28 Transparent Application Failover

Overview of Transparent Application Failover

Transparent Application Failover (TAF) is a feature of the Java Database Connectivity (JDBC) Oracle Call Interface (OCI) driver. It enables the application to automatically reconnect to a database, if the database instance to which the connection is made fails. In this case, the active transactions roll back.

When an instance to which a connection is established fails or is shut down, the connection on the client-side becomes stale and would throw exceptions to the caller trying to use it. TAF enables the application to transparently reconnect to a preconfigured secondary instance, creating a fresh connection, but identical to the connection that was established on the first original instance. That is, the connection properties are the same as that of the earlier connection. This is true regardless of how the connection was lost.

Failover Type Events

The following are possible failover events in the OracleOCIFailover interface:

• FO_SESSION

  Is equivalent to FAILOVER_MODE=SESSION in the tnsnames.ora file CONNECT_DATA flags. This means that only the user session is authenticated again on the server side, while open cursors in the OCI application need to be reprocessed.

• FO_SELECT

  Is equivalent to FAILOVER_MODE=SELECT in tnsnames.ora file CONNECT_DATA flags. This means that not only the user session is re-authenticated on the server side, but open cursors in the OCI can continue fetching. This implies that the client-side logic maintains fetch-state of each open cursor.

• FO_NONE

  Is equivalent to FAILOVER_MODE=NONE in the tnsnames.ora file CONNECT_DATA flags. This is the default, in which no failover functionality is used. This can also be explicitly specified to prevent failover from happening. Additionally, FO_TYPE_UNKNOWN implies that a bad failover type was returned from the OCI driver.

• FO_BEGIN

  Indicates that failover has detected a lost connection and failover is starting.

• FO_END

  Indicates successful completion of failover.

• FO_ABORT

  Indicates that failover was unsuccessful and there is no option of retrying.

• FO_REAUTH

  Indicates that a user handle has been re-authenticated.

• FO_ERROR

  Indicates that failover was temporarily unsuccessful, but it gives the application the opportunity to handle the error and retry failover. The usual method of error handling is to issue the sleep method and retry by returning the value FO_RETRY.

• FO_RETRY

  Indicates that the application should retry failover.

• FO_EVENT_UNKNOWN

  Indicates a bad failover event.

TAF Callbacks

TAF callbacks are used in the event of the failure of one database connection, and failover to another database connection. TAF callbacks are callbacks that are registered in case of failover. The callback is called during the failover to notify the JDBC application of events generated. The application also has some control of failover.

Java TAF Callback Interface

The OracleOCIFailover interface includes the callbackFn method, supporting the following types and events:

public interface OracleOCIFailover{

// Possible Failover Types
public static final int FO_SESSION = 1;
public static final int FO_SELECT  = 2;
public static final int FO_NONE  = 3;
public static final int;

// Possible Failover events registered with callback
public static final int FO_BEGIN   = 1;
public static final int FO_END     = 2;
public static final int FO_ABORT   = 3;
public static final int FO_REAUTH  = 4;
public static final int FO_ERROR  = 5;
public static final int FO_RETRY  = 6;
public static final int FO_EVENT_UNKNOWN = 7;

public int callbackFn (Connection conn,
                       Object ctxt, // ANy thing the user wants to save
                       int type, // One of the possible Failover Types
                       int event ); // One of the possible Failover Events

Handling the FO_ERROR Event

In case of an error while failing over to a new connection, the JDBC application is able to retry failover. Typically, the application sleeps for a while and then it retries, either indefinitely or for a limited amount of time, by having the callback return FO_RETRY.

Handling the FO_ABORT Event

Callback registered should return the FO_ABORT event if the FO_ERROR event is passed to it.