ClearSCADA includes a ViewX Provisional User Logon process, which is initiated when you log on to a server using a client connection. The aim of the Provisional User Logon is to maintain a user’s connection to the system in a multi-server configuration in the event of a change-over. This is important for users that have Exclusive Control of one or more objects within the database.
WebX client, Server Icon, Server Status and System Configuration sessions do not generate a Provisional User Logon.
Example:
The following example illustrates the sequence of events for a Triple-Hot Standby System, but this applies equally to a Hot-Standby Pair. In this example, the Server Hot-Standby 1 initially assumes the role of ‘MAIN’, as a user you connect to the ‘MAIN’ server using a ViewX client (it is assumed that all the correct permissions have been configured).
Once logged on you can take control of one or more database items providing the Exclusive Control permissions have been enabled (see Exclusive Control). When an item is under exclusive control, other users cannot access certain features for that item.
Once you are logged on to the System the Client uses the SCX Manager Link to poll all the servers and establish a Provisional User Logon on all the Servers within the system (see Links (Server)).
You can see when Provisional Users, Provisional Logon sessions are created in the Server Status tool.
In the event of an unauthorized attempt to use the Provisional User logon link to access the system the unauthorized user's name would appear with an asterisk next to it to indicate that an attempt has been made.
When a changeover occurs for any reason (see How Can I Detect that a Server has Failed?), the previously designated ‘MAIN’ server is demoted to Hot-Standby 1 and in this example the Hot-Standby 2 Server is promoted to the role of ‘MAIN’.
You are logged off from Hot-Standby 1 by default, and logged on to the new ‘MAIN’. The Provisional User Logon ensures that any items that were set to Exclusive Control remain under the your control even though the changeover has occurred.
This example illustrates a typical Main server change over, however, a ViewX client may change it's main connection from one server to another (for example if a lower Node Cost server becomes available) and in that instance the Provisional Logon on the Main server is important to maintaining Exclusive Control.
Where a client connects to a system using a DMZ connection, then the Provisional Logon is not available.