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.
LU 6.2 Verbs |
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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.
Basic conversation verbs.
Basic conversation verbs are used to write TPs that provide a service
or system for other TPs. They are also used to write TPs that require
certain privileged functions that only basic conversation verbs
allow. The LU 6.2 data stream (called the general data
stream) requires a unique data format. A programmer
using basic conversation verbs must provide data formatting and
certain error recovery functions in the TP itself.
Mapped conversation verbs.
Mapped conversation verbs are used to simplify the programmatic
interface. A programmer using mapped conversation verbs is freed
from providing formatting functions for the data in the general
data stream. However, data formatting must be provided in an underlying
TP or system.
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.
Transaction Programs |
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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 |
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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 |
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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.
Figure 1-1 Conversation over an LU 6.2 Session
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.
