SPI Common Extensions Manual

Subsystem Control Point
SPI Common Extensions Manual—427508-001
3-3
Message Queueing
Also consider the possible coexistence in a system or network of some combination of
C-series and D-series requesters, SCP processes, and subsystems. This table
summarizes variations in SCP request routing based on these factors:
The necessary information is provided in each subsystem management programming
manual. For example, X25AM uses ZSPI-TKN-MANAGER and assigns it the name of
the communications line associated with the X25AM process. The name of the
communications line is a device name specified for system configuration.
Message Queueing
The SCP process opens a subsystem when the first valid command for that subsystem
is received from an application. This open call specifies nowait-depth as 15, so the
SCP process can send as many as 15 command messages to the subsystem without
waiting for a response. If the SCP process receives more than 15 command messages
for the subsystem, it queues the additional command messages until either the
subsystem finishes processing a command message or one of the command
messages being processed by the subsystem is canceled. Commands are queued in
the order they are received. There is a separate queue for each subsystem.
Subsystem responses to these command messages can be in any order.
Command Cancellation
An application can cancel an outstanding command by using the CANCEL procedure.
When this procedure is called, the file system sends a system message -38 to the SCP
process. The SCP process uses the system message to determine which command
message should be canceled. If the command message was already forwarded to a
XMGR
Token in
Request?
Converted
SCP?
Converted
Subsystem? How SCP Processes the Request
No Irrelevant Irrelevant SCP routes the request based on the MANAGER
or OBJNAME token. The MANAGER token, if
present, is forwarded to the subsystem.
Yes No No If the request contains a valid manager name in
an OBJNAME token, SCP forwards the
unrecognized XMGR token to the subsystem,
which rejects the request because it contains the
unrecognized token.
Yes No Yes If the request contains a valid manager name in
an OBJNAME token, SCP forwards the
unrecognized XMGR token to the subsystem,
which rejects the request because it contains the
old REQID token.
Yes Yes No SCP removes the XMGR token from the request
before forwarding.
Yes Yes Yes SCP forwards the request, including the XMGR
token, to the subsystem.