Oracle DataGuard supports logical and physical standby databases.
Logical Standby: When primary database generates any redo entries it is transferred to standby database and then redo data are converted into SQL statements and then those SQL statements are applied to standby database.
To read more about Physical Standby Database
A thing that got clear to us till now is, Physical Standby works on the redo, while Logical Standby have an extra layer of conversion and it converts redo to SQL statements and those SQL statements then applied to Logical Standby Database. That is called log mining process.
Now, what happens when Redo to SQL conversion takes place.
We can see Six processes for Logical Standby in Above Image.It is categorized in two parts. Log Mining Process and SQL apply process.
1. Log Mining Process
- Reader Process: It reads redo records from the archived redo log files or standby redo log files.
- Preparer Process: This converts block changes contained in redo records into logical change records (LCR). Multiple PREPARER processes can be active for a given redo log file. The LCRs are kept in the system global area (SGA), known as the LCR cache.
- Builder Process: This process groups LCRs into transactions, and performs other tasks, such as memory management in the LCR cache, checkpointing related to SQL Apply, restart and filtering out of uninteresting changes.
2. Apply Process
- Analyzer Process: This process identifies dependencies between different transactions.
- Coordinator Process: This process assigns transactions to different appliers and coordinates among them to ensure that dependencies between transactions are honored. It is also known as LSP process. Main Process in the logical standby database.
- Applier Process: This process applies transactions to the logical standby database under the supervision of the coordinator process.
Stay tuned for More articles on Oracle DataGuard
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