> If we think about developer side, what can we do with SDK that we can not do with Stack?
The SDKs do a significant amount of work that you would otherwise have to implement yourself with just a stack.
In the client: session management, subscription management, address space abstraction, node abstraction, etc... (just look at what's in the client-sdk package)
In the server: implementation of most of the services (HUGE), session management, subscription management, address space abstraction, node abstraction, etc... (again, look at what's in the server-sdk package).
It's kind of a weird, arbitrary line to draw. It's common in the OPC world because there was a time when the stacks were freely available and then toolkit vendors would sell SDKs on top of those stacks. That seems to be less common now.
As a user of Milo none of this matters, you just use the client SDK or the server SDK.
> Also, I want to ask you, is Eclipse Milo always be free? Would you think to add some commercial features for it?
I can never make what's currently available non-free because of the license, and I don't intend to. As far as commercial features, I haven't decided for sure, but some of the code generation and modeling stuff that I'm working on may end up being licensed such that commercial/business users have to pay for it.