XA transaction support for commitment control

 

DB2 Universal Database™ (UDB) for iSeries™ can participate in X/Open global transactions.

The Open Group has defined an industry-standard model for transactional work that allows changes made against unrelated resources to be part of a single global transaction. An example of this is changes to databases that are provided by two separate vendors. This model is called the X/Open Distributed Transaction Processing model.

The following publications describe the X/Open Distributed Transaction Processing model in detail:

• X/Open Guide, February 1996, Distributed Transaction Processing: Reference Model, Version 3 (ISBN:1-85912-170-5, G504), The Open Group.

• X/Open CAE Specification, December 1991, Distributed Transaction Processing: The XA Specification (ISBN:1-872630-24-3, C193 or XO/CAE/91/300), The Open Group.

• X/Open CAE Specification, April 1995, Distributed Transaction Processing: The TX (Transaction Demarcation) Specification (ISBN:1-85912-094-6, C504), The Open Group.

Be familiar with the information in these books, particularly the XA Specification, before attempting to use the XA transaction support provided by DB2^® UDB for iSeries. You can find these books at the Open Group Web site.

There are five components to the DTP model:

Application Program (AP)

It implements the required function of the user by specifying a sequence of operations that involves resources such as databases. It defines the start and end of global transactions, accesses resources within transaction boundaries, and normally makes the decision whether to commit or roll back each transaction.

Transaction Manager (TM)

It manages global transactions and coordinates the decision to start them and commit them, or roll them back in order to ensure atomic transaction completion. The TM also coordinates recovery activities with the RMs after a component fails.

Resource Manager (RM)

It manages a defined part of the computer's shared resources, such as a database management system. The AP uses interfaces defined by each RM to perform transactional work. The TM uses interfaces provided by the RM to carry out transaction completion.

Communications Resource Manager (CRM)

It allows an instance of the model to access another instance either inside or outside the current TM domain. CRMs are outside the scope of DB2 UDB for iSeries and are not discussed here.

Communication Protocol

The protocols used by CRMs to communicate with each other. This is outside the scope of DB2 UDB for iSeries and is not discussed here.

The XA Specification is the part of the DTP model that describes a set of interfaces that is used by the TM and RM components of the DTP model. DB2 UDB for iSeries implements these interfaces as a set of UNIX^® platform-style APIs and exit programs. See XA APIs for detailed documentation of these APIs and for more information about how to use DB2 UDB for iSeries as an RM.

 

iSeries Navigator and XA transactions

iSeries Navigator supports the management of XA transactions as Global Transactions.

A Global Transaction might contain changes both outside and within DB2 UDB for iSeries. A global transaction is coordinated by an external Transaction Manager using the Open Group XA architecture, or another similar architecture. An application commits or rolls back a global transaction using interfaces provided by the Transaction Manager. The Transaction Manager uses commit protocols defined by the XA architecture, or another architecture, to complete the transaction. DB2 UDB for iSeries acts as an XA Resource Manager when participating in a global transaction. There are two types of global transactions:

• Transaction-scoped locks: Locks acquired on behalf of the transaction are scoped to the transaction. The transaction can move from one job or thread to another.

• Job-scoped locks: Locks acquired on behalf of the transaction are scoped to the job. The transaction cannot move from the job that started it.

 

Considerations for XA transactions

The XA APIs for transaction-scoped locks are recommended for new users of the XA transaction support. The XA APIs for job-scoped locks will continue to be supported, but no longer have any advantages over the XA APIs for transaction-scoped locks. The XA APIs for transaction-scoped locks have fewer restrictions and better performance in the following situations:

• If multiple SQL connections are ever used to work on a single XA transaction branch.

• If a single SQL connection is used to work on multiple, concurrent XA transaction branches.

In these situations, a separate job must be started to run XA transaction branches when you use the XA APIs for Job Scoped Locks.

Understand the following considerations and restrictions before using DB2 UDB for iSeries as a RM. The term thread refers to either a job that is not thread capable, or a single thread within a thread capable job.

The following considerations apply to both transactions with transaction-scoped locks and transactions with job-scoped locks unless noted otherwise.

 

DB2 UDB for iSeries considerations

• XA transactions against a local database must be performed in jobs that are running in SQL server mode, which means that applications are limited to SQL interfaces when making changes to DB2 UDB for iSeries during an XA transaction. For such transactions, if the xa_open() or db2xa_open() API is used in a job that is not already running in SQL server mode, SQL server mode is implicitly started.

• XA transactions against a remote database are required to use SQL server mode when you use the XA APIs for job-scoped locks. However, server mode is optional for XA transactions against a remote database when you use the XA APIs for transaction-scoped locks. Furthermore, changes to DDM files using traditional i5/OS^® database access methods are allowed within XA transactions against a remote database when SQL server mode is not used.

• Any errors that are detected by DB2 UDB for iSeries during the XA API invocations are reported through return codes by the XA specification. Diagnostic messages are left in the job log when the meaning of the error can not be clear from the return code alone.

 

Embedded SQL considerations

• In order to use a Structured Query Language (SQL) connection for XA transactions, use the xa_open() or db2xa_open() application programming interface (API) before the SQL connection is made. The relational database that will be connected to must be passed to the xa_open() or db2xa_open() API by the xainfo parameter. The user profile and password to be used in the job that the connection is routed to might be passed to the xa_open() or db2xa_open() API. If it is not passed, the profile uses the one that was specified or used as the default during the connection attempt.

  The following consideration applies only to transactions with job-scoped locks.

• If embedded SQL is used to perform XA transactions, the work performed for each connection is routed to a different job, even if the connections are made in the same thread. This is different than SQL server mode without XA, where work performed for all connections in a single thread is routed to the same job. This is because the XA specification requires a separate prepare, commit or rollback call for each resource manager instance.

  The following consideration applies only to transactions with job-scoped locks.

• If embedded SQL is used to perform XA transactions, only one connection per relational database can be made per thread. Whenever the thread is not actively associated with a transaction branch, work requested over one of the thread's connections will cause the RM to use the TM's ax_reg() exit program to determine whether the work is to start, resume or join a transaction branch.

  If the work is to start a transaction branch, it is performed over that thread's connection to the corresponding relational database.

  If the work is to join a transaction branch, it is rerouted over the connection to the corresponding relational database that was made in the thread that started the transaction branch. Note that the system does not enforce that the user profile for that connection is the same as the one for the connection of the joining thread. The TM is responsible to ensure that this is not a security concern. Typical TMs use the same user profile for all connections. This user profile is authorized to all data that is managed by the TM. Further security of access to this data is managed by the TM or AP instead of using the standard i5/OS security techniques.

  The following consideration applies only to transactions with job-scoped locks.

• If the work is to resume a transaction branch, the connection that is used depends on whether the suspended transaction branch association was established by starting or joining the transaction branch.

  Subsequent work is performed over the same connection until the db2xa_end() API is used to suspend or end the thread's association with that transaction branch.

 

CLI considerations

• If the CLI is used to perform XA transactions, more than one connection might be made in the same thread after the db2xa_open() API is used. The connections can be used in other threads to perform XA transactions, as long as those other threads first use the db2xa_open() API with the same xainfo parameter value.

  The following consideration applies only to transactions with job-scoped locks.

• If the CLI is used to perform XA transactions, the connection that is used to start a transaction branch must be used for all work on that transaction branch. If another thread is to join the transaction branch, the connection handle for the connection used to start the transaction branch must be passed to the joining thread so that it can perform work over that same connection. Likewise, if a thread is to resume the transaction branch, the same connection must be used.

  Because CLI connection handles cannot be used in a different job, the join function is limited to threads running in the same job that started the transaction branch when the CLI is used.

 

Remote relational database considerations

These considerations for a remote relational database apply only to transactions with job-scoped locks.

• XA connections to a remote relational database are supported only if the relational database resides on a system that supports Distributed Unit of Work (DUW) DRDA^® connections. This includes System i™ products that run Distributed Relational Database Architecture™ (DRDA) over SNA LU 6.2 conversations, or that use V5R1 or later when running DRDA using TCP/IP connections. This also includes other platforms that support DRDA over SNA LU 6.2 or that support the XA protocol using DRDA over TCP/IP.

• Before using the XA join function, the db2xa_open() API must be used in the joining thread. The same relational database name and RMID must be specified on the db2xa_open() API in both the thread that started the transaction branch and the joining thread. If the transaction branch is active when a join is attempted, the joining thread is blocked. The joining thread remains blocked until the active thread suspends or ends its association with the transaction branch.

 

Recovery consideration

• The manual heuristic commit and rollback support that is provided for all commitment definitions can be used if it becomes necessary to force a transaction branch to commit or roll back while it is in a prepared state.

 

Transaction branch considerations

• Information about XA transaction branches is shown as part of the commitment control information displayed by iSeries Navigator and the Work with Job (WRKJOB), Display Job (DSPJOB), and Work with Commitment Definition (WRKCMTDFN) commands. The TM name, transaction branch state, transaction identifier and branch qualifier are all shown. The commitment definitions related to all currently active XA transactions can be displayed by using the command WRKCMTDFN JOB(*ALL) STATUS(*XOPEN) or by displaying the Global Transactions in iSeries Navigator.

  The following item applies only to transactions with job-scoped locks.

• If an association between a thread and an existing transaction branch is suspended or ended using the db2xa_end() API, the thread might start a new transaction branch. If the connection used to start the new transaction branch was used earlier to start a different transaction branch and the thread's association with that transaction branch has been ended or suspended by the db2xa_end() API, a new SQL server job might be started. A new SQL server job is needed only if the first transaction branch has not yet been completed by the db2xa_commit() or db2xa_rollback() API. In this case, another completion message SQL7908 is sent to the job log identifying the new SQL server job, just as the connection's original SQL server job was identified when the connection was established. All SQL requests for the new transaction branch are routed to the new SQL server job. When the transaction branch is completed by the db2xa_commit() or db2xa_rollback() API, the new SQL server job is recycled and returned to the prestart job pool.

• A transaction branch is marked Rollback Only in the following situations only for the XA transactions for job-scoped locks:

  • A thread ends when it is still associated with the transaction branch. This includes a thread ending as the result of process termination.

  • The system fails.

• With XA transactions for transaction-scoped locks, a transaction branch is rolled back by the system if any threads are still associated with it when any of the following situations occur:

  • The connection that is related to the transaction branch is ended.

  • The job that started the transaction branch is ended.

  • The system fails.

  The following consideration applies only to transactions with job-scoped locks.

• There is one situation where a transaction branch will be rolled back by the system, regardless of whether there are still associated threads. This occurs when the SQL server job that the connection's work is being routed to is ended. This can only happen when the End Job (ENDJOB) CL command is used against that job.

• A transaction branch is not affected if no threads have an active association with it when any of the following situations occur. The TM can commit or roll back the transaction branch from any thread that has used the xa_open() or db2xa_open() API with the same xainfo parameter value that was specified in the thread that started the transaction branch.

  • The connection that is related to the transaction branch is ended.

  • A thread or job that performed work for the transaction branch uses the xa_close() or db2xa_close() API.

  • The system fails. In this case, the transaction branch is not affected only if it is in prepared state. If it is in idle state, the system rolls it back.

• When the transaction identifier (XID) of two XA transaction branches have the same global transaction identifier (GTRID), but different branch qualifiers (BQUALs), they are said to be loosely coupled. By default, loosely coupled transaction branches do not share locks. However, when using the XA APIs for transaction-scoped locks, there is an option that allows loosely coupled transactions to share locks.

 

Parent topic:

Commitment control concepts

 

Related concepts

Considerations for XA transactions The Open Group Web site
SQL server mode and thread-scoped transactions for commitment control

 

Related tasks

When to force commit and rollback operations and when to cancel resynchronization