Oracle RAC systems provide two methods of failover to provide reliable access to data:
If a connection failure occurs at connect time, the application can fail over the connection to another active node in the cluster. Connection failover ensures that an open route to your data is always available, even when server downtime occurs.
Transparent Application Failover (TAF).
If a communication link failure occurs after a connection is established, the connection fails over to another active node. Any disrupted transactions are rolled back, and session properties and server-side program variables are lost. In some cases, if the statement executing at the time of the failover is a Select statement, that statement may be automatically re-executed on the new connection with the cursor positioned on the row on which it was positioned prior to the failover.
The primary difference between connection failover and TAF is that the former method provides protection for connections at connect time and the latter method provides protection for connections that have already been established. Also, because the state of the transaction must be stored at all times, TAF requires more performance overhead than connection failover.
Enabling connection failover allows a driver to connect to another node if a connection attempt on one node fails. When an application requests a connection to an Oracle database server through the driver, the driver does not connect to the database server directly. Instead, the driver sends a connection request to a listener process, which forwards the request to the appropriate Oracle database instance. In an Oracle RAC system, each active Oracle database instance in the RAC system registers with each listener configured for the Oracle RAC.
Transparent Application Failover (TAF)
With TAF, if a communication link failure occurs after a connection is established, the connection is moved to another active Oracle RAC node in the cluster without the application having to re-establish the connection.
Oracle RAC systems provide two types of load balancing for automatic workload management:
Server load balancing distributes processing workload among Oracle RAC nodes.
Client load balancing distributes new connections among Oracle RAC nodes so that no one server is overwhelmed with connection requests. For example, when a connection fails over to another node because of hardware failure, client load balancing ensures that the redirected connection requests are distributed among the other nodes in the RAC.
The primary difference between these two methods is that the former method distributes processing and the latter method distributes connection attempts.
Server Load Balancing
With Oracle9i RAC systems, a listener service provides automatic load balancing across nodes. The query optimizer determines the optimal distribution of workload across the nodes in the RAC based on the number of processors and current load.
Oracle 10g also provides load-balancing options that allow the database administrator to configure rules for load balancing based on application requirements and Service Level Agreements (SLAs). For example, rules can be defined so that when Oracle 10g instances running critical services fail, the workload is automatically shifted to instances running less critical workloads. Or, rules can be defined so that Accounts Receivable services are given priority over Order Entry services.
The DataDirect Connect for ODBC Oracle drivers can transparently take advantage of server load balancing provided by an Oracle RAC without any changes to the application. If you do not want to use server load balancing, you can bypass it by connecting to the service name that identifies a particular RAC node.
Client Load Balancing
Client load balancing helps distribute new connections in your environment so that no one server is overwhelmed with connection requests. When client load balancing is enabled, connection attempts are made randomly among RAC nodes. You can enable connection failover for DataDirect Connect for ODBC drivers through a driver connection string using the Load Balancing connection string attribute.
Suppose you have the Oracle RAC environment shown in Figure 4 with multiple Oracle RAC nodes, A, B, C, and D. Without client load balancing enabled, connection attempts may be front-loaded, meaning that most connection attempts would try Node A first, then Node B, and so on until a connection attempt is successful. This creates a situation where Node A and Node B can become overloaded with connection requests.
With client load balancing enabled, the driver randomly selects the order of the connection attempts to nodes throughout the Oracle RAC system. For example, Node B may be tried first, followed by Nodes D, C, and A. Subsequent connection retry attempts will continue to use this order. Using a randomly determined order makes it less likely that any one node in the Oracle RAC system will be so overwhelmed with connection requests that it may start refusing connections.
For more details on Oracle RAC you can view on http://www.oracle.com/database/rac_home.html
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