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.