6100 BSC Programming Manual

Writing Applications that Use 6100 BSC
The setting of the BSCNONSTOPON parameter can have a major impact
on error recovery. If BSCNONSTOPON applies and a path switch
occurs in the 6100 subsystem, the protocol state is retained,
i.e., it doesn't become undefined. The application can retry its
request, normally with no fear of loss or duplication of data.
There are several considerations that apply to retries:
• The retry must have the same request ID as the original
request.
• Because the same buffer is used for the write and the
read part of a WRITEREAD request, an error reply may
have overlaid the original request. The application
must rebuild the request buffer before it retries.
• If the path switch lasts long enough that the remote
station perceives a problem, the behavior of that station
can affect recovery. For example, if that station resets
the line, the BSCNONSTOPON setting is no longer helpful.
BSCNONSTOPON does not preserve the state if the application or
its processor fails. In such cases, the backup application
process must take over. To enable the backup process to retry a
request, the primary process should checkpoint all data necessary
to build the request; then, if the primary process fails while
the request is in progress, the backup process can attempt a
retry. Otherwise, the backup process can perform end-to-end
recovery on takeover, or abort the connection and begin again.
Several other SYSGEN parameters have an impact on error recovery.
For example, if the line must be downloaded because of a
hardware problem, AUTOCLOSE determines whether opens are closed.
If so, applications must close the line and open it again to
use it. NOAUTOSTOP determines whether CP6100 makes a STOP
request after closing the opens; if so, a switched connection
is lost. AUTOCONF determines whether after a download, the
configuration block is sent down from the host, replacing the
current configuration; if the application made configuration
changes, it must restore them before continuing. Finally, the
AUTOLOAD parameter selects whether the line is downloaded or
declared down under certain error conditions. For more
information about these parameters, see the CP6100 I/O Process
Programming Manual. To find out how the application knows a
download has occurred, see the description of file system errors
in Appendix A.
There are several requests that can assist the application in
dealing with errors:
• The SETMODE request lets you change the retry count for the
line. Increasing the retry count can reduce the incidence of
3-21