For cases like this, we would introduce GetOrAddNode method (see below).

var storageNode = domain.StorageNodeManager.GetOrAddNode(nodeConfiguration);
using (var session = storageNode.OpenSession()) {
  ...
}

Do you build nodes on demand and use them just after build like in the example you showed?

Yes, we do. On application start, we don't have any nodes added. On the first request, we build a domain and then add the corresponding node. All subsequent requests either use the existing node or add a new one on the first request to that node.

The disadvantage of the suggested approach is that GetOrAddNode method takes node configuration as a parameter but the existing GetNode method gets just node id. I think we would exclude node id from config and pass it separately. So the sample would transform as follows.

var storageNode = domain.StorageNodeManager.GetOrAddNode(nodeId, nodeConfiguration);
using (var session = storageNode.OpenSession()) {
  ...
}
// AddNode method should also be changed for consistency.
_ = domain.StorageNodeManager.AddNode(nodeId, nodeConfiguration);

NodeConfiguration is required to have nodeId anyway, except for default node. It validates before StorageNode build

But it is used only in the validation. The Validate method is called by the internal BuildNode method. In turn, the BuildNode method is called only from the AddNode.
As you can see we can easily transform all this chain.

That's why I added extra check for nodeId to GetOrAdd. Take a look at new new commits.

Can we keep it this way, or even remove the GetOrAddNode method for now?
I'd suggest merging this PR to the 6.0 branch and to the master then.
Once the code is in the master I'd think about further improvements.

I will keep StorageNodeManager API the same. GetOrAdd functionality can be implemented as an extension in your solution. Later we could integrate your implementation to DataObjects.Net