Chapter 11 Transactions
By enclosing one or more
steps in an indivisible unit of work, a transaction ensures data integrity and consistency.
This chapter contains the following sections:
About Transactions
What is a Transaction?
A transaction is a series of discreet actions in an application that must all
complete successfully or else all the changes in each action are backed out. For example,
to transfer funds from a checking account to a savings account is a transaction with
the following steps:
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Check to see if the checking account has enough money to cover the
transfer.
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If there’s enough money in the checking account debit the amount
from the checking account.
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Credit the money to the savings account.
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Record the transfer to the checking account log.
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Record the transfer to the savings account log.
If any of these steps fails, all changes from the preceding steps must be backed
out, and the checking account and savings account must be in the same state as they
were before the transaction started. This event is called a rollback. If all the steps complete successfully,
the transaction is in a committed state. Transactions end in
either a commit or a rollback.
Transactions in J2EE Technology
Transaction processing in J2EE technology involves the following five participants:
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Transaction Manager
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Application Server
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Resource
Manager(s)
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Resource
Adapter(s)
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User Application.
Each of these entities contribute to reliable transaction processing by implementing
the different APIs and functionalities, discussed below:
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The Transaction Manager provides the services and management functions
required to support transaction demarcation, transactional resource management, synchronization,
and transaction context propagation.
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The Application Server provides the infrastructure required to support
the application runtime environment that includes transaction state management.
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The Resource Manager (through a resource adapter) provides the application
access to resources. The resource manager participates in distributed transactions by
implementing a transaction resource interface used by the transaction manager to communicate transaction
association, transaction completion and recovery work. An example of such a resource
manager is a relational database server.
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A Resource Adapter is a system level software library that is used
by the application server or client to connect to a Resource Manager. A Resource Adapter
is typically specific to a Resource Manager. It is available as a library and is used
within the address space of the client using it. An example of such a resource adapter
is a JDBC
driver.
-
A Transactional User Application developed to operate in an application
server environment looks up transactional data sources and, optionally, the transaction
manager, using JNDI. The application may use declarative transaction attribute settings
for enterprise beans or explicit programmatic transaction demarcation.
Admin Console Tasks for Transactions
The Application Server handles transactions based on the settings in the Admin Console.
Configuring Transactions
This section explains how to configure transaction settings:
For additional information about transactions, see these sections:
To configure how the Application Server recovers from transactions
Transactions might be incomplete either because the server crashed or a resource
manager crashed. It is essential to complete these stranded transactions and recover
from the failures. Application Server is designed to recover from these failures and complete
the transactions upon server startup.
While performing the recovery, if some of the resources are unreachable the
server restart may be delayed as it tries to recover the transactions.
When the transaction spans across servers, the server that started the transaction
can contact the other servers to get the outcome of the transactions. If the other
servers are unreachable, the transaction uses the Heuristic Decision field to determine
the outcome.
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In the tree component select the Configurations node.
-
Select the instance
to configure:
-
To configure a particular instance, select the instance’s config
node. For example, the default instance, server, select the server-config node.
-
To configure the default settings for all instances, select the default-config node.
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Select the Transaction Service node.
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To enable the recovery of incomplete transactions, check the Recover in
the On Restart field.
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Set the amount of time, in seconds, the Application Server tries to connect
to the unreachable server in the Retry Timeout field. The default value is 10 minutes
(600 seconds).
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Set the policy for unreachable servers in a transaction in the Heuristic
Decision field.
Unless there is a good reason to set this field to Commit,
leave Heuristic Decision set to Rollback. Committing indeterminate transactions can
compromise the data integrity of your application.
-
Click Save.
-
Restart the Application Server.
To set a transaction timeout value
By default, the server does not timeout a transaction. That is, the server waits
indefinitely for a transaction to complete. If you set a timeout value for transactions,
if a transaction isn’t completed within the configured time, the Application Server rolls
back the transaction.
-
In the tree component, select the Configurations node.
-
Select the instance
to configure:
-
To configure a particular instance, select the instance’s config
node. For example, the default instance, server, select the server-config node.
-
To configure the default settings for all instances, select the default-config node.
-
Select the Transaction Service node.
-
Enter the number of seconds before the transaction times out, in the Transaction
Timeout field.
The default value of Transaction Timeout is 0 seconds.
This disables transaction timeouts.
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Click Save.
-
Restart the Application Server.
To set the location of the transaction logs
The transaction log records the information about each transaction in order
to maintain the data integrity of the resources involved and to recover from failures.
Transaction logs are kept in the tx subdirectory of the directory
specified by the Transaction Log Location field. These logs are not human readable.
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In the tree component, select the Configurations node.
-
Select the instance
to configure:
-
To configure a particular instance, select the instance’s config
node. For example, the default instance, server, select the server-config node.
-
To configure the default settings for all instances, select the default-config node.
-
Select the Transaction Service node.
-
Enter the location of the transaction logs in the Transaction Log Location
field.
A tx subdirectory is created and transaction
logs are kept under that directory.
The default value
is ${com.sun.aas.instanceRoot}/logs. The ${com.sun.aas.instanceRoot} variable is the name of the instance, and is set when you start an Application Server instance.
To see the value of ${com.sun.aas.instanceRoot}, click Actual Values.
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Click Save.
-
Restart the Application Server.
To set the keypoint interval
Keypoint operations
compress the transaction log file. The keypoint interval is the number of transactions between keypoint operations
on the log. Keypoint operations can reduce the size of the transaction log files.
A larger number of keypoint intervals (for example, 2048) results in larger transaction
log files, but fewer keypoint operations, and potentially better performance. A smaller
keypoint interval (for example, 256) results in smaller log files but slightly reduced
performance due to the greater frequency of keypoint operations.
-
In the tree component select the Configurations node.
-
Select the instance
to configure:
-
To configure a particular instance, select the instance’s config
node. For example, the default instance, server, select the server-config node.
-
To configure the default settings for all instances, select the default-config node.
-
Select the Transaction Service node.
-
Enter the number of transactions between keypoint operations in the Keypoint
Interval field.
The default value is 2048.
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Click Save.
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Restart the Application Server.
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