Oracle Data Guard Concepts

Oracle Data Guard Concepts: during this article we are going to discuss about Oracle Data Guard Concepts.

The main features of Oracle data guard is

  • High availablity
  • Data protection
  • Disaster Recovery

A Data guard configuration consists of 1 primary database and one or more standby database. The standby databases are always in sync with primary database. If primary production database is unavailable because of planned or unplanned outage , then we will switch the assembly to standby database, making the appliance less littered with the outage.

But we will use the standby database for taking backups and reporting queries, there by reducing some workload from production.

There are 3 types of standby setup:

  • Physical standby
  • Logical standby
  • Snapshot standby


A physical standby database is an actual, block-for-block copy of a primary database. A physical standby is
maintained as a precise copy through a process called REDO APPLY, during which redo data received from a
primary database is continuously applied to a physical standby database using the database recovery mechanisms. So it’ll be always in sync with primary.

This Standby database will be opened in read only mode( knows as ACTIVE DATA GUARD), for reporting purpose. Most of the companies use physical standby for dataguard configuration.


The logical standby database is kept synchronized with the first database through SQL APPLY, which transforms the info within the redo received from the primary database into SQL statements then executes the SQL statements on the standby database. So it contains same logical information as that of production , but body of information is different.


A snapshot standby database is a fully update-able standby database that is created by converting a physical standby database into a snapshot standby database. During this setup though it will receive logs from primary, But those redo won’t be applied to snapshot standby. Redo data received from the primary database is applied when a snapshot standby database is converted back into a physical standby database, after discarding all local updates to the snapshot standby database.

It provides an exact replica of a production database for development and testing purposes, while maintaining data protection at all times.


It can have 3 protection modes:


During this mode provides maximum protection. It guarantees zero data loss. The redo/transaction data must be written to both primary redo log and standby redo log. For any reason(mostly N/W issue) if it is unable to write to standby, Then primary will get shutdown.


It provides the highest level of data protection that’s possible without affecting the availability of the primary database. Transactions aren’t allowed to commit until all redo data are written to the online redo logs and propagated to a minimum of one synchronized secondary database. If for any reason, the primary database cannot propagate its redo stream to at least one secondary database, the primary won’t shutdown and operates because it were in maximum performance mode until issues are fixed.


This is often the default protection mode. With this protection mode, a transaction is committed as soon as the redo data needed to recover the transaction is written to the local (online) redo log.

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