2. PERFORMANCE REPORTING ANALYSIS › 2.8 Sysplex Analysis › 2.8.4 Parallel Sysplex Concepts › 2.8.4.2 Controlling the Sysplex Environment › 2.8.4.2.1 Planning for Signaling Services

2.8.4.2.1 Planning for Signaling Services

Signaling is the means by which XCF group members communicate
in a sysplex environment.  Signaling can be achieved through
use of Channel-to-Channel (CTC) connections, a coupling
facility (using coupling facility list structures), or a
combination of the two.

All systems in a sysplex must have full signaling
connectivity.  That is, an inbound and outbound signaling
path must exist between each pair of systems in the sysplex.

Signaling paths defined via CTC connections must be
exclusively defined as either inbound or outbound.  In
contrast, a significant advantage of using coupling facility
list structures is that they can be used for both inbound and
outbound paths.  An added advantage is that the same COUPLExx
PARMLIB member an be used for each system in the sysplex.

Transport classes are MVS's way of associating one or more
XCF groups (based on similar signaling requirements) and
then assigning them resources (paths and message buffers).
The idea behind transport class definitions is to segregate
message traffic according to XCF group requirements, the
lengths of the messages, or both.

As with the paths themselves, transport classes are defined
in the COUPLExx member of PARMLIB.  The CLASSDEF statements
assign a class name, a default buffer length, and XCF groups
that will be assigned to the class.

In general, you want to specify a class length (CLASSLEN
keyword) equal to the length of the messages most frequently
sent by the group(s) in the transport class.  Selecting an
appropriate class length represents the classic tradeoff
between storage used and performance for signaling activity.
If messages are usually shorter than the message buffer,
storage is wasted; if longer, performance is degraded.