate monitor to both pool members so that LTM can detect if the primary becomes unavailable. If it does, then it will activate the secondary pool member - but ONLY if the primary is marked down, either by a monitor or by an administrator. You could also still use source address affinity persistence so that, in the event you manually disabled the primary (for example for maintenance), clients would still be allowed to persist to the primary until their session completed, rather than be switched to the secondary immediately. If you force the primary pool member offline, then all new traffic will go to the secondary regardless of whether they were persisting to the primary or not.