Connections between E3 and Database.

For each DBServer created in an E3 domain, there are two possible connections with the Database: one of them is used for E3 services (Formulas, Historics, Storage, and Alarms), and the other one is used for queries.

If DBServer’s EnableLocalCreation property is TRUE, an extra connection will be created for every running Viewer. If the domain has more than one DBServer, and their EnableLocalCreation properties are all TRUE, an extra connection will be created for every Viewer and DBServer, in case there are Viewer queries in each DBServer.

When EnableLocalCreation property is enabled (TRUE), it makes the Viewer access the Database directly, without using the preexisting connection at the Server. Therefore, if the Domain has two DBServers, and if there are queries regarding both DBServers in the Viewer, each active Viewer will create an extra connection for each DBServer, regardless of the number of queries in each one of them.

Examples:

1. Domain with one DBServer, EnableLocalCreation property disabled (FALSE), and 2 running Viewers with 5 different queries to the Database. Formula: number of DBServers * 2 = 1*2 = 2. Number of connections: 2 (two).
2. Domain with one DBServer, EnableLocalCreation property enabled (TRUE), and 2 running Viewers with 5 different queries to the Database. Formula: number of DBServers * 2 + (number of DBServers * number of Viewers) = 1*2+(1*2) = 4. Number of connections: 4 (four).
3. Domain with two DBServers, EnableLocalCreation property enabled (TRUE), and 2 running Viewers with 5 different queries to the Database via both DBServers. Formula: number of DBServers * 2 + (number of DBServers * number of Viewers) = 2*2+(2*2) = 8. Number of connections: 8 (eight).

Note: When setting up EnableLocalCreation property as TRUE, you must make sure the Viewer is in a computer accessing the Database. If the DBServer is not in the same computer as E3, it will not be available outside the company’s intranet. Therefore, you will be able to access the E3 Server via WebViewer (outside the intranet), but not to perform the queries, because it will not be possible to access the DBServer directly.

From version 4.6 on, the EnableLocalCreation property has been removed from the DBServer object. Since then, the Viewers have no longer been able to access the database directly, and all access now take place via server.