TS/MP 2.5 Pathsend and Server Programming Manual

Later parts of this chapter describe how to use these procedures in context-free and context-sensitive
Pathsend programs. chapter 5, “Pathsend Procedure Call Reference” (page 74), gives detailed
syntax and usage considerations for the procedures.
Interprocess Communication in the Pathsend Environment
Communication between requestors and servers in the Pathsend environment differs from the
communication between Guardian requestors and servers. Rather than directly opening a particular
server process, the requestor opens the ACS subsystem ROUT process, which in turn opens a server
process selected by the PATHMON process. The ACS Subsystem ROUT process’s open of the
server process is shared among all requestors running in the same processor as the ACS subsystem
ROUT process.
The communication begins as follows:
1. The requestor calls a Pathsend procedure (for example, SERVERCLASS_SEND_) to request a
server-class send operation.
2. The request goes to the ACS subsystem ROUT process running in the same processor as the
Pathsend requestor process.
3. The ACS subsystem ROUT and PB process checks for a link to a server process in the server-class
specified in the request. If the ACS subsystem ROUT and PB process have no available link
to the server-class, the actions performed depend on the settings of the PATHMON configuration
parameters NUMSTATIC and MAXLINKS. Following steps might occur:
a. The ACS subsystem PB process sends a “get-link” request to the PATHMON process that
manages the server-class specified in the procedure call.
b. The PATHMON process selects a server process in the specified server-class. If necessary,
it starts and initializes a new server process in the server-class.
c. The PATHMON process sends back to the ACS subsystem PB process the Guardian
process name of a server process within the specified server-class.
d. The ACS subsystem ROUT process opens the server process of that name. This open is
shared among all the Pathsend requestor processes running in the same processor as that
ACS subsystem ROUT process; therefore, a close operation does not necessarily occur
when the communication with a particular requestor process is finished.
For more information about the NUMSTATIC and MAXLINKS parameters and their effects on
get-link requests, see the NonStop TS/MP 2.5 System Management Manual.
4. The ACS subsystem ROUT process forwards the send request to the server process.
5. When the server process replies, the ACS subsystem ROUT process replies to the requestor
process, and the server-class send operation is complete.
The action of the PATHMON process in step 3 is called granting a link. The ACS subsystem
processes, which request links and provides access to the server process after the link is granted,
is called the link manager. (For SCREEN COBOL requestors, the TCP serves as the link manager.)
Basic Pathsend Programming
The simplest type of Pathsend program is a context-free program. This subsection provides the
information you need to write a context-free Pathsend program. It also provides information common
to both context-free and context-sensitive programming. The next subsection provides information
about the additional tasks required of a context-sensitive Pathsend program.
Context-free Pathsend programming consists simply of sending messages to a server in a server-class
and receiving the replies. The sending of a message to a Pathway server-class is called a server-class
send operation. In a context-free program, only two Pathsend procedures are used:
SERVERCLASS_SEND_ to send the messages (and to receive the replies if in waited mode) and
SERVERCLASS_SEND_INFO_ to get additional information about the results of the last
SERVERCLASS_SEND_ call if the call failed.
Interprocess Communication in the Pathsend Environment 51