Key points on Read-Only Instance and Read-Only Database

 In Oracle, both Read-Only Instance and Read-Only Database provide mechanisms to limit data modifications, but they are used in different contexts and for different purposes. Here's a comparison:

1. Read-Only Instance

  • Definition: A read-only instance is a configuration in Oracle Real Application Clusters (RAC) where one or more instances of a multi-instance database are set up to operate in read-only mode. This is typically done in environments with high concurrency for both read and write operations.

  • Key Features:

    • Allows real-time query scaling by dedicating specific instances to read-only operations.
    • Write operations are not allowed on the designated read-only instance, but other instances in the cluster can still handle write operations.
    • Useful in RAC configurations for load balancing, where read workloads can be distributed across multiple read-only instances.
    • Instance-specific: The database remains fully functional (read-write) when accessed through other instances.

  • Use Cases:

    • Offloading read-heavy workloads in RAC environments.
    • Supporting real-time analytics without impacting primary write performance.

  • Example:

    • In a 4-instance RAC database, configure Instance 3 and Instance 4 as read-only while Instance 1 and Instance 2 handle regular read-write operations.

2. Read-Only Database

  • Definition: A read-only database is a database-wide mode where the entire database is placed in a state that only allows query and read operations. No DML (INSERT, UPDATE, DELETE) or DDL (ALTER, DROP, etc.) can be performed on the database.

  • Key Features:

    • The entire database is locked for write operations.
    • Typically used for archiving, reporting, or maintenance tasks to ensure the database's data remains consistent and unaltered.
    • Can be achieved using:
      • Setting the database in READ ONLY mode (ALTER DATABASE OPEN READ ONLY).
      • Using a Data Guard physical standby database configured in read-only mode for queries.
    • Not instance-specific: All users across all instances can only perform read operations.

  • Use Cases:

    • Archiving purposes where the database is no longer being updated.
    • Maintenance periods to prevent accidental data modifications.
    • Using a standby database in READ ONLY mode for reporting.

  • Example:

    • A physical standby database opened in READ ONLY mode to run analytics queries without affecting the primary database.

Key Differences

AspectRead-Only InstanceRead-Only Database
ScopeInstance-level in a RAC environmentEntire database
Write OperationsAllowed on other instances in the RACNot allowed across all instances
Use CaseQuery offloading in RACReporting, archiving, or maintenance
ConfigurationSet specific instances in read-only modeOpen database in READ ONLY mode
FlexibilityOther instances can handle writesNo writes allowed, even from privileged users

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