Oracle RAC Load Balancing

Relational databases play a central role in many software systems. In many enterprises, Oracle is the RDBMS of choice. Among those, many mission-critical systems make use of Oracle's Real Application Cluster (RAC), a load-balanced multi-node database that can handle failures of individual nodes without causing an outage. This article provides a decent summary of RAC.

While RAC has many features and advantages (as well as trade-offs), that isn't why this post is here. It is here because the way in which the Java ODBC driver establishes a connection to the RAC is worth understanding in the event that one suspects a connectivity issue between the client code and the database server.

For a Java developer, using RAC is very similar to any other Oracle database. Usually, the only indication that you're using a RAC is the contents of your connect string. It typically lists a number of database server addresses in an ADDRESS_LIST element. For the purposes of illustrating the discussion, an example may be helpful:

jdbc:oracle:thin:@(DESCRIPTION =(ADDRESS_LIST =(ADDRESS = (PROTOCOL = TCP)(HOST = dbserver1.somedomain.com)(PORT = 12345))(ADDRESS = (PROTOCOL = TCP)(HOST = dbserver2.somedomain.com)(PORT = 12345))(LOAD_BALANCE = yes)) (CONNECT_DATA = (SERVER = DEDICATED) (SERVICE_NAME = dbServiceName.somedomain.com) ) )

There are two types of load balancing offered by the RAC: client-based load balancing and server-based load balancing. The system can be configured to use one, both or neither forms of load balancing.

The LOAD_BALANCE element in the connect string above specifies that the client code should perform its own load balancing. Essentially, this means that the client will randomly choose one of the servers from the ADDRESS_LIST whenever a connection is requested.

If server-based load balancing is enabled, a listener service provides automatic load distribution across all nodes in the RAC. The listener will use the query optimizer to determine which node in the RAC should service the request (based on their current workload, machine profiles and, in newer versions of Oracle, admin-specified rules).

If one were to use the connect string above and server-based load balancing were enabled, the sequence of events that would occur when a call to DriverManager.getConnection(...) is invoked are:

• The driver selects dbserver2.somedomain.com (this choice was made at random by the driver) and issues a connection request.
  
• The listener on dbserver2.somedomain.com determines that the most under taxed node in the RAC is dbserver3.somedomain.com and returns it to the client. NOTE: this assumes that the RAC has 3 nodes. It is important to note that all the nodes do NOT need to be included in the connect string... the nodes in the ADDRESS_LIST simply indicate which nodes the client will balance its initial "getConnection" requests against
  
• The client will attempt to establish a connection to dbserver3.somedomain.com.