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APPC Subsystem on MPE XL Node Manager's Guide: HP 3000 MPE/iX Computer Systems > Chapter 1 IntroductionAPPC Overview |
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IBM's Logical Unit 6.2 (LU 6.2) architecture specifies a common set of functions and protocols that application programs running on separate processors can use to communicate with each other. LU 6.2 allows for standardized program-to-program communication, which IBM calls Advanced Program-to-Program Communication, or APPC. The functions and protocols that LU 6.2 architecture specifies are called verbs. Application programs used for communicating in an LU 6.2 environment are called transaction programs, or TPs. When two TPs communicate with each other, they are said to communicate with each other over a conversation. The functions and protocols for communication are specified in the form of programmatic conversation verbs. These conversation verbs allow programs to perform such functions as initiating a conversation with another program, sending and receiving data, requesting confirmation from the partner program, requesting and granting permission to send data, and terminating a conversation. LU 6.2 architecture specifies two types of conversation verbs that TPs can use to communicate: basic conversation verbs and mapped conversation verbs.
Both groups of conversation verbs (basic and mapped) are further divided into base sets and option sets. The base set of a group of verbs must be implemented in all products that use that set, while product developers may or may not choose to implement additional option sets. The APPC subsystem implements the base set of basic conversation verbs and certain option sets. The LU 6.2 API product implements the base set of mapped conversation verbs and certain option sets. There are two kinds of TPs: application TPs and service TPs. 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 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 “Conversation over an LU 6.2 Session”, 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. This section has introduced terms and concepts you need to be familiar with as you work in the APPC environment. The remainder of this chapter describes the APPC subsystem. |
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