Prototype for MES ⇔ L2 OPC UA Communication

This is a simple prototype for communicating in OPC-UA between a MES and a L2 equipment. This implements a very early specification prototype — please beware!

Using

• Install any recent python3 distribution for your system. Linux has it easy, MacOS can use homebrew, and Windows can download it from the website or use choco or the new WSL, if brave enough
• In this folder, create a virtual-env so pip doesn’t install stuff in your system folder: python3 -m venv venv/ and activate it (Linux: source venv/bin/activate, Windows: \Scripts\activate.bat or something?)
• Install dependencies: pip install -r requirements.txt
• Start the server: python EquipmentSidedServer.py
• Let the example client do some magic: python ExampleClient.py
• You should see the client invoking a method, and the server printing the received parameters.
• That’s it!

Interface implemented

Currently, EquipmentSidedServer.py implements a very simple interface. We have:

• A set of methods within a machine-node under the Objects node
• Each method has multiple InputArguments
  • Each InputArgument is of a custom datatype, see below
• Each Method has a ParameterDescriptions child
  • The nodes under ParameterDescriptions must match the InputArguments
  • Static, required-before-runtime self-introduction data is offered in each ParameterDescription
• The InputArguments are of MESServiceMethodParameterStructure, defined in Types/DataTypes/BaseDataType/Structure/MESServiceMethodParameterStructure
• The MESServiceMethodParameterStructure includes information the MES needs to display a »function-block« to its (human) user

Ugly parts

• The MESServiceMethodParameterStructure has a Value property. This property can be of multiple primitives (int, float, string, ...). To realize this, the actual type of that property is set to VariantType.Variant — this also allows nesting ExtensionObjects, Variants, etc - which is ugly. This means we don’t have any type-safety here. On the other hand, it allows Null-ability, which is great.
• The ParameterDescriptions folder is by-convention and not enforced in any technical way. Is there any way to have a HasTypeDefinition reference from a method to some type, where we could enforce this?
• EUInformation should be an EUInformation-type. For that I need a ExtensionObject (I think?), and I couldn’t figure out how to put a non-encoded object into one in my stack. Need a bit more time — and maybe a word from Jouni if this is the right way?

Beware!

Some client (uaexpert / prosys client) don’t correctly display the custom type, neither its definition nor as a method parameter. With these clients you can’t call the methods at all.