To place an order, a user interacts with the order-entry graphical user interface (GUI) on a client machine. This starts the transaction processing flow.
- The GUI program calls the client program with a request to place an order. The request identifies a transaction to be run and passes appropriate parameters to qualify the request.
- The client program calls a CICS (Customer Information Control System) region, passing on the request from the GUI.
- The CICS region verifies that the user is authorized to make the request and that the user's terminal is supported. If not, the user is prevented from using the transaction processing system.
- The CICS region assigns a task to process the new instance of the transaction and schedules the task for processing alongside other current tasks. The CICS region processes and monitors the order-entry request throughout the task.
- The CICS region starts the task, getting storage and other operating system resources for it. It runs the first program for the transaction, the main order-entry program. The program runs on an application server taken from the server's pool of such processes.
- The main program checks and decrements the entry in the relational database. The CICS region and the RDBMS both lock the database entry to isolate it from update by other transactions. Only this one transaction can update the account. The CICS region logs the change but does not commit it until the shipping and billing operations have succeeded.
- The main program adds a shipping request to the central shipping queue. The CICS region logs the change but does not commit it until the billing operation has succeeded.
- The main program invokes a local billing program to route a request to the central billing program, which runs as a remote program on a mainframe host. The local billing program runs on a separate application server.
- The local billing program sends the billing request through a Peer-to-Peer Communication Gateway server to the mainframe host. It waits for the reply and then returns the reply to the main order-entry program. While the billing program waits for a reply from the mainframe host, in the meantime the main program can be doing other work.
- Having successfully accessed the database, queued the shipping request, and processed the billing request, the main order-entry program now issues a *syncpoint. The CICS region commits and logs the changes made to the database, shipping queue, and billing data. This makes the changes durable.
- The CICS region releases the resources used for the order-entry request and makes the application server available for other tasks.
- The CICS region returns an appropriate success message to the user through the client program and the order-entry GUI.
*: Synchronization (sync) points divide a task, usually a long – running one, into smaller logical units of work (LUWs). Each syncpoint command establishes a syncpoint to mark the completion of a LUW.
EMERGENT PROPERTIES OF THE SYSTEM
Functional Properties:
The above depicted system can be said to achieve its goals in the sense that when a user makes a request, he gets a response for the request he has made. In other words, he has been able to do what he wanted and the system has also been able to achieve what it is meant to achieve.
The system also supports such user tasks as querying details of items in a catalog, ordering items and shipping items to customers. To order an item, the application checks for the item in its database, changes the inventory and sends requests to billing and shipping applications.
Non-Functional Properties:
Security and Performance are two non-functional properties of this system.
Security: The CICS region verifies that the user is authorized to make the request and that the user’s terminal is supported and unless that is the case, the user is prevented from using the transaction processing system.
Performance: The gap between the time a user makes a request to the time he receives a response is fairly short, implying the client-server model depicted in the system is more rapid than other models like, for instance, teleprocessing and file-server models.
THE SYSTEM ENVIRONMENT
The order-entry application and associated programs described in the application are implemented on several client and server machines.
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