In Oracle Database, what is Real Application Clusters (RAC) and how is it different from a non RAC database configuration?

Introduction / Context:
Oracle Real Application Clusters, or RAC, is a high availability and scalability feature that allows several Oracle instances to run on separate servers while accessing a shared database. It is widely used in mission critical environments where continuous availability and horizontal scalability are important. This question assesses whether you understand the core difference between RAC and a traditional non RAC single instance database configuration.

Given Data / Assumptions:

• We are comparing Oracle RAC with a non RAC configuration.
• In both cases, the data is stored in Oracle data files.
• RAC uses multiple instances on separate nodes.
• A non RAC configuration uses a single instance on a single node.

Concept / Approach:
In a non RAC database, one Oracle instance, consisting of memory structures and background processes, mounts and opens a database that resides on disk. All user sessions connect to this single instance. In RAC, several instances on different servers mount and open the same database files on shared storage. Oracle Clusterware coordinates access and ensures cache coherency among the instances. Users can connect to any instance in the cluster, which improves scalability and provides instance level failover. A correct answer must mention multiple instances, shared storage, and the contrast with a single instance configuration.

Step-by-Step Solution:
Step 1: Recall that RAC involves more than one instance running on separate nodes, all accessing the same database files. Step 2: Recall that a non RAC database configuration uses a single instance running on one server. Step 3: Examine Option A, which states that RAC allows multiple database instances running on different servers to access the same shared database files, while a non RAC database has a single instance accessing the data files. This matches the textbook definition. Step 4: Examine Options B, C, D, and E, which make incorrect or misleading statements about browsers, in memory storage only, or report versus transaction usage. Step 5: Conclude that Option A is the only accurate description of RAC and its difference from non RAC.

Verification / Alternative check:
If you look at RAC architecture diagrams, you will see multiple nodes, each with an Oracle instance, connected to shared storage where the database files reside. Clusterware manages instance membership, and Global Cache Service coordinates data blocks in the buffer caches. In documentation for non RAC databases, only one instance is shown. There is no suggestion that RAC is a browser library or that it stores data only in memory, which confirms that the other options are incorrect.

Why Other Options Are Wrong:

• Option B is wrong because RAC is a server side cluster technology, not a browser library.
• Option C is wrong because both RAC and non RAC databases store data on disk; memory is used for caching only.
• Option D is wrong because both types of configurations can handle transactional and reporting workloads.
• Option E is wrong because Oracle explicitly documents architectural differences between RAC and single instance databases.

Common Pitfalls:
A common pitfall is to think that RAC alone solves all performance problems. Poorly written SQL and bad logical design can still cause issues even in a RAC environment. Another misconception is that RAC is the same as data guard; in reality, data guard provides disaster recovery by replicating data to standby databases, while RAC provides clustering for a single database. Understanding these distinctions is important for designing high availability solutions.

Final Answer:
The correct explanation is Option A: RAC allows multiple database instances running on different servers to access the same shared database files, while a non RAC database has a single instance accessing the data files.