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convertersystems / opc-ua-client

Visualize and control your enterprise using OPC Unified Architecture (OPC UA) and Visual Studio.
MIT License
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Linq to OPC query service #195

Closed quinmars closed 3 years ago

quinmars commented 3 years ago

UaClient provides access to the plain query service set. The query service set is not very easy to use, at least for me who has never used it. 😄 I think it'd be really great if we would have support for IAsyncQueryable to ease the use of this service. So querying all nodes that have a node class of DataType:

var allDataTypes = channel.Nodes.Where(x => x.NodeClass == NodeClass.DataType);

await foreach (var datatype in allDataTypes)
{
    // ...
}

Nodes or another fitting name would return an IAsyncQueryable and the Where-clause will be evaluated, due to the underlying mechanics, on the OPC server. This is not a finalized proposal. I know that this will not be simple to implement and there are probably many LINQ operators that cannot be translate into a query request, but I think that would be really great to support as much as possible.

djonasdev commented 3 years ago

If that were possible, that would be awesome! 😃👍 If you need help at this point for further implementation, I am happy to support you.

I am currently searching the entire OPC server to instantiate my software accordingly. That would surely save me a lot of time!

quinmars commented 3 years ago

If you need help at this point for further implementation, I am happy to support you.

Thanks, at the moment I'm still familiarize myself with the query service and C#'s query expressions. But once I have something to show, I'll let you know.

quinmars commented 3 years ago

Here is a short update on my progress on that issue. I have good and bad news.

Let's start with the good news. I studied the OPC UA query service set and C# query expressions. The query service set is indeed very powerful. I think it is possible to support the following operators: Select, Where, Take, TypeOf. Combining operators like SelectMany, Join, Zip and aggregating operators like Min, Max, etc. are probabliy not possible. Due to the filter abilities of the query service set, the Where operator could support very complex expressions, which are server-side evaluated. I already started with the implementation.

But here comes the bad news. While playing with the query service set, I recognized that the UaCPPServer apparently does not support the query service set. After some researched I found the following paper: Querying OPC UA information models with SPARQL. There the authors state:

The OPC UA Query ServiceSet is probably the only OPC UA ServiceSet which has no publicly available implementation up to now. Even the fact that a whole annex was introduced in OPC UA Part 4, which added a lot of examples for this Service, has not fulfilled its goal.

That curbed my enthusiasm a bit. Without a server supporting the query service set, it is more like an academic finger exercise. Nonetheless, it is possible to implement the query methods with unit tests only.