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Archive Gap

An archive gap is a range of archived redo logs created whenever the standby system in unable to receive the next archived redo log generated by the primary database. This might happen if the network connection is broken.
Data guard is able to detect archive gaps automatically and can fetch the missing logs as soon as the connection is re-established.
v$archive_gap can be used to determine if there are gaps on a physical standby database. Use dba_logstdby_log on physical standby databases.