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The WLE 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 WLE system.
Overview of Transactions in the WLE System
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.
For example, consider an internet-based online shopping application. The user of the client application browses through an online catalog and makes multiple purchase selections. When the user is done choosing all the items he or she wants to buy, the user clicks on a button to make the purchase, where the user may enter credit card information. If the credit card check fails (for example, the user cannot provide valid credit card information) the shopping application needs a way to cancel all the pending purchase selections or roll back any purchase transactions made during the conversation.
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:
If the credit invocation on the bank database fails, the banking application needs a way to roll back the previous debit invocation.
The WLE system supports two transaction API models:
Integrating Transactions in a WLE Client and Server Application
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 WLE Javadoc and the CORBA Java Programming Reference.
The WLE system supports transactions in the following ways:
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.
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.
Assigning the always
transactional policy to an object's interface is appropriate when:
Making an Object Automatically Transactional
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 |
---|---|
Note:
Database cursors cannot span transactions. For an example, see Creating C++ Server Applications.
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:
Enabling an Object to Participate in a Transaction
Activation Policy |
Transaction Policy |
---|---|
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 WLE 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 WLE 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.
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.
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:
Preventing an Object from Being Invoked While a Transaction Is Scoped
Activation Policy |
Transaction Policy |
---|---|
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:
Excluding an Object from an Ongoing Transaction
Activation Policy |
Transaction Policy |
---|---|
For information about how to create a Server Description File and specify policies on objects, see the section Step 5: Define the object activation and transaction policies..
The XA Bankapp sample application in the drive:\M3dir\samples\corba\bankapp_java\XA
directory uses the Oracle7 Transaction Manager Server (TMS) as an example of a relational database management service (RDBMS). TMS 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:
Assigning Policies
Using an XA Resource Manager
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.
If an object's interface has the always
or optional
transaction policy, you must invoke the com.beasys.Tobj.TP.open_xa_rm
method in the com.beasys.Tobj.Server.initialize
method in the Server object that supports this object. You must build a special version of the JavaServer by using the buildXAJS
command, if your object performs database operations.
In the SERVERS
section of the application's UBBCONFIG
file, you must use the JavaServerXA
element in place of JavaServer
to associate the XA resource manager with a specified server group. (JavaServer
uses the null RM.)
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 com.beasys.Tobj.Server.initialize
method automatically opens the resource manager.
If you have an object that participates in a transaction but does not actually perform database operations (the object typically has the optional
transaction policy), you still need to include an invocation to the com.beasys.Tobj.TP.open_xa_rm
method.
If your Server object's com.beasys.Tobj.Server.initialize
method opens an XA resource manager, you must include the following invocation in the com.beasys.Tobj.Server.release
method:
com.beasys.Tobj.TP.close_xa_rm(); 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.
The following sections address some key points regarding transactions and object state management.
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.
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:
Opening an XA Resource Manager
Closing an XA Resource Manager
Transactions and Object State Management
Delegating Object State Management to an XA Resource Manager
Waiting Until Transaction Work Is Complete Before Writing to the Database
When the transaction work is complete, the WLE 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:
This introduces a performance efficiency because it reduces the number of database write operations that may need to be rolled back.
If the WLE 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.
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 |
---|---|
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 and Closing an XA Resource Manager.
Note the following about integrating transactions into your WLE client/server applications:
Notes on Using Transactions in the WLE System
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.)
A transactional object cannot call a second object, which in turn calls the first object.
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
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.
If an exception is raised inside an operation on that object, the client application receives an OBJ_ADAPTER exception. In this situation, the WLE system automatically rolls back the transaction. However, the client application is completely unaware that a transaction has been scoped in the WLE domain.
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Copyright © 1999 BEA Systems, Inc. All rights reserved.
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