You can configure synchronization updates to send only the necessary operations to another replica. Although sending an operation multiple times does no harm, packet creation and transmission is more efficient if you exclude operations that have already been imported at the receiving replica.
The chepoch –actual and sync_export_list –update commands contact a remote replica and update your current replica’s record of the state of the remote replica. The primary use of these commands is to resend lost packets, but you can also use them to increase synchronization efficiency. However, depending on your synchronization pattern and schedule, these commands can decrease efficiency. The following sections describe two examples: one in which efficiency is increased, and one in which it is decreased.
You have three replicas in a VOB family, and a subset of the synchronization pattern and schedule is shown in Figure 19. All replicas use receipt handlers, so incoming packets are imported immediately. First, replica sanfran_hub sends a packet to replica boston_hub. Next, boston_hub sends a packet to bangalore. This packet includes operations from sanfran_hub.
At 8:00 GMT, sanfran_hub sends a packet to bangalore. This packet contains operations originating at sanfran_hub that bangalore has already received from boston_hub. In this case, use the command chepoch –actual bangalore at sanfran_hub before generating an update packet for bangalore. When you generate the packet, the operations already imported at bangalore are excluded from the packet.
In this example, two replicas in a VOB family, sanfran_hub and sydney, exchange update packets every 15 minutes. At some point during the day, packets may start accumulating at one of the replicas because the imports are taking a long time. For example, there is a lot of development activity in the sydney VOB, and four packets are waiting to be imported.
In this case, if you run chepoch –actual at sanfran_hub before generating a packet for sydney, the update packet will contain all the operations that are already in the packets waiting to be imported at sydney. You can use the chepoch –actual –raise_only command to avoid sending operations multiple times. With –raise_only, chepoch does not lower epoch numbers for sibling replicas.