When it comes to limiting data modifications in Oracle, two options are available: Read-Only Instances and Read-Only Databases. While both options restrict data changes, they serve different purposes and are used in distinct contexts.

Read-Only Instances:

A Read-Only Instance is a configuration in Oracle Real Application Clusters (RAC) where one or more instances are set to operate in read-only mode. This setup is ideal for environments with high concurrency for both read and write operations.

Key features of Read-Only Instances include:

• Real-time query scaling by dedicating specific instances to read-only operations
• Write operations are not allowed on designated read-only instances, but other instances can still handle writes
• Useful for load balancing in RAC configurations
• Read-Only Instances are suitable for offloading read-heavy workloads in RAC environments and supporting real-time analytics without impacting primary write performance.

Read-Only Databases

A Read-Only Database, on the other hand, is a database-wide mode that restricts all write operations. This setup is typically used for archiving, reporting, or maintenance tasks. 

Key features of Read-Only Databases include:

• The entire database is locked for write operations
• Used for archiving, reporting, or maintenance tasks
• Can be achieved using the ALTER DATABASE OPEN READ ONLY command or a Data Guard physical standby database
• Read-Only Databases are ideal for archiving purposes, maintenance periods, or using a standby database for reporting.

Choosing the Right Option:

When deciding between Read-Only Instances and Read-Only Databases, consider the following:

• If you have a RAC environment and need to offload read-heavy workloads, Read-Only Instances might be the better choice.
• If you need to restrict, write operations across the entire database, a Read-Only Database is the way to go.

Ultimately, understanding the differences between Read-Only Instances and Read-Only Databases will help you make informed decisions about managing your Oracle database.

Hope this helps.