Data Guard
Oracle Active Data Guard
Data Guard is Oracle’s technology for maintaining one or more synchronized copies (standby) of a production database (primary). The standby continuously receives and applies redo logs generated by the primary, staying aligned in real time or near-real time.
How it works #
The primary generates redo logs with every transaction. These logs are transmitted to the standby over the network, where they are applied in one of two ways:
- Physical standby: applies redo at block level (exact replica, byte for byte)
- Logical standby: reconstructs SQL statements from the redo and re-executes them
If the primary fails, the standby can become the new primary via switchover (planned) or failover (emergency).
Active Data Guard #
The Active Data Guard variant allows the standby to be opened in read-only mode while it continues applying redo. This enables using it for reports, backups and analytical queries, offloading the primary.
Protection modes #
| Mode | Behaviour | Data loss |
|---|---|---|
| MaxPerformance | Asynchronous replication, no impact on primary performance | Possible (a few seconds) |
| MaxAvailability | Synchronous replication, degrades to MaxPerformance if standby unreachable | Zero under normal conditions |
| MaxProtection | Synchronous replication, primary halts if standby doesn’t confirm | Guaranteed zero |