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Integrating Transactions into a CORBA Server Application

This chapter describes how to integrate transactions into a CORBA server application, using the Transactions University sample application as an example. The Transactions sample application encapsulates the process of a student registering for a set of courses. The Transactions sample application does not show all the possible ways to integrate transactions into a CORBA server application, but it does show two models of transactional behavior, showing the impact of transactional behavior on the application in general and on the durable state of objects in particular.

This topic includes the following sections:

This chapter also presents a section on user-defined exceptions. The Transactions sample application introduces a user-defined exception, which can be returned to the client application and that potentially causes a client-initiated transaction to be rolled back.

 

Overview of Transactions in the BEA Tuxedo System

The BEA Tuxedo 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 BEA Tuxedo system.

 

Designing and Implementing Transactions in a CORBA Server Application

This section explains how to design and implement transactions in a CORBA server application using the Transactions University sample application as an example. This section also describes how the Transactions sample application works, and discusses the design considerations for implementing transactions in it. For additional general information about transactions, see the section "Integrating Transactions in a CORBA Client and Server Application" on page -9.

The Transactions sample application uses transactions to encapsulate the task of a student registering for a set of courses. The transactional model used in this application is a combination of the conversational model and the model in which a single invocation makes multiple individual operations on a database, as described in the preceding section.

The Transactions sample application builds on the Security sample application by adding the following capabilities:

The Transactions sample application shows two ways in which a transaction can be rolled back:

The sections that follow explain:

How the Transactions University Sample Application Works

To implement the student registration process, the Transactions sample application does the following:

Transactional Model Used by the Transactions University Sample Application

The basic design rationale for the Transactions sample application is to handle course registrations in groups, as opposed to one at a time. This design helps to minimize the number of remote invocations on the Registrar object.

In implementing this design, the Transactions sample application shows one model of the use of transactions, which were described in the section Overview of Transactions in the BEA Tuxedo System on page 6-2. The model is as follows:

Object State Considerations for the University Server Application

Because the Transactions University sample application is transactional, the University server application generally needs to consider the implications on object state, particularly in the event of a rollback. In cases where there is a rollback, the server application must ensure that all affected objects have their durable state restored to the proper state.

Because the Registrar object is being used for database transactions, a good design choice for this object is to make it transactional; that is, assign the always transaction policy to this object's interface. If a transaction has not already been scoped when this object is invoked, the BEA Tuxedo system will start a transaction automatically.

By making the Registrar object automatically transactional, all database write operations performed by this object will always be done within the scope of a transaction, regardless of whether the client application starts one. Since the server application uses an XA resource manager, and since the object is guaranteed to be in a transaction when the object writes to a database, the object does not have any rollback or commit responsibilities because the XA resource manager takes responsibility for these database operations on behalf of the object.

The RegistrarFactory object, however, can be excluded from transactions because this object does not manage data that is used during the course of a transaction. By excluding this object from transactions, you minimize the processing overhead implied by transactions.

Object Policies Defined for the Registrar Object

To make the Registrar object transactional, the ICF file specifies the always transaction policy for the Registrar interface. Therefore, in the Transaction sample application, the ICF file specifies the following object policies for the Registrar interface:

Activation Policy

Transaction Policy

process

always


 

Object Policies Defined for the RegistrarFactory Object

To exclude the RegistrarFactory object from transactions, the ICF file specifies the ignore transaction policy for the Registrar interface. Therefore, in the Transaction sample application, the ICF file specifies the following object policies for the RegistrarFactory interface:

Activation Policy

Transaction Policy

process

ignore


 

Using an XA Resource Manager in the Transactions Sample Application

The Transactions sample application uses the Oracle Transaction Manager Server (TMS), which handles object state data automatically. Using any XA resource manager imposes specific requirements on how different objects managed by the server application may read and write data to that database, including the following:

Configuration Requirements for the Transactions Sample Application

The University sample applications use an Oracle transaction manager server (TMS). To use an Oracle database, you must include specific Oracle-provided files in the server application build process.

For details about building, configuring, and running the Transactions sample application, see the Guide to the CORBA University Sample Applications. This online document also contains the UBBCONFIG files for each sample application and explains the entries in that file.

 

Integrating Transactions in a CORBA Client and Server Application

The BEA Tuxedo system supports transactions in the following ways:

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 TP::open_xa_rm() and TP::close_xa_rm() operations. For more information about database connections, see Opening an XA Resource Manager on page 6-13.

Making an Object Automatically Transactional

The BEA Tuxedo system provides the always transactional policy, which you can define on an object's interface to have the BEA Tuxedo 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 BEA Tuxedo 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 BEA Tuxedo 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:

If you want an object to be automatically transactional, assign the following policies to that object's interface in the Implementation Configuration File (ICF file):

Activation Policy

Transaction Policy

process, method, or transaction

always


 

Note: Database cursors cannot span transactions. The CourseSynopsisEnumerator object in the CORBA University sample applications uses a database cursor to find matching course synopses from the University database. Because database cursors cannot span transactions, the activate_object() operation on the CourseSynopsisEnumerator object reads all matching course synopses into memory. Note that the cursor is managed by an iterator class and is thus not visible to the CourseSynopsisEnumerator object.

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 specified in the ICF file for that object's interface, to make an object optionally transactional:

Activation Policy

Transaction Policy

process, method, or transaction

optional


 

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 BEA Tuxedo 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 an Oracle TMS with the BEA Tuxedo 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 BEA Tuxedo 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, even if the current transaction is suspended.

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 ICF 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 BEA Tuxedo 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 ICF file:

Activation Policy

Transaction Policy

process or method

ignore


 

Assigning Policies

For information about how to create an ICF file and specify policies on objects, see the section Step 4: Define the In-memory Behavior of Objects on page 2-12.

Opening an XA Resource Manager

If an object's interface has the always or optional transaction policy, you must invoke the TP::open_xa_rm() operation in the Server::initialize() operation in the Server object. The resource manager is opened using the information provided in the OPENINFO parameter, which is in the GROUPS section of the UBBCONFIG file. Note that the default version of the Server::initialize() operation automatically opens the resource manager.

If you have an object that does not write data to disk and that participates in a transaction—the object typically has the optional transaction policy—you still need to include an invocation to the TP::open_xa_rm() operation. In that invocation, specify the NULL resource manager.

Closing an XA Resource Manager

If your Server object's Server::initialize() operation opens an XA resource manager, you must include the following invocation in the Server::release() operation:

TP::close_xa_rm();

 

Transactions and Object State Management

If you need transactions in your CORBA client and server application, you can integrate transactions with object state management in a few different ways. In general, the BEA Tuxedo 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 an object state management.

Delegating Object State Management to an XA Resource Manager

Using an XA resource manager, such as Oracle which is used in the CORBA University sample applications, 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:

When the transaction work is complete, the BEA Tuxedo system invokes each transaction-bound object's Tobj_ServantBase::deactivate_object() operation, passing a reason code that can be either DR_TRANS_COMMITTING or DR_TRANS_ABORT. If the variable is DR_TRANS_COMMITTING, the object can invoke its database write operations. If the variable is DR_TRANS_ABORT, the object skips its write operations.

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

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 ICF file:

Activation Policy

Transaction Policy

transaction

always or optional


 

Note: Transaction-bound objects cannot start a transaction or invoke other objects from inside the Tobj_ServantBase::deactivate_object() operation. The only valid invocations transaction-bound objects can make inside the Tobj_ServantBase::deactivate_object() operation 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, respectively, the XA resource manager, even if the object does not write any data to disk. (If you have a transactional object that does not write data to disk, you specify the NULL resource manager.) For more information about opening and closing an XA resource manager, see the sections Opening an XA Resource Manager on page 6-13 and Closing an XA Resource Manager on page 6-13.

 

Notes on Using Transactions in the BEA Tuxedo System

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

Note: In the WebLogic Enterprise version 4.2 software, no workaround exists for this situation. We recommend that applications perform as much data validation as possible before starting a transaction.

 

User-defined Exceptions

The Transactions sample application includes an instance of a user-defined exception, TooManyCredits. This exception is thrown by the server application when the client application tries to register a student for a course, and the student has exceeded the maximum number of courses for which he or she can register. When the client application catches this exception, the client application rolls back the transaction that registers a student for a course. This section explains how you can define and implement user-defined exceptions in your CORBA client/server application, using the TooManyCredits exception as an example.

Including a user-defined exception in a CORBA client/server application involves the following steps:

  1. In your OMG IDL file, define the exception and specify the operations that can use it.
  2. In the implementation file, include code that throws the exception.
  3. In the client application source file, include code that catches and handles the exception.

The sections that follow explain and give examples of the first two steps.

Defining the Exception

In the OMG IDL file for your client/server application:

  1. Define the exception and define the data sent with the exception. For example, the TooManyCredits exception is defined to pass a short integer representing the maximum number of credits for which a student can register. Therefore, the definition for the TooManyCredits exception contains the following OMG IDL statements:
    2. exception TooManyCredits
    {
        unsigned short maximum_credits;
    };
  2. In the definition of the operations that throw the exception, include the exception. The following example shows the OMG IDL statements for the register_for_courses() operation on the Registrar interface:
    3. NotRegisteredList register_for_courses(
        in StudentId        student,
        in CourseNumberList courses)
     raises (TooManyCredits);

Throwing the Exception

In the implementation of the operation that uses the exception, write the code that throws the exception, as in the following example.

if ( ... ) {
    UniversityZ::TooManyCredits e;
    e.maximum_credits = 18;
    throw e;
    }

 

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