What is CICS/400? - IBM - AS/400

CICS/400 brings to the iSeries range of computers the benefits of mainframe CICS, but it has been specially designed for the easy-to-use OS/400 environment. CICS/400 extends the OS/400 transaction processing capability by providing CICS functions through the CICS application programming interface, building on the native OS/400 function. It is a CICS family product, having many of the familiar features of mainframe CICS, but adapted to be familiar to the OS/400 user as well.The external interfaces for the administration tasks required for CICS/400 have the look and feel of OS/400.

CICS/400 can run in two environments:

  1. Stand-alone system
  2. In this type of environment, new applications can be developed on the OS/400, taking full advantage of existing CICS skills. Existing mainframe CICS applications can be run on the OS/400 provided that they conform to the requirements of CICS/400.

  3. Networked system
  4. Over a network of interconnected CICS systems, you can develop distributed applications. Typically, you would choose to place the presentation logic close to the user, and the business logic close to the data it requires. The systems themselves could all be CICS/400s, or may include other CICS family members, for example CICS for MVS/ESA, CICS for OS/2, or CICS workstation-based clients. You could use CICS/400 as a front-end system connected to CICS on a mainframe,a CICS workstation-based client as a front end to CICS/400, or even a mainframe CICS as a front end to CICS/400. Each processor—mainframe,iSeries, or workstation—could then be used for the purposes for which it is best suited.Its suitability may be by virtue of its proximity to data or users, or by virtue of the interface it is able to provide as a front-end processor.

In a distributed processing environment, you can also place your data in accordance with your business needs. Data relevant to only one part of your business can be placed on the user’s computer, thereby improving response times and reducing communications costs. Databases that are going to be used by your whole business might be placed on a mainframe, or a centralized OS/400 system.

With the CICS transaction processing family, you can locate programs and data on the most appropriate processor. When a transaction is started, CICS responds to the request, and the end user is unaware of the location of the data or the programs being run. This is achieved using CICS intersystem communication(ISC).

Server support for CICS clients
CICS/400 provides support for those CICS clients that can connect to the iSeries over Advanced Program-to-Program Communication (APPC) or TCP/IP communications. These include:

  • AIX
  • Microsoft® Windows
  • OS/2
  • Solaris
  • HP-UX
  • Linux 390

CICS client applications are written using programming interfaces provided on the workstation:

  • External Presentation Interface (EPI)
  • External Call Interface (ECI)

The EPI allows existing CICS applications to exploit user-friendly graphical user interfaces(GUIs) on the workstation without the need for changes.
The ECI allows the design of fully distributed applications. Typically the business logic is kept on a CICS server, while the presentation logic can be implemented on the workstation to take advantage of available GUIs.

CICS/400 architecture
A CICS/400 system is a collection of OS/400 jobs. One type of job, a control region, provides the control, scheduling, and work management mechanisms necessary to coordinate all the shared resources of a CICS environment. The other type of job, a shell, provides COBOL/400 and ILE C programs with an interface to the CICS-managed resources and maintains the application environment required to execute CICS transactions.

Before you can run CICS transactions you must start a CICS/400 control region and at least one CICS/400 shell associated with that control region. You may start more than one control region, each of which will independently manage its own set of resources and provide services to a set of shells. Usually a control region will have many shells associated with it, but a shell can only be associated with one control region.

Transactions run as CICS/400 tasks within a shell. The separation of duties between a control region and its shells, together with the separation of shared and nonshared storage within the control region and the shells, provides protection to the overall CICS system in the event of failure. If a shell ends, either normally or abnormally, the other shells and the control region should be unaffected. Similarly, if a control region ends, other control regions on the same iSeries system are unaffected.


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