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.
- Setting the database in
- 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- Archiving purposes where the database is no longer being updated.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.
- A physical standby database opened in
Key Differences
Aspect | Read-Only Instance | Read-Only Database |
---|---|---|
Scope | Instance-level in a RAC environment | Entire database |
Write Operations | Allowed on other instances in the RAC | Not allowed across all instances |
Use Case | Query offloading in RAC | Reporting, archiving, or maintenance |
Configuration | Set specific instances in read-only mode | Open database in READ ONLY mode |
Flexibility | Other instances can handle writes | No writes allowed, even from privileged users |
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