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Integrating Transactions into a Java Server Application

This chapter describes how to integrate transactions into a WebLogic Enterprise server application and covers the following topics:

Overview of Transactions in the WebLogic Enterprise System

The WebLogic Enterprise system provides transactions as a means to guarantee that database transactions are completed accurately and that they take on all the ACID properties (atomicity, consistency, isolation, and durability) of a high-performance transaction. That is, you have a requirement to perform multiple write operations on durable storage, and you must be guaranteed that the operations succeed; if any one of the operations fails, the entire set of operations is rolled back.

Transactions typically are appropriate in the situations described in the following list. Each situation encapsulates a transactional model supported by the WebLogic Enterprise system.

The client application needs to make invocations on several different objects, which may involve write operations to one or more databases. If any one invocation is unsuccessful, any state that is written (either in memory or, more typically, to a database) must be rolled back.

For example, consider a travel agent application. The client application needs to arrange for a journey to a distant location; for example, from Strasbourg, France, to Alice Springs, Australia. Such a journey would inevitably require multiple individual flight reservations. The client application works by reserving each individual segment of the journey in sequential order; for example, Strasbourg to Paris, Paris to New York, New York to Los Angeles. However, if any individual flight reservation cannot be made, the client application needs a way to cancel all the flight reservations made so far. For example, if the client application cannot book a flight from Los Angeles to Honolulu on a given date, the client application needs to cancel the flight reservations made up to that point.
The client needs a conversation with an object managed by the server application, and the client needs to make multiple invocations on a specific object instance. The conversation may be characterized by one or more of the following:
Within the scope of a single client invocation on an object, the object performs multiple edits to data in a database. If one of the edits fails, the object needs a mechanism to roll back all the edits. (And in this situation, the individual database edits are not necessarily CORBA invocations.)

For example, consider a banking application. The client invokes the transfer operation on a teller object. The transfer operation requires the teller object to make the following invocations on the bank database:
- Invoking the debit method on one account
- Invoking the credit method on another account
  
  If the credit invocation on the bank database fails, the banking application needs a way to roll back the previous debit invocation.

Integrating Transactions in a WebLogic Enterprise Client and Server Application

The WebLogic Enterprise system supports two transaction API models:

The Java Transaction Service defined by Sun Microsystems-This service is the Java mapping of the Object Transaction Service (OTS) that is specified as part of CORBA: org.omg.CosTransactions.Current.
The Java Transaction API defined by Sun Microsystems-Only the application-level transaction demarcation interface is supported: javax.transaction.UserTransaction.

In this document, we refer generically to these mappings as the TransactionCurrent object. For specifics about org.omg.CosTransactions.Current and javax.transaction.UserTransaction, see the Java API Reference and the Java Programming Reference.

The WebLogic Enterprise system supports transactions in the following ways:

The client or the server application can begin and end transactions explicitly by using calls on the TransactionCurrent object. For details about the TransactionCurrent object, see Creating Client Applications.
You can assign transactional policies to an object's interface so that when the object is invoked, the WebLogic Enterprise system can start a transaction automatically for that object, if a transaction has not already been started, and commit or roll back the transaction when the method invocation is complete. You use transactional policies on objects in conjunction with an XA resource manager and database when you want to delegate all the transaction commit and rollback responsibilities to that resource manager.
Objects involved in a transaction can force a transaction to be rolled back. That is, after an object has been invoked within the scope of a transaction, the object can invoke the rollback_only method on the TransactionCurrent object to mark the transaction for rollback only. This prevents the current transaction from being committed. An object may need to mark a transaction for rollback if an entity, typically a database, is otherwise at risk of being updated with corrupt or inaccurate data.
Objects involved in a transaction can be kept in memory from the time they are first invoked until the moment when the transaction is ready to be committed or rolled back. In the case of a transaction that is about to be committed, these objects are polled by the WebLogic Enterprise system immediately before the resource managers prepare to commit the transaction. (In this sense, polling means invoking the object's com.beasys.Tobj_Servant.deactivate_object method and passing a reason value.)

When an object is polled, the object may veto the current transaction by invoking the rollback_only method on the TransactionCurrent object. In addition, if the current transaction is to be rolled back, objects have an opportunity to skip any writes to a database. If no object vetos the current transaction, the transaction is committed.

The following sections explain how you can use object activation policies and transaction policies to get the transactional behavior you want in your objects. Note that these policies apply to an interface and, therefore, to all operations on all objects implementing that interface.

Note: If a server application manages an object that you want to be able to participate in a transaction, the Server object for that application must invoke the com.beasys.Tobj.TP.open_xa_rm and com.beasys.Tobj.TP.close_xa_rm methods. For more information about database connections, see the section "Opening an XA Resource Manager" on page 3-9.

Making an Object Automatically Transactional

The WebLogic Enterprise system provides the always transactional policy, which you can define on an object's interface to have the WebLogic Enterprise system start a transaction automatically when that object is invoked and a transaction has not already been scoped. When an invocation on that object is completed, the WebLogic Enterprise system commits or rolls back the transaction automatically. Neither the server application, nor the object implementation, needs to invoke the TransactionCurrent object in this situation; the WebLogic Enterprise system automatically invokes the TransactionCurrent object on behalf of the server application.

Assigning the always transactional policy to an object's interface is appropriate when:

The object writes to a database and you want all the database commit or rollback responsibilities delegated to an XA resource manager whenever this object is invoked.
You want to give the client application the opportunity to include the object in a larger transaction that encompasses invocations on multiple objects, and the invocations must all succeed or be rolled back if any one invocation fails.

If you want an object to be automatically transactional, assign the following policies to that object's interface in the XML-based Server Description File:


Activation Policy	Transaction Policy
`process`, `method`, or `transaction`	`always`

Note: Database cursors cannot span transactions. For an example, see Creating C++ Server Applications.

Enabling an Object to Participate in a Transaction

If you want an object to be able to be invoked within the scope of a transaction, you can assign the optional transaction policies to that object's interface. The optional transaction policy may be appropriate for an object that does not perform any database write operations, but that you want to have the ability to be invoked during a transaction.

You can use the following policies, when they are specified in the XML-based Server Description File for that object's interface, to make an object optionally transactional:


Activation Policy	Transaction Policy
`process`, `method`, or `transaction`	`optional`

When the transaction policy is optional, if the AUTOTRAN parameter is enabled in the application's UBBCONFIG file, the implementation is transactional. Servers containing transactional objects must be configured within a group associated with an XA-compliant resource manager.

If the object does perform database write operations, and you want the object to be able to participate in a transaction, assigning the always transactional policy is generally a better choice. However, if you prefer, you can use the optional policy and encapsulate any write operations within invocations on the TransactionCurrent object. That is, within your operations that write data, scope a transaction around the write statements by invoking the TransactionCurrent object to, respectively, begin and commit or roll back the transaction, if the object is not already scoped within a transaction. This ensures that any database write operations are handled transactionally. This also introduces a performance efficiency: if the object is not invoked within the scope of a transaction, all the database read operations are nontransactional, and, therefore, more streamlined.

Note: Some XA resource managers used in the WebLogic Enterprise system require that any object participating in a transaction scope their database read operations, in addition to write operations, within a transaction. (However, you can still scope your own transactions.) For example, using the Oracle7 TMS with the WebLogic Enterprise system has this requirement. When choosing the transaction policies to assign to your objects, make sure you are familiar with the requirements of the XA resource manager you are using.

Preventing an Object from Being Invoked While a Transaction Is Scoped

In many cases, it may be critical to exclude an object from a transaction. If such an object is invoked during a transaction, the object returns an exception, which may cause the transaction to be rolled back. The WebLogic Enterprise system provides the never transaction policy, which you can assign to an object's interface to specifically prevent that object from being invoked within the course of a transaction.

This transaction policy is appropriate for objects that write durable state to disk that cannot be rolled back; for example, for an object that writes data to a disk that is not managed by an XA resource manager. Having this capability in your client/server application is crucial if the client application does not or cannot know if some of its invocations are causing a transaction to be scoped. Therefore, if a transaction is scoped, and an object with this policy is invoked, the transaction can be rolled back.

To prevent an object from being invoked while a transaction is scoped, assign the following policies to that object's interface in the XML-based Server Description File:


Activation Policy	Transaction Policy
`process` or `method`	`never`

Excluding an Object from an Ongoing Transaction

In some cases, it may be appropriate to permit an object to be invoked during the course of a transaction but also keep that object from being a part of the transaction. If such an object is invoked during a transaction, the transaction is automatically suspended. After the invocation on the object is completed, the transaction is automatically resumed. The WebLogic Enterprise system provides the ignore transaction policy for this purpose.

The ignore transaction policy may be appropriate for an object such as a factory that typically does not write data to disk. By excluding the factory from the transaction, the factory can be available to other client invocations during the course of a transaction. In addition, using this policy can introduce an efficiency into your server application because it minimizes the overhead of invoking objects transactionally.

To prevent any transaction from being propagated to an object, assign the following policies to that object's interface in the Server Description File:


Activation Policy	Transaction Policy
`process` or `method`	`ignore`

com.beasys.Tobj.TP.close_xa_rm();

Transactions and Object State Management

If you need transactions in your WebLogic Enterprise client and server application, you can integrate transactions with object state management in a few different ways. In general, the WebLogic Enterprise system can automatically scope the transaction for the duration of an operation invocation without requiring you to make any changes to your application's logic or the way in which the object writes durable state to disk.

The following sections address some key points regarding transactions and object state management.

Delegating Object State Management to an XA Resource Manager

Using an XA resource manager, such as Oracle7, generally simplifies the design problems associated with handling object state data in the event of a rollback. Transactional objects can always delegate the commit and rollback responsibilities to the XA resource manager, which greatly eases the task of implementing a server application. This means that process- or method-bound objects involved in a transaction can write to a database during transactions, and can depend on the resource manager to undo any data written to the database in the event of a transaction rollback.

Waiting Until Transaction Work Is Complete Before Writing to the Database

The transaction activation policy is a good choice for objects that maintain state in memory that you do not want written, or that cannot be written, to disk until the transaction work is complete. When you assign the transaction activation policy to an object, the object:

Is brought into memory when it is first invoked within the scope of a transaction
Remains in memory until the transaction is either committed or rolled back

When the transaction work is complete, the WebLogic Enterprise system invokes each transaction-bound object's com.beasys.Tobj_Servant.deactivate_object method, passing a reason code that can be either DR_TRANS_COMMITTING or DR_TRANS_ABORTED. If the variable is DR_TRANS_COMMITTING, the object can invoke its database write operations. If the variable is DR_TRANS_ABORTED, the object skips its write operations.

Assigning the transaction activation policy to an object may be appropriate in the following situations:

You want the object to write its durable state to disk at the time that the transaction work is complete.

This introduces a performance efficiency because it reduces the number of database write operations that may need to be rolled back.
You want to provide the object with the ability to veto a transaction that is about to be committed.

If the WebLogic Enterprise system passes the reason DR_TRANS_COMMITTING, the object can, if necessary, invoke the rollback_only method on the TransactionCurrent object. Note that if you do make an invocation to the rollback_only method from within the com.beasys.Tobj_Servant.deactivate_object method, the deactivate_object method is not invoked again.
You have an object that is likely to be invoked multiple times during the course of a single transaction, and you want to avoid the overhead of continually activating and deactivating the object during that transaction.

To give an object the ability to wait until the transaction is committing before writing to a database, assign the following policies to that object's interface in the XML-based Server Description File:


Activation Policy	Transaction Policy
`transaction`	`always` or `optional`

Note: Transaction-bound objects cannot start a transaction or invoke other objects from inside the com.beasys.Tobj_Servant.deactivate_object method. The only valid invocations transaction-bound objects can make inside the deactivate_object method are write operations to the database. Also, if you have an object that is involved in a transaction, the Server object that manages that object must include invocations to open and close the XA resource manager, even if the object does not write any data to disk. For more information about opening and closing an XA resource manager, see the sections "Opening an XA Resource Manager" on page 3-9 and "Closing an XA Resource Manager" on page 3-10.

Notes on Using Transactions in the WebLogic Enterprise System

Note the following about integrating transactions into your WebLogic Enterprise client/server applications:

The following transactions are not permitted in the WebLogic Enterprise system:
- Nested transactions
You cannot start a new transaction if an existing transaction is already active. (You may start a new transaction if you first suspend the existing one; however, the object that suspends the transaction is the only object that can subsequently resume the transaction.)
- Recursive transactions
A transactional object cannot call a second object, which in turn calls the first object.
The object that starts a transaction is the only entity that can end the transaction. (In a strict sense, the object can be the client application, the TP Framework, or an object managed by the server application.) An object that is invoked within the scope of a transaction may suspend and resume the transaction (and while the transaction is suspended, the object can start and end other transactions). However, you cannot end a transaction in an object unless you began the transaction there.
Objects can be involved with only one transaction at one time. The WebLogic Enterprise system does not support concurrent transactions.
The WebLogic Enterprise system does not queue requests to objects that are currently involved in a transaction. If a nontransactional client application attempts to invoke an operation on an object that is currently in a transaction, the client application receives the following error message:
```
org.omg.CORBA.OBJ_ADAPTER
```
If a client that is in a transaction attempts to invoke an operation on an object that is currently in a different transaction, the client application receives the following error message:
```
org.omg.CORBA.INVALID_TRANSACTION
```
For transaction-bound objects, you might consider doing all state handling in the com.beasys.Tobj_Servant.deactivate_object method. This makes it easier for the object to handle its state properly, since the outcome of the transaction is known at the time that the com.beasys.Tobj_Servant.deactivate_object method is invoked.
For method-bound objects that have several operations, but only a few that affect the object's durable state, you may want to consider the following:
- Assign the optional transaction policy.
- Scope any write operations within a transaction, by making invocations on the TransactionCurrent object.
  
  If the object is invoked outside a transaction, the object does not incur the overhead of scoping a transaction for reading data. This way, regardless of whether the object is invoked within a transaction, all the object's write operations are handled transactionally.
Transaction rollbacks are asynchronous. Therefore, it is possible for an object to be invoked while its transactional context is still active. If you try to invoke such an object, you receive an exception.
If an object with the always transaction policy is involved in a transaction that is started by the WebLogic Enterprise system, and not the client application, note the following:

If an exception is raised inside an operation on that object, the client application receives an OBJ_ADAPTER exception. In this situation, the WebLogic Enterprise system automatically rolls back the transaction. However, the client application is completely unaware that a transaction has been scoped in the WebLogic Enterprise domain.
If the client application initiates a transaction, and the server application marks the transaction for a rollback and returns a CORBA exception, the client application receives only a transaction rollback exception but not the CORBA exception.

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Last update: July 01, 1999.