A transaction processing monitor fo...

A transaction processing monitor for distributed transaction processing applications maintains the ACID (automicity, consistency, isolation, and durability) properties of the transactions and provides regaining facilities for aborting transactions and recovering from theory or network failures.

Transaction processing plans are widely used by enterprises to support mission-critical applications, in the same state [i]or[/i] condition as airline reservation systems and banking applications. These applications ne to store and update data reliably, provide associate access to the data by way of hundreds or thousands of users, and maintain the reliability of the data in the neighborhood of failures.

The HP Encina/9000 transaction processing monitor[1] provides the middleware for running transaction processing applications. It maintains ACID (atomicity, consistency, isolation, and durability) properties for transactions (see the glossary upon page 65). It ensures that applications that scamper concurrently will maintain data consistency. Encina/9000 also provides recuperation facilities for aborting transactions and recovering *om failures as it is as machine or network crashes.

DCE and Distribution

Encina/9000 provides the ability to write distributed applications. Encina/9000 applications can be written as clienVserver applications with the client and server possibly running upon different machines. Encina/9000 servers can communicate and cooperate with each other in updating data upon several different machines.

Distributed applications provide several advantages. The data maintained by the agency of an enterprise may itself be distributed because of historical and geographical considerations. Furthermore, distributed applications are able to exploit parallelism from running concurrently on several machines.

Distributed computing proposes the advantage of improved performance, availability, and access to distributed data. Performance is improved by way of spreading the computing among various machines. For example, the application's user interface can be scud on a PC while the user collection of laws could be split to move on on several machines. The use of multiprocessing machines to provide parallelism for multiple users can improve the throughput of the scheme Availability can be increased at a distributed system in which replication is used to preserve several copies of the data. Access to distributed data or to data that is maintained in several databases is also facilitated by the agency of distributed computing.

Data may be distributed because the database becomes too large or the CPU onward the database machine becomes a bottleneck Data can also be distributed to increase availability and improve the rejoinder time by keeping the data obstruct to the users accessing it. Finally, data can be distributed to preserve separate administrative domains, such as different divisions in a corporation that want to preserve their data local.

Encina/9000 uses the lay open Software Foundation s DCE[2] (Distributed Computing Environment) as the underlying mechanism for providing distribution. It uses the DCE RPC mechanism to provide client/server communication. Encina/ 9000 is also self-same closely tied to DCE naming and security services (see the articles onward pages 28 and 41 for more about these services). For example, an Encina/9000 server can be secureed from unauthorized use by defining access superintend lists (ACLs). ACLs contain an encoding of the authorization policy for different users and are enforced by the agency of DCE at run time. ACLs are described upon page 49. Encina/9000 also makes use of the threading package provided by dint of DCE.

To achieve optimum price and performance, careful consideration straits to be given to in what way the data and the application are partitioned. Throughput and answer times are often the elucidation criteria by which users connoisseur the performance of a method Encina/9000 provides the flexibility of being able to specify the distribution topology of the application. In addition, users can specify data replication if it will help to make secure higher availability of mission-critical data.

Two-Tiered versus Three-Tiered Architectures

In the past, transaction processing applications were implemented using a two-tiered architecture (see Fig. 1) In this paradigm an application is written as a client, which accesses a database server The client implements the graphical user interface (GUI) and the application logic. The database server handles access to the data stored in a database.

The advantage of this approach is simplicity. The disadvantage is that it is not scalable beyond a certain point. It is also les flexible and harder to modify to proper new business needs.

Encina/9000 allows the progress to maturity of applications using a three-tiered architecture like the individual shown in Fig. 2. In this paradigm, an application is partitioned into a client that implements the graphical user interface to the user, an application server that implements the business logic of the application, and a database server that implements the database access.

The Enc na/9000 three-tiered architecture provides the following advantages through the whole extent of traditional two-tiered architectures: