Program guide > Programming with system APIs and plug-ins > Transaction life cycle plug-ins

External transaction managers

Typically, eXtreme Scale transactions begin with the Session.begin method and end with the Session.commit method. However, when an ObjectGrid is embedded, an external transaction coordinator can start and end transactions. In this case, you do not need to call the begin or commit methods.

External transaction coordination

The TransactionCallback plug-in is extended with the isExternalTransactionActive(Session session) method that associates the eXtreme Scale session with an external transaction. The method header follows:

public synchronized boolean isExternalTransactionActive(Session session)

For example, eXtreme Scale can be set up to integrate with WebSphere Application Server and WebSphere Extended Deployment.

Also, eXtreme Scale provides a built in plug-in called the Plug-ins for managing transaction life cycle events, which describes how to build the plug-in for WebSphere Application Server environments, but you can adapt the plug-in for other frameworks.

The key to this seamless integration is the exploitation of the ExtendedJTATransaction API in WebSphere Application Server v5.x and v6.x. However, if you are using WebSphere Application Server v6.0.2, apply APAR PK07848 to support this method. Use the following sample code to associate an ObjectGrid session with a WebSphere Application Server transaction ID:

/**
* This method is required to associate an objectGrid session with a WebSphere 
* Application Server transaction ID.
*/
Map/**/ localIdToSession;
public synchronized boolean isExternalTransactionActive(Session session)
{
    // remember that this localid means this session is saved for later.
    localIdToSession.put(new Integer(jta.getLocalId()), session);
    return true;
}

Retrieve an external transaction

Sometimes you might need to retrieve an external transaction service object for the TransactionCallback plug-in to use. In the WebSphere Application Server server, look up the ExtendedJTATransaction object from its namespace as shown in the following example:

public J2EETransactionCallback() {
    super();
    localIdToSession = new HashMap();
    String lookupName="java:comp/websphere/ExtendedJTATransaction";
    try
    {
        InitialContext ic = new InitialContext();
        jta = (ExtendedJTATransaction)ic.lookup(lookupName);
        jta.registerSynchronizationCallback(this);
    }
    catch(NotSupportedException e)
    {
        throw new RuntimeException("Cannot register jta callback", e);
    }
    catch(NamingException e){
        throw new RuntimeException("Cannot get transaction object");
    }
}

For other products, you can use a similar approach to retrieve the transaction service object.

Control commit by external callback

The TransactionCallback plug-in must receive an external signal to commit or roll back the eXtreme Scale session. To receive this external signal, use the callback from the external transaction service. Implement the external callback interface and register it with the external transaction service. For example, with WebSphere Application Server, implement the SynchronizationCallback interface, as shown in the following example:

public class J2EETransactionCallback implements 
    com.ibm.websphere.objectgrid.plugins.TransactionCallback, SynchronizationCallback {
   public J2EETransactionCallback() {
      super();
      String lookupName="java:comp/websphere/ExtendedJTATransaction";
      localIdToSession = new HashMap();
      try {
         InitialContext ic = new InitialContext();
         jta = (ExtendedJTATransaction)ic.lookup(lookupName);
         jta.registerSynchronizationCallback(this);
      } catch(NotSupportedException e) {
         throw new RuntimeException("Cannot register jta callback", e);
      }
      catch(NamingException e) {
         throw new RuntimeException("Cannot get transaction object");
      }
   }

   public synchronized void afterCompletion(int localId, byte[] arg1,boolean didCommit) {
      Integer lid = new Integer(localId);
      // find the Session for the localId
      Session session = (Session)localIdToSession.get(lid);
      if(session != null) {
             try {
            // if WAS is committed when 
            // hardening the transaction to backingMap.
            // We already did a flush in beforeCompletion
            if(didCommit) {
               session.commit();
            } else {
               // otherwise rollback
               session.rollback();
            }
         } catch(NoActiveTransactionException e) {
            // impossible in theory
         } catch(TransactionException e) {
            // given that we already did a flush, this should not fail
         } finally {
            // always clear the session from the mapping map.
            localIdToSession.remove(lid);
         }
      }
   }

   public synchronized void beforeCompletion(int localId, byte[] arg1) {
      Session session = (Session)localIdToSession.get(new Integer(localId));
      if(session != null) {
         try {
            session.flush();
         } catch(TransactionException e) {
            // WAS  does not formally define 
            // a way to signal the
            // transaction has failed so do this
            throw new RuntimeException("Cache flush failed", e);
         }
      }
   }
}

Use eXtreme Scale APIs with the TransactionCallback plug-in

The TransactionCallback plug-in disables autocommit in eXtreme Scale. The normal usage pattern for an eXtreme Scale follows:

Session ogSession = ...;
ObjectMap myMap = ogSession.getMap("MyMap");
ogSession.begin();
MyObject v = myMap.get("key");
v.setAttribute("newValue");
myMap.update("key", v);
ogSession.commit();

When this TransactionCallback plug-in is in use, eXtreme Scale assumes that the application uses the eXtreme Scale when a container-managed transaction is present. The previous code snippet changes the following code in this environment:

public void myMethod() {
   UserTransaction tx = ...;
   tx.begin();
   Session ogSession = ...;
   ObjectMap myMap = ogSession.getMap("MyMap");
   yObject v = myMap.get("key");
   v.setAttribute("newValue");
   myMap.update("key", v);
   tx.commit();
}

The myMethod method is similar to a Web application scenario. The application uses the normal UserTransaction interface to begin, commit, and roll back transactions. The eXtreme Scale automatically begins and commits around the container transaction. If the method is an Enterprise JavaBeans™ (EJB) method that uses the TX_REQUIRES attribute, then remove the UserTransaction reference and the calls to begin and commit transactions and the method works the same way. In this case, the container is responsible for starting and ending the transaction.

Parent topic:

Plug-ins for managing transaction life cycle events

Related concepts

Transaction processing overview

Introduction to plug-in slots

WebSphereTransactionCallback plug-in

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