Key concepts

It is essential to understand the following concepts:
Regions
A region is an instance of a CICS® server. This includes all the listeners configured for that region.
Programs
CICS programs are executable units that can be started within a CICS region. A particular CICS program might be available on multiple CICS regions. The Workload Manager has a list of CICS regions on which the program is defined (program affinity).
TCP/IP address
An address that uniquely identifies a machine on a particular network. Each machine may have many threads of execution running simultaneously; a TCP/IP port uniquely identifies a particular thread of execution.
TCP/IP port
A numeric TCP/IP end-point in the range of 0-65535. This port number is defined in the configuration file (ctg.ini) file for each instance of a server definition.

The number of client requests that can be handled on a particular port varies with the CICS server platform. For example, a TXSeries® for AIX® or TXSeries for Windows® may handle client requests via a number of ports, but only about 250 client requests can be handled per port. You might need to define multiple server instances.

Server instance
An entry in ctg.ini that corresponds to an instance of a CICS server. For a particular CICS region there may be multiple instances of CICS server definitions, each with different port numbers. The names provided by the Server name configuration setting (Server=servername in the configuration file) are used in the actual ECI requests. In the Workload Manager section of the Configuration Tool, the server instances are gathered together in Server Groups.

In CICS Universal Client Version 3.0 this was known as a Listener.

Server Group
The Configuration Tool uses Server Groups to group together server instances. A Server Group can correspond to one CICS region on a server, or to several physical servers.

The first time an instance of the Client daemon makes a request, it selects a server instance from the group at random. For the duration of the life of that Client daemon instance, all subsequent requests to that Server Group are sent to the same server instance.

An ECI program can be associated with a Server Group. The Workload Manager assumes that the program exists on all server instances in the Server Group.

Workload management
The ability of the Workload Manager to select a suitable CICS server as a target for a client request. The load balancing algorithms try to spread the workload over various server instances on a particular region (via random selection) and they allow the user to effectively specify a bias for the selection of a particular region. Known inactive regions are not considered for selection.
Note: Apart from TCP/IP, other supported protocols can be used to communicate with CICS servers. For example, SNA can be used, and in this case the server instances can be identified by Logical Units (LU), rather than by TCP/IP ports.