C/C++ Programmer's Guide (G06.27+, H06.03+)

Using the C Run-Time Library
HP C/C++ Programmer’s Guide for NonStop Systems—429301-010
4-9
Active Backup Programming Functions
An active backup program executes as a primary and backup process pair running the
same program file. The primary and backup processes perform interprocess
communication. The primary process sends critical data to the backup process. This
critical data serves two purposes: to provide sufficient information to enable the backup
process to resume application processing (file state and application state information),
and to indicate to the backup process where it should logically resume application
processing (control state information).
The backup process receives messages from two sources. It receives critical
information from the primary process (state information), which it must record for future
use in the event it must take over processing from the primary process. It can also
receive messages from the operating system indicating that the primary process or
processor has failed. If the primary process fails, the backup process takes over
processing at the logical point in the application indicated by the most recent control
state information received from the primary process, and it continues processing using
the most recent file state and application state information.
Table 4-3 lists the functions used to write active backup programs.
These functions are described in the Guardian TNS C Library Calls Reference Manual
and Guardian Native C Library Calls Reference Manual. See the Guardian
Programmer’s Guide for a description of active backup programming, including:
•
An overview of the activities an active backup program must perform
•
Detailed explanations of how to code an active backup program
•
Examples of active backup programs
Table 4-3. C Functions for Active Backup Programming
Function Use
__ns_start_backup() Called by the primary process to create and
initialize the backup process.
__ns_backup_fopen() Called by the backup process to open files that
have already been opened by the primary
process.
__ns_fget_file_state() Called by the primary process to obtain file state
information.
__ns_fset_file_state() Called by the backup process to update file state
information.
__ns_fget_file_open_state() Called by the primary process to obtain open
state information for a file.
__ns_fopen_special() Called by the primary process to open a file with
a specified sync depth.