How to define a class and method in DDS idl file?

Question

I am new to DDS.... so far I have little experience in OpenDDS and CycloneDDS

Is it possible to define a class inside the idle file and have member variables and member methods? or only structure and primitive data types are supported in DDS standards?

Answer 1

The IDL language is defined in the OMG IDL specification. It consists of a number of building blocks that include Core Data Types, like the structures and primitive data types that you mentioned, and Interfaces, that includes the methods you asked about.

However, only a subset of those building blocks is used by DDS. For the current version 4.2, section 9.3 DDS Profiles defines which of them are relevant for three different levels of support by DDS: Plain DDS, Extensible DDS and DDS over RPC.

You will see that the latter indeed includes Building Block Interfaces - Basic, as you might expect from RPC. However, not all DDS implementations support RPC. Plain DDS and Extensible DDS are more commonly supported and interfaces are not part of that functionality.

Since you asked about this in another question: note that the interface functionality as captured in DDS over RPC is not for the purpose of distributing objects with their methods, but for invoking methods on objects remotely -- as the name Remote Procedure Call implies.

Answer 2

Another answer to your question is that you are, perhaps, asking the follow-up question as if it is the initial one. There are many different ways of building distributed systems, and given your question, three examples seem appropriate:

• those designed around remote procedure calls/remote method invocation: in this context, CORBA is the perfect reference, but there are many (RPC, gRPC, DCOM, you name it);
• those designed around shipping objects with their implementation across: one example is Java/JINI, but there are many others (JavaScript in a browser could be considered one);
• those designed around shipping state (a.k.a. plain old data) and adding/transforming that state: SPLICE in the ancient history, DDS today.

Your question suggests that you are looking for middleware for doing distributed object computing. If that's indeed what you are looking for, DDS is a very suboptimal choice. Yes, RPC can be built on top of it (RPC-over-DDS simply makes it a bit easier to do it) and in a system predominantly built around distributing state it makes sense to do that.

If you can serialise objects with their methods then of course you can use DDS to distribute them in the network (there are fun things you can do that way). However, that's more a function of the programming language you use than of the middleware and IDL won't help you with that.