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How does HADR work?


Let explain this way. 
  1. HADR is based on log replay. 
  2. Initialize the standby with a backup or split mirror image of the primary. 
  3. You then configure and start HADR on the primary and standby databases. 
  4. The primary ships its transaction log data to the standby via a TCP connection. 
  5. The standby continuously replays the log records to keep itself in sync with the primary.
If the primary fails, you issue a "takeover HADR by force" command on the standby to make the standby the new primary. A single command does it all. For planned maintenance, you issue a "takeover HADR" (without "by force" option) command to switch primary and standby's roles. HADR management is so simple. There are only 3 commands: Start HADR, stop HADR, and takeover HADR.
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