Closing Dataguard Transfer and Apply Gaps
In the last week, I had two customers that had some failures with their standby databases and contacted me about closing their DG gaps. Since this kind of problems is common, and since the solutions are fairly easy I thought it worth a post to document this for their and your use.
Before we begin, let’s understand what dataguard gaps are. There are two types of gaps: transport and apply gaps. The transport gaps problem usually starts after a network disconnection between the primary and standby databases. At this time, the archive logs are not being shipped between the databases – and this is called a transport gap. The other case is the apply gap: in this case, we have all the files but the standby didn’t finish applying them all yet.
Our problem begins when during a transport gap, the archives we need to close the gap (in the primary) are removed. In this case the dataguard will not be able to continue rolling the log files and will hang. This is exactly what happen to my customers, and their question was what to do next – preferably, without rebuilding the entire standby database.