NetBase for MPE Reference Guide

Chapter 5
5-2
Process Overview
NetBase operates on the principle of master and shadow copies of files and databases. Each file that
is to be shadowed will have a master copy to which all updates are performed. The updates are then
transported and posted to the shadow copies. This guarantees that all copies will be exact duplicates
of each other. In addition, updates occur to the shadow copy in the exact order that they occurred
against the master.
Be default, shadow copies of files and databases are available for read-only access. When an
application is run on the shadow system and it opens the shadow copy in a read mode, then the
application will read the data locally. If an application opens the shadow copy in an update mode
and NetBase NFA has been purchased, NetBase will automatically NFA the access to the master
system since all updates must occur on the master copy of the data. This default access can be
overridden with options specified on the FILE statement.
NetBase shadowing is designed to function through all modes of operation and failure, including
system backups, posting interruptions, and hardware or communication failures. If the shadow
computer is unavailable, NetBase's export process will automatically queue all updates on the
master computer until the shadow machine becomes available.
User exits are also available as a part of shadowing. Custom user exit procedures can be written for
both the exporting and posting of data. Export user exits are available on the master system to allow
the customization of shadowing by preprocessing the data before it is sent. Post user exits are
available on the shadow system and provide the ability to do customized shadowing. Turn to the
“Export User Exits” and “Post User Exits” sections in this chapter for more information.
Master System
The NetBase Shell captures updates on the master system as they occur. These updates are written
to the shadow transport file, NBM.IPC.NETBASE. There are two methods for writing the captured
transactions into the shadow transport file.
NetBase has been integrated with TurboIMAGE so that transactions to IMAGE databases are
written to the shadow transport file during the IMAGE transactions by IMAGE. This is known as
third party shadowing (TPS). The IMAGE procedure will call a NetBase routine from
XLNETBSE.PUB.SYS during the IMAGE transaction which will write the transaction into the shadow
transport file. For non-IMAGE files, the NetBase Shell will write the transactions to the shadow
transport file.