The XDASrv XML DA server is working in different operating modes, depending on the values returned in the GetServerParameters() method:
| Browse Mode |
Defines how client browse calls are handled.
| Real |
All browse calls are handled in the generic server according he items defined in the server cache.
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| Virtual |
All client browse calls are handled in this plug-in and typically return the items that are or could be dynamically added to the server cache.
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| Item Validate Mode |
Controls how the generic server handles the ValidateItems client calls.
| UnknownItems |
The plug-in ValidateItems method is called for items that are not found in the generic server cache.
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| Never |
The plug-in ValidateItems method is NEVER called. Requested items that are not defined in the generic server cache are treated as unknown items.
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| Always |
The plug-in ValidateItems method is ALWAYS called. This allows the plug-in to determine the items access rights based on the client's credentials and return OPC_E_UNKNOWNITEMID for clients without the required privileges.
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| Client Update Handling |
The client update callbacks are handled in the generic server and can be selected to be done in two different ways to achieve maximum performance for the application. The DANSrv Standard Edition supports only the Cache Poll mode.
| Cache Poll |
The client update thread runs periodically according the client update intervall definition and checks the cache for changed values or items that need to be handled for other reasons.
This mode is optimal if there are less than 1000 items in client groups and there are rather many value changes.
If no client is connected or if no client has callback handlers then the update thread is idle and causes no CPU load even if there are many item value changes from the device. With a large number of items in OPC groups ther can be a high CPU load even if there are no value changes.
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| Queued |
The cache write method queues items for each active OPC group that has the item attached. The client update thread transfers the values of the queued items to the client.
This mode is optimal if there are a large number (e.g. > 10000) of items in OPC groups and there are not too many value changes (e.g.<1000/sec). The client update handling causes no CPU load as long as there no value changes. With a large number of value changes the queue handling causes additional overhead.
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| Write Cache Update |
Defines how the generic server handles the cache in OPC client write calls. The DANSrv Standard Edition supports only the Generic mode.
| Generic |
The generic server writes the item values into the cache.
This is done after calling the customization WriteItems method and is done only for items that were successfully handled in the customization module and no error was returned.
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| Custom |
The generic server does NOT write the value into the cache. The customization module WriteItems method must update the cache by calling the SetItemValue callback method. This allows the customization module to modify the value and update the cache with the value that is actually written to the device.
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