30 Distributed Transactions

Distributed Transaction Components and Scenarios

In reading the remainder of the distributed transactions section, it will be helpful to keep the following points in mind:

• A distributed transaction system typically relies on an external transaction manager, such as a software component that implements standard JTA functionality, to coordinate the individual transactions.

  Many vendors offer XA-compliant JTA modules, including Oracle, which includes JTA in Oracle9i Application Server and Oracle Application Server 10g.

• XA functionality is usually isolated from a client application, being implemented instead in a middle-tier environment, such as an application server.

  In many scenarios, the application server and transaction manager will be together on the middle tier, possibly together with some of the application code as well.

• Discussion throughout this section is intended mostly for middle-tier developers.

• The term resource manager is often used in discussing distributed transactions. A resource manager is simply an entity that manages data or some other kind of resource. Wherever the term is used in this chapter, it refers to a database.

  Note:

  Using JTA functionality requires jta.jar to be in the CLASSPATH environment variable. This file is located at ORACLE_HOME/jlib. Oracle includes this file with the JDBC product.

Distributed Transaction Concepts

When you use XA functionality, the transaction manager uses XA resource instances to prepare and coordinate each transaction branch and then to commit or roll back all transaction branches appropriately.

XA functionality includes the following key components:

• XA data sources

  These are extensions of connection pool data sources and other data sources, and similar in concept and functionality.

  There will be one XA data source instance for each resource manager that will be used in the distributed transaction. You will typically create XA data source instances in your middle-tier software.

  XA data sources produce XA connections.

• XA connections

  These are extensions of pooled connections and similar in concept and functionality. An XA connection encapsulates a physical database connection. Individual connection instances are temporary handles to these physical connections.

  An XA connection instance corresponds to a single Oracle session, although the session can be used in sequence by multiple logical connection instances, as with pooled connection instances.

  You will typically get an XA connection instance from an XA data source instance in your middle-tier software. You can get multiple XA connection instances from a single XA data source instance if the distributed transaction will involve multiple sessions in the same database.

  XA connections produce OracleXAResource instances and JDBC connection instances.

• XA resources

  These are used by a transaction manager in coordinating the transaction branches of a distributed transaction.

  You will get one OracleXAResource instance from each XA connection instance, typically in your middle-tier software. There is a one-to-one correlation between OracleXAResource instances and XA connection instances. Equivalently, there is a one-to-one correlation between OracleXAResource instances and Oracle sessions.

  In a typical scenario, the middle-tier component will hand off OracleXAResource instances to the transaction manager, for use in coordinating distributed transactions.

  Each OracleXAResource instance corresponds to a single Oracle session. So, there can be only a single active transaction branch associated with an OracleXAResource instance at any given time. However, there can be additional suspended transaction branches.

  Each OracleXAResource instance has the functionality to start, end, prepare, commit, or roll back the operations of the transaction branch running in the session with which the OracleXAResource instance is associated.

  The prepare step is the first step of a two-phase commit operation. The transaction manager will issue a PREPARE to each OracleXAResource instance. Once the transaction manager sees that the operations of each transaction branch have prepared successfully, it will issue a COMMIT to each OracleXAResource instance to commit all the changes.

• Transaction IDs

  These are used to identify transaction branches. Each ID includes a transaction branch ID component and a distributed transaction ID component. This is how a branch is associated with a distributed transaction. All OracleXAResource instances associated with a given distributed transaction would have a transaction ID that includes the same distributed transaction ID component.

• OracleXAResource.ORATRANSLOOSE

  Start a loosely coupled transaction with transaction ID xid.

Switching Between Global and Local Transactions

Applications can share connections between local and global transactions. Applications can also switch connections between local transactions and global transactions.

A connection is always in one of the following modes:

• NO_TXN

  No transaction is actively using this connection.

• LOCAL_TXN

  A local transaction with auto-commit turned off or disabled is actively using this connection.

• GLOBAL_TXN

  A global transaction is actively using this connection.

Each connection switches automatically between these modes depending on the operations carried out on the connection. A connection is always in NO_TXN mode when it is instantiated.

Table 30-1 describes the connection mode transition rules.

If none of these rules is applicable, then the mode does not change.

Mode Restrictions on Operations

The current connection mode restricts which operations are valid within a transaction.

• In the LOCAL_TXN mode, applications must not call prepare, commit, rollback, forget, or end on an XAResource. Doing so causes an XAException to be thrown.

• In the GLOBAL_TXN mode, applications must not call commit, rollback, rollback(Savepoint), setAutoCommit(true), or setSavepoint on a java.sql.Connection, and must not call OracleSetSavepoint or oracleRollback on an oracle.jdbc.OracleConnection. Doing so causes a SQLException to be thrown.

  Note:

  This mode-restriction error checking is in addition to the standard error checking on the transaction and savepoint APIs.

Oracle XA Packages

Oracle supplies the following three packages that have classes to implement distributed transaction functionality according to the XA standard:

• oracle.jdbc.xa

• oracle.jdbc.xa.client

• oracle.jdbc.xa.server

Classes for XA data sources, XA connections, and XA resources are in both the client package and the server package. An abstract class for each is in the top-level package. The OracleXid and OracleXAException classes are in the top-level oracle.jdbc.xa package, because their functionality does not depend on where the code is running.

In middle-tier scenarios, you will import OracleXid, OracleXAException, and the oracle.jdbc.xa.client package.

If you intend your XA code to run in the target Oracle Database, however, you will import the oracle.jdbc.xa.server package instead of the client package.

If code that will run inside a target database must also access remote databases, then do not import either package. Instead, you must fully qualify the names of any classes that you use from the client package to access a remote database or from the server package to access the local database. Class names are duplicated between these packages.

XADatasource Interface and Oracle Implementation

The javax.sql.XADataSource interface outlines standard functionality of XA data sources, which are factories for XA connections. The overloaded getXAConnection method returns an XA connection instance and optionally takes a user name and password as input:

public interface XADataSource
{
   XAConnection getXAConnection() throws SQLException;
   XAConnection getXAConnection(String user, String password)
      throws SQLException;
   ...
}

Oracle JDBC implements the XADataSource interface with the OracleXADataSource class, located both in the oracle.jdbc.xa.client package and the oracle.jdbc.xa.server package.

The OracleXADataSource classes also extend the OracleConnectionPoolDataSource class, which extends the OracleDataSource class, and therefore, include all the connection properties.

The getXAConnection methods of the OracleXADataSource class returns the Oracle implementation of XA connection instances, which are OracleXAConnection instances.

XAConnection Interface and Oracle Implementation

An XA connection instance, as with a pooled connection instance, encapsulates a physical connection to a database. This would be the database specified in the connection properties of the XA data source instance that produced the XA connection instance.

Each XA connection instance also has the facility to produce the OracleXAResource instance that will correspond to it for use in coordinating the distributed transaction.

An XA connection instance is an instance of a class that implements the standard javax.sql.XAConnection interface:

public interface XAConnection extends PooledConnection
{
   javax.jta.xa.XAResource getXAResource() throws SQLException;
}

As you see, the XAConnection interface extends the javax.sql.PooledConnection interface, so it also includes the getConnection, close, addConnectionEventListener, and removeConnectionEventListener methods.

Oracle JDBC implements the XAConnection interface with the OracleXAConnection class, located both in the oracle.jdbc.xa.client package and the oracle.jdbc.xa.server package.

The OracleXAConnection classes also extend the OraclePooledConnection class.

The OracleXAConnection class getXAResource method returns the Oracle implementation of an OracleXAResource instance, which is an OracleXAResource instance. The getConnection method returns an OracleConnection instance.

A JDBC connection instance returned by an XA connection instance acts as a temporary handle to the physical connection, as opposed to encapsulating the physical connection. The physical connection is encapsulated by the XA connection instance. The connection obtained from an XAConnection object behaves exactly like a regular connection, until it participates in a global transaction. At that time, auto-commit status is set to false. After the global transaction ends, auto-commit status is returned to the value it had before the global transaction. The default auto-commit status on a connection obtained from XAConnection is false in all releases prior to Oracle Database 10g. Starting from Oracle Database 10g, the default status is true.

Each time an XA connection instance getConnection method is called, it returns a new connection instance that exhibits the default behavior, and closes any previous connection instance that still exists and had been returned by the same XA connection instance. However, it is advisable to explicitly close any previous connection instance before opening a new one.

Calling the close method of an XA connection instance closes the physical connection to the database. This is typically performed in the middle tier.

XAResource Interface and Oracle Implementation

The transaction manager uses OracleXAResource instances to coordinate all the transaction branches that constitute a distributed transaction.

Each OracleXAResource instance provides the following key functionality, typically invoked by the transaction manager:

• It associates and disassociates distributed transactions with the transaction branch operating in the XA connection instance that produced this OracleXAResource instance. Essentially, it associates distributed transactions with the physical connection or session encapsulated by the XA connection instance. This is done through use of transaction IDs.

• It performs the two-phase commit functionality of a distributed transaction to ensure that changes are not committed in one transaction branch before there is assurance that the changes will succeed in all transaction branches.

  Note:

  • Because there must always be a one-to-one correlation between XA connection instances and OracleXAResource instances, an OracleXAResource instance is implicitly closed when the associated XA connection instance is closed.

  • If a transaction is opened by a given OracleXAResource instance, then it must also be closed by the same OracleXAResource instance.

An OracleXAResource instance is an instance of a class that implements the standard javax.transaction.xa.XAResource interface. Oracle JDBC implements the XAResource interface with the OracleXAResource class, located both in the oracle.jdbc.xa.client package and the oracle.jdbc.xa.server package.

Oracle JDBC driver creates and returns an OracleXAResource instance whenever the getXAResource method of the OracleXAConnection class is called, and it is Oracle JDBC driver that associates an OracleXAResource instance with a connection instance and the transaction branch being run through that connection.

This method is how an OracleXAResource instance is associated with a particular connection and with the transaction branch being run in that connection.

OracleXAResource Method Functionality and Input Parameters

The OracleXAResource class has several methods to coordinate a transaction branch with the distributed transaction with which it is associated. This functionality usually involves two-phase commit operations.

A transaction manager, receiving OracleXAResource instances from a middle-tier component, such as an application server, typically invokes this functionality.

Each of these methods takes a transaction ID as input, in the form of an Xid instance, which includes a transaction branch ID component and a distributed transaction ID component. Every transaction branch has a unique transaction ID, but transaction branches belonging to the same global transaction have the same global transaction component as part of their transaction IDs.

start

Starts work on behalf of a transaction branch, associating the transaction branch with a distributed transaction.

void start(Xid xid, int flags)

The flags parameter must be one or more of the following values:

• XAResource.TMNOFLAGS

  Flags the start of a new transaction branch for subsequent operations in the session associated with this XA resource instance. This branch will have the transaction ID xid, which is an OracleXid instance created by the transaction manager. This will map the transaction branch to the appropriate distributed transaction.

• XAResource.TMJOIN

  Joins subsequent operations in the session associated with this XA resource instance to the existing transaction branch specified by xid.

• XAResource.TMRESUME

  Resumes the transaction branch specified by xid.

  Note:

  A transaction branch can be resumed only if it had been suspended earlier.

• OracleXAResource.TMPROMOTE

  Promotes a local transaction to a global transaction

• OracleXAResource.ORATMSERIALIZABLE

  Starts a serializable transaction with transaction ID xid.

• OracleXAResource.ORATMREADONLY

  Starts a read-only transaction with transaction ID xid.

• OracleXAResource.ORATMREADWRITE

  Starts a read/write transaction with transaction ID xid.

• OracleXAResource.ORATRANSLOOSE

  Starts a loosely coupled transaction with transaction ID xid.

TMNOFLAGS, TMJOIN, TMRESUME, TMPROMOTE, ORATMSERIALIZABLE, ORATMREADONLY, and ORATMREADWRITE are defined as static members of the XAResource interface and OracleXAResource class. ORATMSERIALIZABLE, ORATMREADONLY, and ORATMREADWRITE are the isolation-mode flags. The default isolation behavior is READ COMMITTED.

Note that to create an appropriate transaction ID in starting a transaction branch, the transaction manager must know to which distributed transaction the transaction branch belongs. The mechanics of this are handled between the middle tier and transaction manager.

end

Ends work on behalf of the transaction branch specified by xid, disassociating the transaction branch from its distributed transaction.

void end(Xid xid, int flags)

The flags parameter can have one of the following values:

• XAResource.TMSUCCESS

  This is to indicate that this transaction branch is known to have succeeded.

• XAResource.TMFAIL

  This is to indicate that this transaction branch is known to have failed.

• XAResource.TMSUSPEND

  This is to suspend the transaction branch specified by xid. By suspending transaction branches, you can have multiple transaction branches in a single session. Only one can be active at any given time, however. Also, this tends to be more expensive in terms of resources than having two sessions.

TMSUCCESS, TMFAIL, and TMSUSPEND are defined as static members of the XAResource interface and OracleXAResource class.

prepare

Prepares the changes performed in the transaction branch specified by xid. This is the first phase of a two-phase commit operation, to ensure that the database is accessible and that the changes can be committed successfully.

int prepare(Xid xid)

This method returns an integer value as follows:

• XAResource.XA_RDONLY

  This is returned if the transaction branch runs only read-only operations such as SELECT statements.

• XAResource.XA_OK

  This is returned if the transaction branch runs updates that are all prepared without error.

• NA (no value returned)

  No value is returned if the transaction branch runs updates and any of them encounters errors during preparation. In this case, an XA exception is thrown.

XA_RDONLY and XA_OK are defined as static members of the XAResource interface and OracleXAResource class.

commit

Commits prepared changes in the transaction branch specified by xid. This is the second phase of a two-phase commit and is performed only after all transaction branches have been successfully prepared.

void commit(Xid xid, boolean onePhase)

Set the onePhase parameter as follows:

• true

  This is to use one-phase instead of two-phase protocol in committing the transaction branch. This is appropriate if there is only one transaction branch in the distributed transaction; the prepare step would be skipped.

• false

  This is to use two-phase protocol in committing the transaction branch.

rollback

Rolls back prepared changes in the transaction branch specified by xid.

void rollback(Xid xid)

forget

Tells the resource manager to forget about a heuristically completed transaction branch.

public void forget(Xid xid)

recover

The transaction manager calls this method during recovery to obtain the list of transaction branches that are currently in prepared or heuristically completed states.

public Xid[] recover(int flag)

The resource manager returns zero or more Xids for the transaction branches that are currently in a prepared or heuristically completed state. If an error occurs during the operation, then the resource manager throws the appropriate XAException.

isSameRM

To determine if two OracleXAResource instances correspond to the same resource manager, call the isSameRM method from one OracleXAResource instance, specifying the other OracleXAResource instance as input. In the following example, presume xares1 and xares2 are OracleXAResource instances:

boolean sameRM = xares1.isSameRM(xares2);

Xid Interface and Oracle Implementation

The transaction manager creates transaction ID instances and uses them in coordinating the branches of a distributed transaction. Each transaction branch is assigned a unique transaction ID, which includes the following information:

• Format identifier

  A format identifier specifies a Java transaction manager. For example, there could be a format identifier orcl. This field cannot be null. The size of a format identifier is 4 bytes.

• Global transaction identifier

  It is also known as a distributed transaction ID component. The size of a global transaction identifier is 64 bytes.

• Branch qualifier

  It is also known as transaction branch ID component. The size of a branch qualifier is 64 bytes.

The 64-byte global transaction identifier value will be identical in the transaction IDs of all transaction branches belonging to the same distributed transaction. However, the overall transaction ID is unique for every transaction branch.

An XA transaction ID instance is an instance of a class that implements the standard javax.transaction.xa.Xid interface, which is a Java mapping of the X/Open transaction identifier XID structure.

Oracle implements this interface with the OracleXid class in the oracle.jdbc.xa package. OracleXid instances are employed only in a transaction manager, transparent to application programs or an application server.

A transaction manager may use the following in creating an OracleXid instance:

public OracleXid(int fId, byte gId[], byte bId[]) throws XAException

fId is an integer value for the format identifier, gId[] is a byte array for the global transaction identifier, and bId[] is a byte array for the branch qualifier.

The Xid interface specifies the following getter methods:

• public int getFormatId()

• public byte[] getGlobalTransactionId()

• public type[] getBranchQualifier()

XAException Classes and Methods

XA methods throw XA exceptions, as opposed to general exceptions or SQLExceptions. An XA exception is an instance of the standard class javax.transaction.xa.XAException or a subclass.

An Oracle XAException is an instance that consists of an Oracle error portion and an XA error portion. Oracle provides the oracle.jdbc.xa.OracleXAException subclasses of the standard javax.transaction.xa.XAException class. An OracleXAException instance is constructed using one of the following constructors:

public OracleXAException()

public OracleXAException(int error)

The error value is an error code that combines an Oracle SQL error value and an XA error value. The JDBC driver determines exactly how to combine the Oracle and XA error values.

The OracleXAException class has the following methods:

• public int getOracleError()

  This method returns the Oracle SQL error code pertaining to the exception, a standard ORA error number or 0 if there is no Oracle SQL error.

• public int getXAError()

  This method returns the XA error code pertaining to the exception. XA error values are defined in the javax.transaction.xa.XAException class.

Mapping Between Oracle Errors and XA Errors

Oracle errors correspond to XA errors in OracleXAException instances as documented in Table 30-2.

XA Error Handling

The following example uses the OracleXAException class to process an XA exception:

try {
   ...
   ...Perform XA operations...
   ...
} catch(OracleXAException oxae) { 
  int oraerr = oxae.getOracleError();
  System.out.println("Error " + oraerr);
} 
  catch(XAException xae)
{...Process generic XA exception...}

In case the XA operations did not throw an Oracle-specific XA exception, the code drops through to process a generic XA exception.

Oracle XA Optimizations

Oracle JDBC has functionality to improve performance if two or more branches of a distributed transaction use the same database instance, meaning that the OracleXAResource instances associated with these branches are associated with the same resource manager.

In such a circumstance, the prepare method of only one of these OracleXAResource instances will return XA_OK or will fail. The rest will return XA_RDONLY, even if updates are made. This allows the transaction manager to implicitly join all the transaction branches and commit or roll back, in case of failure, the joined transaction through the OracleXAResource instance that returned XA_OK or failed.

The transaction manager can use the OracleXAResource class isSameRM method to determine if two OracleXAResource instances are using the same resource manager. This way it can interpret the meaning of XA_RDONLY return values.

Summary of Imports for Oracle XA

You must import the following for Oracle XA functionality:

import oracle.jdbc.xa.OracleXid;
import oracle.jdbc.xa.OracleXAException;
import oracle.jdbc.pool.*;
import oracle.jdbc.xa.client.*;
import javax.transaction.xa.*;

The oracle.jdbc.pool package has classes for connection pooling functionality, some of which have XA-related classes as subclasses.

Alternatively, if the code will run inside Oracle Database and access that database for SQL operations, you must import oracle.jdbc.xa.server instead of oracle.jdbc.xa.client.

import oracle.jdbc.xa.server.*;

If your application must access another Oracle Database as part of an XA transaction using the server-side Thin driver, then your code can use the fully qualified names of the oracle.xa.client classes.

The client and server packages each have versions of the OracleXADataSource, OracleXAConnection, and OracleXAResource classes. Abstract versions of these three classes are in the top-level oracle.jdbc.xa package.

Oracle XA Code Sample

This example uses a two-phase distributed transaction with two transaction branches, each to a separate database.

Note that for simplicity, this example combines code that would typically be in a middle tier with code that would typically be in a transaction manager, such as the OracleXAResource method invocations and the creation of transaction IDs.

For brevity, the specifics of creating transaction IDs and performing SQL operations are not shown here. The complete example is shipped with the product.

This example performs the following sequence:

1. Start transaction branch #1.

2. Start transaction branch #2.

3. Execute DML operations on branch #1.

4. Execute DML operations on branch #2.

5. End transaction branch #1.

6. End transaction branch #2.

7. Prepare branch #1.

8. Prepare branch #2.

9. Commit branch #1.

10. Commit branch #2.

// You need to import the java.sql package to use JDBC
import java.sql.*;
import javax.sql.*;
import oracle.jdbc.*;
import oracle.jdbc.pool.*;
import oracle.jdbc.xa.OracleXid;
import oracle.jdbc.xa.OracleXAException;
import oracle.jdbc.xa.client.*;
import javax.transaction.xa.*;

class XA4
{
  public static void main (String args [])
       throws SQLException 
  {

    try
    {
        String URL1 = "jdbc:oracle:oci:@";
        // You can put a database name after the @ sign in the connection URL.
        String URL2 ="jdbc:oracle:thin:@(description=(address=(host=localhost)
                     (protocol=tcp)(port=5521))(connect_data=(service_name=orcl)))";
        // Create first DataSource and get connection
        OracleDataSource ods1 = new OracleDataSource();
        ods1.setURL(URL1);
        ods1.setUser("HR");
        ods1.setPassword("hr");
        Connection conna = ods1.getConnection();

        // Create second DataSource and get connection
        OracleDataSource ods2 = new OracleDataSource();
        ods2.setURL(URL2);
        ods2.setUser("HR");
        ods2.setPassword("hr");
        Connection connb = ods2.getConnection();

        // Prepare a statement to create the table
        Statement stmta = conna.createStatement ();

        // Prepare a statement to create the table
        Statement stmtb = connb.createStatement ();

        try
        {
          // Drop the test table
          stmta.execute ("drop table my_table");
        }
        catch (SQLException e)
        {
          // Ignore an error here
        }

        try
        {   
          // Create a test table
          stmta.execute ("create table my_table (col1 int)");
        }
        catch (SQLException e)
        {
          // Ignore an error here too
        }

        try
        {
          // Drop the test table
          stmtb.execute ("drop table my_tab");
        }
        catch (SQLException e)
        {
          // Ignore an error here
        }

        try
        {   
          // Create a test table
          stmtb.execute ("create table my_tab (col1 char(30))");
        }
        catch (SQLException e)
        {
          // Ignore an error here too
        }

        // Create XADataSource instances and set properties.
        OracleXADataSource oxds1 = new OracleXADataSource();
        oxds1.setURL("jdbc:oracle:oci:@");
        oxds1.setUser("HR");
        oxds1.setPassword("hr");

        OracleXADataSource oxds2 = new OracleXADataSource();

        oxds2.setURL("jdbc:oracle:thin:@(description=(address=(host=localhost)
                   (protocol=tcp)(port=5521))(connect_data=(service_name=orcl)))");
        oxds2.setUser("HR");
        oxds2.setPassword("hr");
    
        // Get XA connections to the underlying data sources
        XAConnection pc1  = oxds1.getXAConnection();
        XAConnection pc2  = oxds2.getXAConnection();

        // Get the physical connections
        Connection conn1 = pc1.getConnection();
        Connection conn2 = pc2.getConnection();

        // Get the XA resources
        XAResource oxar1 = pc1.getXAResource();
        XAResource oxar2 = pc2.getXAResource();

        // Create the Xids With the Same Global Ids
        Xid xid1 = createXid(1);
        Xid xid2 = createXid(2);

        // Start the Resources
        oxar1.start (xid1, XAResource.TMNOFLAGS);
        oxar2.start (xid2, XAResource.TMNOFLAGS);

        // Execute SQL operations with conn1 and conn2
        doSomeWork1 (conn1);
        doSomeWork2 (conn2);

        // END both the branches -- IMPORTANT
        oxar1.end(xid1, XAResource.TMSUCCESS);
        oxar2.end(xid2, XAResource.TMSUCCESS);

        // Prepare the RMs
        int prp1 =  oxar1.prepare (xid1);
        int prp2 =  oxar2.prepare (xid2);

        System.out.println("Return value of prepare 1 is " + prp1);
        System.out.println("Return value of prepare 2 is " + prp2);

        boolean do_commit = true;

        if (!((prp1 == XAResource.XA_OK) || (prp1 == XAResource.XA_RDONLY)))
           do_commit = false;

        if (!((prp2 == XAResource.XA_OK) || (prp2 == XAResource.XA_RDONLY)))
           do_commit = false;

       System.out.println("do_commit is " + do_commit);
        System.out.println("Is oxar1 same as oxar2 ? " + oxar1.isSameRM(oxar2));

        if (prp1 == XAResource.XA_OK)
          if (do_commit)
             oxar1.commit (xid1, false);
          else
             oxar1.rollback (xid1);

        if (prp2 == XAResource.XA_OK)
          if (do_commit)
             oxar2.commit (xid2, false);
          else
             oxar2.rollback (xid2);

         // Close connections
        conn1.close();
        conn1 = null;
        conn2.close();
        conn2 = null;

        pc1.close();
        pc1 = null;
        pc2.close();
        pc2 = null;

        ResultSet rset = stmta.executeQuery ("select col1 from my_table");
        while (rset.next())
          System.out.println("Col1 is " + rset.getInt(1));
  
        rset.close();
        rset = null;

        rset = stmtb.executeQuery ("select col1 from my_tab");
        while (rset.next())
          System.out.println("Col1 is " + rset.getString(1));
  
        rset.close();
        rset = null;

        stmta.close();
        stmta = null;
        stmtb.close();
        stmtb = null;

        conna.close();
        conna = null;
        connb.close();
        connb = null;

    } catch (SQLException sqe)
    {
      sqe.printStackTrace();
    } catch (XAException xae)
    {
      if (xae instanceof OracleXAException) {
        System.out.println("XA Error is " +
                      ((OracleXAException)xae).getXAError());
        System.out.println("SQL Error is " +
                      ((OracleXAException)xae).getOracleError());
      }
    }
  }

  static Xid createXid(int bids)
    throws XAException
  {...Create transaction IDs...}

  private static void doSomeWork1 (Connection conn)
   throws SQLException
  {...Execute SQL operations...}

  private static void doSomeWork2 (Connection conn)
   throws SQLException
  {...Execute SQL operations...}
}

OCI Native XA

Native XA is enabled through the use of the tnsEntry and nativeXA properties of the OracleXADataSource class.

Configuration and Installation

On a Solaris or Linux system, you need the libheteroxa11.so shared library to enable the Native XA feature. This library must be installed and available in the search path for the Native XA feature to work properly.

On a Microsoft Windows system, you need the heteroxa11.dll file to enable the Native XA feature. This file must be installed and available in the Windows DLL path for the Native XA feature to work properly.

Exception Handling

When using the Native XA feature in distributed transactions, it is recommended that the application simply check for XAException or SQLException, rather than OracleXAException or OracleSQLException.

Native XA Code Example

The following portion of code shows how to enable the Native XA feature:

...
// Create a XADataSource instance
OracleXADataSource oxds = new OracleXADataSource();
oxds.setURL(url);

// Set the nativeXA property to use Native XA feature
oxds.setNativeXA(true);

// Set the tnsEntry property to an older DB as required
oxds.setTNSEntryName("ora805");
...

Thin Native XA

Like the JDBC OCI driver, the JDBC Thin driver also provides support for Native XA. However, the JDBC Thin driver provides support for Native XA by default. This is unlike the case of the JDBC OCI driver in which the support for Native XA is not enabled by default.

You can disable Native XA by calling setNativeXA(false) on the XA data source as follows:

...
// Create a XADataSource instance
OracleXADataSource oxds = new OracleXADataSource();
...
// Disabling Native XA
oxds.setNativeXA(false);
...

For example, you may need to disable Native XA as a workaround for a bug in the Native XA code.