When using XML-DA to access the database, an XML-DA subscription is required. The XML-DA subscription contains information about the XML-DA tags that are being requested by a client. By using the Server Status Tool’s XML-DA Subscriptions category, you can view the status of the XML-DA subscriptions currently in use by clients (that are connected to the server.)
You can view status information about the individual XML-DA tags for each subscription in the XML-DA Items category (see XML-DA Items)
The XML-DA Subscriptions category provides the following information:
- Handle—A unique identifier that is used to identify the subscription internally.
- Items—The number of items in the XML-DA group to which the XML-DA client is subscribed.
- Ping Rate—The maximum amount of time permitted between each 'ping' attempt to communicate with the XML-DA client. If the client is not updated within the Ping Rate time, the server assumes the client cannot be contacted and so frees some of its resources.
If a client is not updating within the Ping Rate time, the server and XML-DA client may be unable to communicate with each other. Alternatively, the ping time may be too small (the ping time is fixed for ClearSCADA, but may be user-defined in some other OPC based programs).
- Last Refresh—A time stamp of the last update to the items in the XML-DA group.