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Transactions in CORBA Server Applications

This topic includes the following sections:

These sections describe how to integrate transactions into a WebLogic Enterprise (WLE) server application. Before you begin, you should read Introducing Transactions.

Integrating Transactions in a WLE Client and Server Application

This topic includes the following sections:

Transaction Support in CORBA Applications

WLE 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 Transactions in CORBA Client Applications.
You can assign transactional policies to an object's interface so that when the object is invoked, the WLE 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 rollback_only() 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 WLE 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 (in Java) or Tobj_ServantBase::deactivate_object() operation (in C++) and passing a reason value.
When an object is polled, the object may veto the current transaction by invoking rollback_only() 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 vetoes the current transaction, the transaction is committed.

The following sections explain how you can use object activation policies and transaction policies to determine 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 (in Java), or the TP::open_xa_rm() and TP::close_xa_rm() operations (in C++). For more information about database connections, see Opening an XA Resource Manager.

Making an Object Automatically Transactional

The WLE system provides the always transactional policy, which you can define on an object's interface to have the WLE 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 WLE 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 WLE system automatically invokes the TransactionCurrent object on behalf of the server application.

Assign the always transactional policy to an object's interface 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 (in Java) or Implementation Configuration File (in C++):

Activation Policies

Transaction Policy

process
method
transaction

always

Activation Policies	Transaction Policy
process method transaction	always

Note: Database cursors cannot span transactions. However, in C++, the CourseSynopsisEnumerator object in the WLE 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 they are specified in the XML-based Server Description File (in Java) or Implementation Configuration File (in C++) for that object's interface, to make an object optionally transactional:

Activation Policies

Transaction Policy

process
method
transaction

optional

Activation Policies	Transaction Policy
process method 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: 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. For example, some XA Resource Managers (such as the Oracle 7 Transaction Manager Server) require that any object participating in a transaction scope their database read operations, in addition to write operations, within a transaction (you can still scope your own transactions, however). Other Resource Managers, such as Oracle8i, do not require a transaction context for read and write operations. If an application attempts a write operation without a transaction context, Oracle8i will start a local transaction implicitly, in which case the application needs to commit the local transaction explicitly.

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 WLE 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, such as 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 (in Java) or Implementation Configuration File (in C++):

Activation Policies

Transaction Policy

process
method

never

Activation Policies	Transaction Policy
process 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 WLE 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 (in Java) or Implementation Configuration File (in C++):

Activation Policies

Transaction Policy

process
method

ignore

Activation Policies	Transaction Policy
process method	ignore

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

C++:

TP::close_xa_rm();

Transactions and Object State Management

This topic includes the following sections:

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

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. (The Oracle7 Resource Manager is used in the WLE University sample C++ applications). Transactional objects can always delegate the commit and rollback responsibilities to the XA Resource Manager, which greatly simplifies 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 WLE system invokes each transaction-bound object's com.beasys.Tobj_Servant.deactivate_object method (in Java) or Tobj_ServantBase::deactivate_object() operation (in C++), 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.

When to Assign the Transaction Activation Policy

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

You want the object to write its persistent 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 WLE system passes the reason DR_TRANS_COMMITTING , the object can, if necessary, invoke rollback_only() on the TransactionCurrent object. Note that if you do make an invocation to rollback_only() from within the com.beasys.Tobj_Servant.deactivate_object method (in Java) or Tobj_ServantBase::deactivate_object() operation (in C++), then deactivate_object() is not invoked again.
You want to provide the object with the ability to perform batch updates.
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.

Transaction Policies to Use with the Transaction Activation Policy

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 (in Java) or Implementation Configuration File (in C++):

Activation Policy

Transaction Policy

transaction

always or optional

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 (in Java) or Tobj_ServantBase::deactivate_object() operation (in C++). The only valid invocations transaction-bound objects can make inside deactivate_object() 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. (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 Opening an XA Resource Manager and Closing an XA Resource Manager.

User-Defined Exceptions

This topic includes the following sections:

About User-Defined Exceptions

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

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

For example, the Transactions sample C++ 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 WLE client/server application, using the TooManyCredits exception as an example.

Defining the Exception

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

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:
exception TooManyCredits
{
unsigned short maximum_credits;
};
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:
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 C++ example.

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

How the Transactions University Sample Application Works (C++ Only)

This topic includes the following sections:

About the Transactions University Sample Application

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

The client application obtains a reference to the TransactionCurrent object from the Bootstrap object.
When the student submits the list of courses for which he or she wants to register, the client application:
1. Begins a transaction by invoking the Current::begin() operation on the TransactionCurrent object
2. Invokes the register_for_courses() operation on the Registrar object, passing a list of courses
The register_for_courses() operation on the Registrar object processes the registration request by executing a loop that does the following iteratively for each course in the list:
1. Checks to see how many credits the student is already registered for
2. Adds the course to the list of courses for which the student is registered
  The Registrar object checks for the following potential problems, which prevent the transaction from being committed:
As defined in the application's OMG IDL, the register_for_courses() operation returns a parameter to the client application, NotRegisteredList , which contains a list of the courses for which the registration failed.
If the NotRegisteredList value is empty, the client application commits the transaction.
If the NotRegisteredList value contains any courses, the client application queries the student to indicate whether he or she wants to complete the registration process for the courses for which the registration succeeded. If the user chooses to complete the registration, the client application commits the transaction. If the user chooses to cancel the registration, the client application rolls back the transaction.
If the registration for a course has failed because the student exceeds the maximum number of credits he or she can take, the Registrar object returns a TooManyCredits exception to the client application, and the client application rolls back the entire transaction.

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 Integrating Transactions in a WLE Client and Server Application. The model is as follows:

The client begins a transaction by invoking the begin() operation on the TransactionCurrent object, followed by making an invocation to the register_for_courses() operation on the Registrar object.
The Registrar object registers the student for the courses for which it can, and then returns a list of courses for which the registration process was unsuccessful. The client application can choose to commit the transaction or roll it back. The transaction encapsulates this conversation between the client and the server application.
The register_for_courses() operation performs multiple checks of the University database. If any one of those checks fail, the transaction can be rolled back.

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 (assign the always transaction policy to this object's interface). If a transaction has not already been scoped when this object is invoked, the WLE 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

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

Activation Policy	Transaction Policy
process	ignore

Using an XA Resource Manager in the Transactions Sample Application

The Transactions sample application uses the Oracle7 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:

Some XA Resource Managers, such as Oracle7, require that all database operations be scoped within a transaction. This means that the CourseSynopsisEnumerator object needs to be scoped within a transaction because this object reads from a database.
When a transaction is committed or rolled back, the XA Resource Manager automatically handles the durable state implied by the commit or rollback. That is, if the transaction is committed, the XA Resource Manager ensures that all database updates are made permanent. Likewise, if there is a rollback, the XA Resource Manager automatically restores the database to its state prior to the beginning of the transaction.
This characteristic of XA Resource Managers actually makes the design problems associated with handling object state data in the event of a rollback much simpler. Transactional objects can always delegate the commit and rollback responsibilities to the XA Resource Manager, which greatly simplifies the task of implementing a server application.

Configuration Requirements for the Transactions Sample Application

The University sample applications use an Oracle7 Transaction Manager Server (TMS). To use the Oracle7 database, you must include specific Oracle-provided files in the server application build process. For more information about building, configuring, and running the Transactions sample application, see Transactions Sample CORBA C++ XA Application. For more information about the configurable settings in the UBBCONFIG file, see Modifying the UBBCONFIG File to Accommodate Transactions.