APPC Subsystem on MPE/XL Node Manager's Guide (30294-90007)

Chapter 1 19
Introduction
APPC Overview
Transaction Programs
There are two kinds of TPs: application TPs and service TPs.
• Application TPs are programs that perform services for end users.
They typically use mapped conversation verbs. An application TP
written with mapped conversation verbs must rely on a service TP or
underlying system to perform the data formatting and error
recovery functions.
• Service TPs are programs that provide a service or system to other
TPs. For example, a service TP might provide data formatting and
error recovery for application TPs. Service TPs are generally written
in assembly language, and they usually use basic conversation
verbs.
Conversations
TPs are said to communicate with each other over conversations. Just
as in human conversations, one TP must initiate the conversation. Once
the conversation is established, a TP may perform such functions as
sending or receiving data, asking for or giving confirmation of data
received or sent, asking for or giving permission to send data, and
informing the partner TP of an error condition. When the transaction is
complete, one of the TPs must terminate the conversation.
The APPC Session
The APPC session is the logical connection between LUs that allows a
conversation to take place. It is independent of any conversation
conducted over it. Just as two people cannot have a telephone
conversation until a phone connection is established, TPs cannot
conduct a conversation until a session is established. After a
conversation using a session has stopped, the session may be
terminated or allowed to remain active.
In Figure 1-1, two sessions are active between node A and node B. One
of these sessions is being used for a conversation between transaction
program A (TP A) and transaction program B (TP B). The other session
is active, but it is not being used.