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Connection Sharing in GlassFish Application Server

When multiple connections x and y acquired by an application are used within a transaction scope, application server will provide shared connection behavior when the following set of properties are satisfied.

• x and y are collocated in a single Java Virtual Machine process address space.
• x and y are using a single transactional resource manager.
• x and y have identical properties.
• x and y are marked as shareable.
• x and y are used within a container-managed or bean-managed transaction scope.

When multiple connection requests to same data-source happen within a transaction, application server may re-use the connection already acquired as a part of the transaction.
Let us consider the following scenario :

	Tx Scope Begins
		get Connection1
		do DB operation
		get Connection2
		do DB operation
	Tx Scope Ends

How do I enable/disable Connection Sharing ?

By specifying the following highlighted property in deployment descriptor (web.xml / ejb-jar.xml). Default value is Shareable.

 <resource-ref>
        <res-ref-name>jdbc/EmployeeAppDB</res-ref-name>
        <res-type>javax.sql.DataSource</res-type>
        <res-auth>Container</res-auth>
        <res-sharing-scope>Shareable</res-sharing-scope>
    </resource-ref>

        <res-sharing-scope>Unshareable</res-sharing-scope>

Connection Sharing in BMPs

a) Container Managed Transaction:

   ( <transaction-type>Container</transaction-type>)
		 BMP-Bean-1
			businessMethod(){   // Transaction attribute "Required"
				con1 = dataSource.getConnection();  
				doDBOperation();
				bean2 = lookupBean2;
				bean2.businessMethod();
				con1.close();
			}

		BMP-Bean-2
			businessMethod(){ // Transaction Attribute "Required"
				con2 = dataSource.getConnection(); // same data-source as that of con1 in bean-1
				doDBOperation();
				con2.close();
  			}

During Bean2's method call, connection sharing happens. Application server first acquires a free connection from the connection pool. During connection enlistment, connection association happens which will map the ManagedConnection (and hence physical connection) of con-1 to con-2. Thus DBOperation happens through same physical connection (connection sharing).

What happens to the actual Physical Connection acquired by conn-2 ?

By the end of transaction, physical connection of con-1 is dissociated from conn-2 and conn-2 is mapped back with its actual physical connection. The purpose of this connection association and dissociation can be seen in the following sample.

		   (<transaction-type>Bean</transaction-type>)
			businessMethod(){   
				UserTransaction uTx = entityContext.getUserTransaction(); 
				try{
					uTx.begin();
					con1 = dataSource.getConnection(); 
					doDBOperation() using con1;

					con2 = dataSource.getConnection(); 
					doDBOperation() using con2; //using physical connection of conn-1, connection sharing.
					uTx.commit();
				}
				catch(SQLException sqle){
					uTx.rollback();
				}
					con1.close();
					doDBOperation() using con2; // using actual physical connection,  no sharing involved.
					con2.close();
			}

How do I know that connection-sharing has actually happened?

In the sample above, dataSource ( com.sun.appserv.jdbc.DataSource ) has a method to get the physical connection.

			Connection pc1 = dataSource.getConnection(con1);
			Connection pc2 = dataSource.getConnection(con2);

• Picture shows connections' state in Application Server, Resource Adapter (Physical Connection), within and outside the transaction scope.
• Log shows the connection objects' hash-code value within and outside the transaction scope.

Few interesting scenarios to see whether connection sharing happens or not is,

			a) 
				con1 = dataSource.getConnection();  
				UserTransaction uTx = entityContext.getUserTransaction(); 
				uTx.begin();
					con2 = dataSource.getConnection();  
 					doDBOperation() using connection-1
 					doDBOperation() using connection-2
				uTx.commit();
				con1.close();
				con2.close();
			
			b)
				con1 = dataSource.getConnection();  
				con2 = dataSource.getConnection();  
				UserTransaction uTx = entityContext.getUserTransaction(); 
				uTx.begin();
 					doDBOperation() using connection-1
 					doDBOperation() using connection-2
				uTx.commit();
				con1.close();
				con2.close();
 

Are there any restrictions when connections are used in sharing scope?

Yes, When connection sharing is enabled, certain properties should not be modified. Suppose connection-1's property is changed and when sharing happens, connection-2 will have the modified property. Application Server will throw SharingViolationException. More details can be found in the J2EE Connector Architecture 1.5 Specification, section 7.9 - Transaction Management > Connection Sharing.