27 Fast Connection Failover

Fast Connection Failover Prerequisites

Fast Connection Failover is available under the following circumstances:

• The implicit connection cache is enabled.

  Fast Connection Failover works in conjunction with the JDBC connection caching mechanism. This helps applications manage connections to ensure high availability.

• The application uses service names to connect to the database.

  The application cannot use service identifiers.

• The underlying database has Oracle Database 11g Real Application Clusters (Oracle RAC) capability or Oracle Data Guard configured with either single instance Databases or Oracle RAC.

  If failover events are not propagated, then connection failover cannot occur.

• Oracle Notification Service (ONS) is configured and available on the node where JDBC is running.

  JDBC depends on ONS to propagate database events and notify JDBC of them.

• The Java Virtual Machine (JVM) in which your JDBC instance is running must have oracle.ons.oraclehome set to point to your ORACLE_HOME.

Configuring ONS for Fast Connection Failover

In order for Fast Connection Failover to work, you must configure ONS correctly. ONS is shipped as part of Oracle Database 11g. For more information, refer to "Configuration of ONS".

setONSConfiguration(String remoteONSConfig)

ods.setONSConfiguration("nodes=racnode1.example.com:4200,racnode2.example.com:4200");

ods.setONSConfiguration("nodes=racnode1:4200,racnode2:4200\nwalletfile=/mydir/Wallet\nwalletpassword=mypasswd");

ods.setONSConfiguration("nodes=racnode1:4200,racnode2:4200\nwalletfile=/mydir/conf/Wallet");

Enabling Fast Connection Failover

An application enables Fast Connection Failover by calling setFastConnectionFailoverEnabled(true) on a DataSource instance, before retrieving any connections from that instance.

You cannot enable Fast Connection Failover when reinitializing a connection cache. You must enable it before using the OracleDataSource instance.

Example 27-1 illustrates how to enable Fast Connection Failover.

To enable Fast Connection Failover, you must perform the following:

• Configure and start ONS. If ONS is not correctly set up, then implicit connection cache creation fails and an ONSException is thrown at the first getConnection request.

• Set the FastConnectionFailoverEnabled property before making the first getConnection request to an OracleDataSource. When Fast Connection Failover is enabled, the failover applies to all connections in the connection cache. If your application explicitly creates a connection cache using the Connection Cache Manager, then you must first set FastConnectionFailoverEnabled before retrieving any connections.

• Use a service name rather than a service identifier when setting the OracleDataSource url property.

// declare datasource
ods.setUrl(
"jdbc:oracle:oci:@(DESCRIPTION=
  (ADDRESS=(PROTOCOL=TCP)(HOST=cluster_alias)
    (PORT=1521))
    (CONNECT_DATA=(SERVICE_NAME=service_name)))");
ods.setUser("scott");
ods.setConnectionCachingEnabled(true);
ods.setFastConnectionFailoverEnabled(true):
ctx.bind("myDS",ods);
ds=(OracleDataSource) ctx.lookup("MyDS");
try {
 ds.getConnection();  // transparently creates and accesses cache
 catch (SQLException SE {
  }
}
...

Querying Fast Connection Failover Status

An application determines if Fast Connection Failover is enabled by calling OracleDataSource.getFastConnectionFailoverEnabled, which returns true if failover is enabled, false otherwise.

What the Application Sees

By the time an Oracle RAC service failure is propagated to the JDBC application, the database already rolls back the local transaction. The cache manager then cleans up all invalid connections. When an application holding an invalid connection tries to do work through that connection, it is possible to receive SQLException, ORA-17008, Closed Connection.

When an application receives a Closed Connection error message, it should do the following:

1. Retry the connection request. This is essential, because the old connection is no longer open.

2. Replay the transaction. All work done before the connection was closed has been lost.

How It Works

Under Fast Connection Failover, each connection in the cache maintains a mapping to a service, instance, database, and host name.

When a database generates an Oracle RAC event, that event is forwarded to the JVM in which JDBC is running. A daemon thread inside the JVM receives the Oracle RAC event and passes it on to the Connection Cache Manager. The Connection Cache Manager then throws SQL exceptions to the applications affected by the Oracle RAC event.

A typical failover scenario may work like the following:

1. A database instance fails, leaving several stale connections in the cache.

2. The Oracle RAC mechanism in the database generates an Oracle RAC event which is sent to the JVM containing JDBC.

3. The daemon thread inside the JVM finds all the connections affected by the Oracle RAC event, notifies them of the closed connection through SQL exceptions, and rolls back any open transactions.

4. Each individual connection receives a SQL exception and must retry.