All the mechanisms described in this book follow a similar pattern. A client is the source of the external request which comes into CICS® over a network using a variety of transport protocols, or from another CICS region, using Inter Region Communication (IRC). CICS (or another product) provides a transport-specific listener (a long-running task) that starts another task (a facilitator such as an alias or a mirror), to process the incoming request. The facilitator uses CICS services to access the application.
The priorities of different alias transactions can be adjusted to determine the service that a client request receives. There must be enough free tasks to service the alias transactions as they are started by the listener. The CICS programs that service the client requests are subject to contention for resources in the CICS system, and to transmission delays if they are remote from the CICS system, or if they request the use of remote resources by function shipping or distributed program link.
The CICS server is independent of the application model (2/3-tier, 2/3 platforms). The listener/facilitator deals with the different transports used and sets the rules for which programming models are supported.