Dataguard's Log Gap detection and processing principles

When talking about the Gap Processing Mechanism of dataguard, the measure most people can say is to set fal_server and fal_client. From 9i, Oracle provides two types of log gap detection and processing mechanisms. For gap processing, the fal _ * parameter is not required in some cases.

1. Automatic Gap Resolution
2. FAL Gap Resolution

1. Automatic Gap Resolution

Starting from 9i, Dataguard introduces an Automatic Log missing detection mechanism. Datguard runs under this mechanism without any fal _ * parameter.
When the Lgwr and Arch processes send redo/archive to the standby end, the current log sequence is compared with the log sequence received by the RFS process on the standby end. If the two are found to have a broken file, RFS sends a request to the primary end, requiring the master database to send the missing log. The Automatic gap resolution function has been enhanced since 9iR2. The ARCH process on the master database checks the log gap in the slave database every minute and processes it accordingly.

2. FAL Gap Resolution

FAL is short for Fetch Archive Log. It is a mechanism for implementing Gap detection by configuring FALserver and FALclient. When the RFS process on the standby end receives
When archivelog is used, the standby control file is updated to record the archive information. Once the MRP detects that the control file is updated, the Recover/Apply log is performed. If MRP finds that the required logs are missing or the required log files are unavailable (damaged or physically removed), FAL will be used to send corresponding processing requests. MRP is the recovery process on the standby side. Unlike the RFS process, MRP is directly associated with parimary and actively requests primary to process the gap through FAL parameter configuration.

FAL_Server and fal_client are parameter configurations on the standby side. Considering the smoothness of switchover, you can also set them on the primary side.
FAL_SERVER: Oracle Net service pointing to the primary end
FAL_CLIENTL: Oracle Net service pointing to standby

In Versions later than 9iR2, Oracle first tries to use FAL Gap Resolution for GAP processing. When it finds that the FAL mechanism does not take effect,
Then try to use Automatic Gap Resolution for processing.

For some cascade javasuard architectures, FAL Gap Resolution is a better method of gap processing. In addition, the Automatic gap resolution
In some versions of dg environments, there are bugs (such as bug 5929647) and you have to configure the FAL parameter.