Oracle9i JDBC Developer's Guide and Reference
Release 1 (9.0.1)
Part Number A90211-01
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15
Distributed Transactions

This chapter discusses the Oracle JDBC implementation of distributed transactions. These are multi-phased transactions, often using multiple databases, that must be committed in a coordinated way. There is also related discussion of XA, which is a general standard (not specific to Java) for distributed transactions.

The following topics are discussed:

For further introductory and general information about distributed transactions, refer to the Sun Microsystems specifications for the JDBC 2.0 Optional Package and the Java Transaction API (JTA).

For information on the OCI-specific HeteroRM XA feature, see "OCI HeteroRM XA".

Overview

A distributed transaction, sometimes referred to as a global transaction, is a set of two or more related transactions that must be managed in a coordinated way. The transactions that constitute a distributed transaction might be in the same database, but more typically are in different databases and often in different locations. Each individual transaction of a distributed transaction is referred to as a transaction branch.

For example, a distributed transaction might consist of money being transferred from an account in one bank to an account in another bank. You would not want either transaction committed without assurance that both will complete successfully.

In the JDBC 2.0 extension API, distributed transaction functionality is built on top of connection pooling functionality, described under "Connection Pooling". This distributed transaction functionality is also built upon the open XA standard for distributed transactions. (XA is part of the X/Open standard and is not specific to Java.)

The remainder of this overview covers the following topics:

For further introductory and general information about distributed transactions and XA, refer to the Sun Microsystems specifications for the JDBC 2.0 Optional Package and the Java Transaction API.

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:

Distributed Transaction Concepts

Software that uses distributed transactions cannot use normal connection instance COMMIT, auto-commit, or ROLLBACK functionality, because all COMMIT or ROLLBACK operations in a distributed transaction must be coordinated. Any attempt to use the commit() or rollback() method or enable the auto-commit flag of a connection instance would result in a SQL exception.

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:

Oracle XA Packages

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

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.

XA Components

This section discusses the XA components--standard XA interfaces specified in the JDBC 2.0 Optional Package, and the Oracle classes that implement them. The following topics are covered:

XA Data Source 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), so include all the connection properties described in "Data Source Properties".

The OracleXADataSource class getXAConnection() methods return the Oracle implementation of XA connection instances, which are OracleXAConnection instances (as the next section discusses).

Note:

You can register XA data sources in JNDI using the same naming conventions as discussed previously for non-pooling data sources in "Register the Data Source". 

XA Connection 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 XA resource 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 listed in "Pooled Connection Interface and Oracle Implementation".

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 XA resource instance, which is an OracleXAResource instance (as the next section discusses). 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.

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. It is advisable to explicitly close any previous connection instance before opening a new one, however.

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

XA Resource Interface and Oracle Implementation

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

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

An XA resource instance is an instance of a class that implements the standard javax.transaction.xa.XAResource interface: 

public interface XAResource
{
   void commit(Xid xid, boolean onePhase) throws XAException;
   void end(Xid xid, int flags) throws XAException;
   void forget(Xid xid) throws XAException;
   int prepare(Xid xid) throws XAException;
   Xid[] recover(int flag) throws XAException;
   void rollback(Xid xid) throws XAException;
   void start(Xid xid, int flags) throws XAException;
   boolean isSameRM(XAResource xares) throws XAException;
}

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.

The Oracle JDBC driver creates and returns an OracleXAResource instance whenever the OracleXAConnection class getXAResource() method is called, and it is the Oracle JDBC driver that associates an XA resource instance with a connection instance and the transaction branch being executed through that connection.

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

XA Resource 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.

"XA ID Interface and Oracle Implementation" discusses the OracleXid class and the standard interface upon which it is based.

Following is a description of key XA resource functionality, the methods used, and additional input parameters. Each of these methods throws an XA exception if an error is encountered. See "XA Exception Classes and Methods".

Start

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

void start(Xid xid, int flags)

The flags parameter can have one of the following values:

TMNOFLAGS, TMJOIN, and TMRESUME are defined as static members of the XAResource interface and OracleXAResource class.

Note:

Instead of using the start() method with TMRESUME, the transaction manager can cast to an OracleXAResource instance and use the resume(Xid xid) method, an Oracle extension. 

Note that to create an appropriate transaction ID in starting a transaction branch, the transaction manager must know which distributed transaction the transaction branch should belong to. The mechanics of this are handled between the middle tier and transaction manager and are beyond the scope of this document. Refer to the Sun Microsystems specifications for the JDBC 2.0 Optional Package and the Java Transaction API.

End

End 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:

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

Notes:

  • Instead of using the end() method with TMSUSPEND, the transaction manager can cast to an OracleXAResource instance and use the suspend(Xid xid) method, an Oracle extension.

  • This XA functionality to suspend a transaction provides a way to switch between various transactions within a single JDBC connection. You can use the XA classes to accomplish this, even if you are not in a distributed transaction environment and would otherwise have no need for the XA classes.

 
Prepare

Prepare 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:

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

Notes:

  • Always call the end() method on a branch before calling the prepare() method.

  • If there is only one transaction branch in a distributed transaction, then there is no need to call the prepare() method. You can call the XA resource commit() method without preparing first.

 
Commit

Commit 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:

Roll back

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)

Note:

The flag parameter is ignored and therefore not implemented since the scan option (flag parameter) is not meaningful without a count parameter. See the Sun Microsystems Java Transaction API (JTA) Specification for more detail. 

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, the resource manager throws the appropriate XAException.

Check for same RM

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

boolean sameRM = xares1.isSameRM(xares2);

A transaction manager can use this method regarding certain Oracle optimizations, as "Oracle XA Optimizations" explains.

XA ID 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:

The 64-byte global transaction identifier value will be identical in the transaction IDs of all transaction branches belonging to the same distributed transaction. The overall transaction ID, however, 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.

Note:

OracleXid is not required for Oracle XA resource calls. Instead, use any class that implements javax.transaction.xa.Xid interface. 

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

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

Where 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:

Error Handling and Optimizations

This section has two focuses: 1) the functionality of XA exceptions and error handling; and 2) Oracle optimizations in its XA implementation. The following topics are covered:

The exception and error-handling discussion includes the standard XA exception class and the Oracle-specific XA exception class, as well as particular XA error codes and error-handling techniques.

XA Exception Classes and Methods

XA methods throw XA exceptions, as opposed to general exceptions or SQL exceptions. An XA exception is an instance of the standard class javax.transaction.xa.XAException or a subclass. Oracle subclasses XAException with the oracle.jdbc.xa.OracleXAException class.

An OracleXAException instance consists of an Oracle error portion and an XA error portion and is constructed as follows by the Oracle JDBC driver: 

public OracleXAException()

or: 

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:

Mapping between Oracle Errors and XA Errors

Oracle errors correspond to XA errors in OracleXAException instances as documented in Table 15-1.

Table 15-1 Oracle-XA Error Mapping 
Oracle Error Code  XA Error Code 

ORA 3113 

XAException.XAER_RMFAIL 

ORA 3114 

XAException.XAER_RMFAIL 

ORA 24756 

XAException.XAER_NOTA 

ORA 24764 

XAException.XA_HEURCOM 

ORA 24765 

XAException.XA_HEURRB 

ORA 24766 

XAException.XA_HEURMIX 

ORA 24767 

XAException.XA_RDONLY 

ORA 25351 

XAException.XA_RETRY 

all other ORA errors 

XAException.XA_RMERR 

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 XA resource instances associated with these branches are associated with the same resource manager.

In such a circumstance, the prepare() method of only one of these XA resource instances will return XA_OK (or failure); 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, if failure) the joined transaction through the XA resource instance that returned XA_OK (or failure).

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

Implementing a Distributed Transaction

This section provides an example of how to implement a distributed transaction using Oracle XA functionality.

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 are subclassed by XA-related classes.

In addition, if the code will run inside an Oracle database and access that database for SQL operations, you must import the following: 

import oracle.jdbc.xa.server.*;

(And if you intend to access only the database in which the code runs, you would not need the oracle.jdbc.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 XA resource method invocations and the creation of transaction IDs).

For brevity, the specifics of creating transaction IDs (in the createID() method) and performing SQL operations (in the doSomeWork1() and doSomeWork2() methods) are not shown here. The complete sample is in "XA with Two-Phase Commit Operation--XA4.java".

For another complete sample, showing how to use XA resource functionality to suspend and resume a transaction, see "XA with Suspend and Resume--XA2.java".

This example executes 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:oci8:@";
        String URL2 ="jdbc:oracle:thin:@(description=(address=(host=dlsun991)
                     (protocol=tcp)(port=5521))(connect_data=(sid=rdbms2)))";

        DriverManager.registerDriver(new OracleDriver());

        // You can put a database name after the @ sign in the connection URL.
        Connection conna =
          DriverManager.getConnection (URL1, "scott", "tiger");

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

        Connection connb =
          DriverManager.getConnection (URL2, "scott", "tiger");

        // 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:oci8:@");
        oxds1.setUser("scott");
        oxds1.setPassword("tiger");

        OracleXADataSource oxds2 = new OracleXADataSource();

        oxds2.setURL("jdbc:oracle:thin:@(description=(address=(host=dlsun991)
                   (protocol=tcp)(port=5521))(connect_data=(sid=rdbms2)))");
        oxds2.setUser("scott");
        oxds2.setPassword("tiger");
    
        // 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...}

}

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