HP 3000 Manuals

Silhouette Reference Manual

How Silhouette Works 

Silhouette duplicates TurboIMAGE databases by transmitting the logfile on
the primary system over data communications lines to the secondary
system(s).  The database on the secondary system(s) is automatically
updated with all data applied to the primary system.  To begin using
Silhouette, the Operator:

 *  Specifies the databases to be duplicated.

 *  Enables database logging on the primary system.

 *  Specifies how frequently Silhouette will examine the logfile.

 *  Specifies how far Silhouette may get behind in logfile transmission
    before a request is issued to mount a magnetic tape for storing
    records.

 *  Specifies the data communications line(s) to be used by Silhouette.

Figure 1-6 shows the internal operation of Silhouette on the primary
system and on the secondary system.


          Figure 1-6.  How Silhouette Works 

If the Primary System Fails 

If the primary system fails, Silhouette issues a message to the Operator
on the secondary system, who then issues a >SWITCH command to change the
status of the secondary system to become the primary system.  Users then
switch over to the new primary system and continue processing as usual.

This process ensures data integrity by automatically removing all
incomplete transactions when a failure occurs.

A report is produced showing each individual user's incomplete
transactions and each user's last complete transaction so they know where
to resume processing.

Figure 1-7 shows the process that occurs if the primary system fails.


          Figure 1-7.  Primary System Failure 

If the Secondary System Fails 

If the secondary system fails, Silhouette notifies the Operator on the
primary system that the secondary system is not receiving records and
then begins logging the untransmitted records to a buffer.  When the
number of records in the buffer exceeds the high water mark, (a value
defined in the configuration file), Silhouette automatically writes all
the log records, not previously transmitted, to magnetic tape.

This logging, or taping of records, is a precaution to guard against the
primary system failing while the secondary system is unavailable.  If the
primary system remains operational when the secondary system is brought
back up, the database on the secondary system is synchronized with the
database on the primary system.  Figure 1-8 shows the sequence of events
if the secondary system fails.


          Figure 1-8.  Secondary System Failure 

If Data Communication Lines Fail 

If a system has multiple data communications lines, and one fails,
Silhouette automatically distributes record transmission over the
remaining lines.  If the last line fails, Silhouette notifies the
Operator on the primary system and begins logging records to the buffer.
If the number of records exceeds the high water mark, (a value defined in
the configuration file), Silhouette automatically writes all records to
magnetic tape.  Either the buffer or magnetic tape may be used to
update the duplicate database when the data communications line is
re-established.

Figure 1-9 shows what occurs if a data communication line fails.


          Figure 1-9.  Data Communications Failure 

For additional information on how Silhouette operates and how to perform
recovery procedures refer to Chapter 3, "Silhouette Overview" , and
Chapter 5, "Silhouette Recovery Procedures" .

MPE/iX 5.0 Documentation