X/Open distributed transaction processing model

Application program (AP)

The application program (AP) defines transaction boundaries, and defines the application-specific actions that make up the transaction.

For example, a CICS® application program might want to access resource managers (RMs), such as a database and a CICS Transient Data Queue, and use programming logic to manipulate the data. Each access request is passed to the appropriate resource managers through function calls specific to that RM. In the case of Db2® products, these could be function calls generated by the Db2 database precompiler for each SQL statement, or database calls coded directly by the programmer using the APIs.

A transaction manager (TM) product usually includes a transaction processing (TP) monitor to run the user application. The TP monitor provides APIs to allow an application to start and end a transaction, and to perform application scheduling and load balancing among the many users who want to run the application. The application program in a distributed transaction processing (DTP) environment is really a combination of the user application and the TP monitor.

To facilitate an efficient online transaction processing (OLTP) environment, the TP monitor pre-allocates a number of server processes at startup, and then schedules and reuses them among the many user transactions. This conserves system resources, by allowing more concurrent users to be supported with a smaller number of server processes and their corresponding RM processes. Reusing these processes also avoids the overhead of starting up a process in the TM and RMs for each user transaction or program. (A program invokes one or more transactions.) This also means that the server processes are the actual "user processes" to the TM and the RMs. This has implications for security administration and application programming.

The following types of transactions are possible from a TP monitor:

• Non-XA transactions

  These transactions involve RMs that are not defined to the TM, and are therefore not coordinated under the two-phase commit protocol of the TM. This might be necessary if the application needs to access an RM that does not support the XA interface. The TP monitor simply provides efficient scheduling of applications and load balancing. Since the TM does not explicitly "open" the RM for XA processing, the RM treats this application as any other application that runs in a non-DTP environment.

• Global transactions

  These transactions involve RMs that are defined to the TM, and are under the TM's two-phase commit control. A global transaction is a unit of work that could involve one or more RMs. A transaction branch is the part of work between a TM and an RM that supports the global transaction. A global transaction could have multiple transaction branches when multiple RMs are accessed through one or more application processes that are coordinated by the TM.

  Loosely coupled global transactions exist when each of a number of application processes accesses the RMs as if they are in a separate global transaction, but those applications are under the coordination of the TM. Each application process will have its own transaction branch within an RM. When a commit or rollback is requested by any one of the APs, TM, or RMs, the transaction branches are completed altogether. It is the application's responsibility to ensure that resource deadlock does not occur among the branches. (Note that the transaction coordination performed by the Db2 transaction manager for applications prepared with the SYNCPOINT(TWOPHASE) option is roughly equivalent to these loosely coupled global transactions.

  Tightly coupled global transactions exist when multiple application processes take turns to do work under the same transaction branch in an RM. To the RM, the two application processes are a single entity. The RM must ensure that resource deadlock does not occur within the transaction branch.

Transaction manager (TM)

The transaction manager (TM) assigns identifiers to transactions, monitors their progress, and takes responsibility for transaction completion and failure. The transaction branch identifiers (known as XIDs) are assigned by the TM to identify both the global transaction, and the specific branch within an RM. This is the correlation token between the log in a TM and the log in an RM. The XID is needed for two-phase commit, or rollback, to perform the resynchronization operation (also known as a resync) on system startup, or to let the administrator perform a heuristic operation (also known as manual intervention), if necessary.

After a TP monitor is started, it asks the TM to open all the RMs that a set of application servers have defined. The TM passes xa_open calls to the RMs, so that they can be initialized for DTP processing. As part of this startup procedure, the TM performs a resync to recover all indoubt transactions. An indoubt transaction is a global transaction that was left in an uncertain state. This occurs when the TM (or at least one RM) becomes unavailable after successfully completing the first phase (that is, the prepare phase) of the two-phase commit protocol. The RM will not know whether to commit or roll back its branch of the transaction until the TM can reconcile its own log with the RM logs when they become available again. To perform the resync operation, the TM issues a xa_recover call one or more times to each of the RMs to identify all the indoubt transactions. The TM compares the replies with the information in its own log to determine whether it should inform the RMs to xa_commit or xa_rollback those transactions. If an RM has already committed or rolled back its branch of an indoubt transaction through a heuristic operation by its administrator, the TM issues an xa_forget call to that RM to complete the resync operation.

When a user application requests a commit or a rollback, it must use the API provided by the TP monitor or TM, so that the TM can coordinate the commit and rollback among all the RMs involved. For example, when a WebSphere® application issues a request to commit a transaction, the WebSphere XA TM will in turn issue XA calls, such as xa_end, xa_prepare, xa_commit, or xa_rollback to request the RM to commit or roll back the transaction. The TM could choose to use one-phase instead of two-phase commit if only one RM is involved, or if an RM replies that its branch is read-only.

Resource managers (RM)

A resource manager (RM) provides access to shared resources, such as databases.

The Db2 system, as resource manager of a database, can participate in a global transaction that is being coordinated by an XA-compliant TM. As required by the XA interface, the database manager provides a db2xa_switch external C variable of type xa_switch_t to return the XA switch structure to the TM. This data structure contains the addresses of the various XA routines to be invoked by the TM, and the operating characteristics of the RM.

There are two methods by which the RM can register its participation in each global transaction: static registration and dynamic registration:

• Static registration requires the TM to issue (for every transaction) the xa_start, xa_end, and xa_prepare series of calls to all the RMs defined for the server application, regardless of whether a given RM is used by the transaction. This is inefficient if not every RM is involved in every transaction, and the degree of inefficiency is proportional to the number of defined RMs.
• Dynamic registration (used by Db2) is flexible and efficient. An RM registers with the TM using an ax_reg call only when the RM receives a request for its resource. Note that there is no performance disadvantage with this method, even when there is only one RM defined, or when every RM is used by every transaction, because the ax_reg and the xa_start calls have similar paths in the TM.

The XA interface provides two-way communication between a TM and an RM. It is a system-level interface between the two DTP software components, not an ordinary application program interface to which an application developer codes. However, application developers should be familiar with the programming restrictions that the DTP software components impose.

Although the XA interface is invariant, each XA-compliant TM may have product-specific ways of integrating an RM. For information about integrating your Db2 product as a resource manager with a specific transaction manager, see the appropriate TM product documentation.