NonStop Systems Introduction

The NonStop Kernel
NonStop Systems Introduction—527825-001
6-11
Easy Transition to Distributed Processing
In Figure 6-7 on page 6-11, the same processes are spread over two network nodes
(system A and system B), but they continue to communicate through the distributed
message system and Expand software.
Processes cooperate to accomplish work in the same way in both the stand-alone
configuration shown in Figure 6-6 on page 6-10 and in the network configuration
shown in Figure 6-7.
1. Suppose P1 is a user process and P8 is a system I/O process. If P1 wants to
communicate with P8, it sends a request message, along with the name of the
destination process, to the file system (the operating system component
responsible for overall management of I/O operations such as opening, reading,
writing, and closing files).
2. The file system validates the message, looks up the processor location of the
destination process in a system table, and gives the message to the message
system.
3. The message system delivers the request to I/O process P8.
4. I/O process P8 performs the requested service (such as the update of a disk file)
by using a ServerNet I/O controller.
5. After completing the service, process P8 formulates a reply message for process
P1 and sends the message and the name of the destination process to the file
system.
6. The file system validates the message, looks up the location of P1, and passes the
message to the message system.
7. The message system delivers the reply to process P1.
The only real difference between the stand-alone system and the network is that the
message exchange between process P1 and process P8 travels over the dual
Figure 6-7. Interprocess Communication in an Expand Network
Processor 0 Processor 1 Processor 0 Processor 1
Dual ServerNet fabrics
File system
Message system
P1
P2
File system
Message system
P3
P4
File system
Message system
P5
P6
File system
Message system
P7
P8
Dual ServerNet fabrics
Expand
network
connection
System A System B
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