Archive Gap: It means set of Archive Logs are not transmitted from Primary Database to Standby Database for some reason. The main reason could be your Network Connectivity. When network connectivity resumes data guard resumes redo data transmission from the Primary to Standby site.

Causes of Archive Gaps :

  1. Network Disconnection.
  2. Standby database outage.
  3. I/O issues at the standby database.
  4. Insufficient Bandwith in the Network between the Primary and Standby Site

Oracle Data Guard has two mechanisms for Gap Detection and  Resolution.

  1. Automatic Gap Resolution
  2. FAL configuration

Automatic Gap Resolution :

We don’t need any extra configuration for this. Let us understand this with an example.

To simulate this situation, I have stopped listener at the standby database.

Step 1 : Stop listener at standby database.

Step 2: Check archive log sequence on both sides.

Primary :

Standby :

Step 3: Generate archive logs using “ALTER SYSTEM SWITCH LOGFILE” command.

Step 4: We can check error at the alert log that listener is not up at standby side so, there is trouble creating archive logs at the standby site.

We can check error: “Heartbeat failed to connect to standby ‘std_testdb”.

Step 5: Start listener at Standby :

Step 6: Check alert log of standby database  :

We can see here Old archive logs are automatically transmitted to Standby Database.

FAL Configuration :

Step 1: Configure FAL_SERVER:

Note: FAL_SERVER accepts the name that is associated in tnsnames.ora entry.

Step 2: Generate archive logs in primary using alter system switch logfile.

Step 3: Check maximum applied log at the standby database.

Step 4: To simulate this situation manually delete archive logs from standby.

Step 5: Start recovery

Step 6: Check alert log file.

We can see here Error of deleted file: Error opening /u01/arc/testdb/stdby/1_76_973537980.arc and then it is fetched by FAL server – Fetching gap sequence in thread 1, gap sequence 76-76. So it is automatically detected and resolved by FAL.

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