X/Open DTP (DTP)
The X/Open specification - also X/Open DTP - is an important and general specification for distributed transaction processing. The specification defines a model, system architecture, and interfaces for Distributed Transaction Processing (DTP). X/Open is an industry consortium as well as a standards body. The specification is supported by all major vendors of databases and transaction monitors to enable them to participate in distributed transactions.
The X/Open XA specification is based on the two- phase commit protocol( 2PC protocol). XA includes additional methods for committing and resuming a transaction as well as methods for concurrent use of a transaction by multiple processes. Development of the specification began in the 1980s and its first version was released in 1991.
The development of X/Open DTP
The model developed first on the basis of older transaction monitors and the thereby aimed goal was the unification of the interfaces between the components and the application. For the distributed processing of transactions, X/Open created an overarching framework that combines several standards for the various subject areas. The standardization of system interfaces for distributed transaction processing is based by the X/Open on the model of a distributed transaction system shown in the adjacent figure.
There the components of a so-called centralized transaction system are visible in each system involved. These include a transaction manager( TM), various resource managers( RM) and the respective application programs. In the case of a distributed transaction model, a communications manager( CM) is added for each node, which maps a special resource manager (RM) for the realization of the communication processes. The essential aspect of standardization is the definition of the interfaces between the components. The resource managers provide access to shared resources such as databases or general communication services. The transaction manager (TM) is responsible for the complete handling of transactions( ACID paradigms) - i.e. it coordinates all RMs and controls the recovery functions. The application program initiates the transactions and uses corresponding services of the RMs within their scope. Between the applications and RMs a subset of SQL has been agreed by X/Open. These statements are supported by most SQL implementations. The background is that not all SQL implementations clearly integrate the agreed ISO standard of SQL.
X/Open DTP and its interfaces
The X/Open DTP model differentiates with respect to the following sub-specifications:
XA interface A bidirectional interface, allowing different resource managers - which can be databases that have implemented the XA interface, for example - to work on a common, global basis. The X/Open XA specification is based on the two-phase commit protocol, which allows globally correct transactions to be performed. To do this, the modified data is first secured in each participating node. In the next step, the modified data is then confirmed and released in each participating node.
CPI-C (Common Programming Interface for Communication) This defines an interface for application programs so that program-to-program communication is possible. The interface abstracts the services of the communication layer.
CPI-RR (Common Program Interface for Resource Recovery) Also implements an interface for application programs and supports resource recovery.
TX specification Is the programmable interface( API) through which an application program communicates with the transaction manager. This is used to delimit global transactions against each other and to control their completeness.
TxRPC (Transactional Remote Procedure Call) Provides the application program with an RPC interface to communicate with a resource manager.
ATMI Implements a client- server interface between an application program and the resource manager in question.
The objective of the X/Open DTP architectural model is :
- Collaboration in distributed heterogeneous systems based on standardized protocols.
- The interchangeability of components based on standardized interfaces.
- The portability of application programs based on the standards of the X/Open.