AVEVA Global User Guide
Concepts : Global Daemons : How the Global Database is Updated
How the Global Database is Updated
Administrative changes to databases in a Global project (allocation, setting primary locations, etc.) involve changes to the Global database. Administrative changes can also be made to a Satellite’s System database from the primary location for the System database, which can be the Satellite itself, another Satellite, or the Hub. Whenever changes are made to the Global or System databases, the updated databases are propagated around the communications network by the Global daemon.
The Global daemon checks the Global and System databases at intervals to see if there have been any changes: the changes are not propagated instantaneously. If a Satellite’s System database is primary at a remote location, once any changes are completed, they are propagated automatically from the remote location back to the Satellite.
It is important to be aware that:
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Some commands cannot be executed until conditions are right (for example, a command may not be executed while users are still accessing the target database).
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Commands executed by the daemon will require a GETWORK command to see the results, as the daemon is effectively another base product user.
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All commands which involve daemon activity will take some time, normally a few minutes, to execute. If the delay is more than this, and the above two conditions have been observed, check the transaction database. A few commands use the pending file. Refer to The Transaction Database and the Pending File for further information.
Another fact that influences what happens when commands are executed via the daemon is that commands are processed in parallel:
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In a standard project, commands are processed one at a time, so that the next command cannot begin until the previous one has finished. In principle, the state of the system is therefore always known.
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In Global, commands which are given at one location but have an effect at another location are processed in parallel, and so the next command may be initiated before the previous one has finished. This mode of operation is called non-blocking, and its advantage is that it prevents a slow, long-transaction command from blocking the user.
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However, there are situations when issuing a series of commands, it is essential that one command has executed completely before the next one can be carried out.
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The user can monitor the progress of commands that operate over Global locations, by using the Command Transactions window. Refer to Monitoring Command Progress for further information.
More information about specific operations is given in the appropriate sections of this manual.
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