Re: Turing the NM UI into an agent

[Marcel Holtmann <marcel holtmann org>]

Hi Dan,

> > since you are re-working most of the D-Bus APIs based around Network
> > Manager, I like to propose a radical change on how you handle the UI
> > part. Personally I think that the current UI inside Network Manager
> > (even with 0.7) has too much tasks to fulfill. I prefer having a
> > "stupid" UI.
> >
> > So my main concern is that replacing the current UI is too
> > complicated. One reason is that you need a well known bus name for  
> > the
> > UI on the system bus. This makes it impossible to install two UI
> > components or split tasks between two applications. We did the same  
> > in
> > the early days of BlueZ and I think it is not a good design to hard-
> > code a well known bus name into the daemon and have the UI use the
> > same one. So for BlueZ we came up with the D-Bus agent concept. So  
> > let
> > me try to explain it.
> >
> > So the UI only gets a unique name (no need for a D-Bus security  
> > file).
> > Then it registers itself a object path (the actual path value is
> > unimportant). After that it calls a RegisterAgent(object path) method
> > inside the daemon. Now the daemon knows where to find an agent  
> > (object
> > path and the unique name from the message sender value) and can call
> > into this object for additional information. It really serves as a
> > callback interface and the only things to agree on is the interface
> > description. The bus name and object path are kept random.
> >
> > This concept allows an easy way of monitoring agents. So you know the
> > case when you have no agent (meaning no UI is running) or when an
> > agent dies. No unneeded D-Bus calls that might fails etc. The other
> > advantage is that in theory you can stack agents and call them in
> > order. This is useful if you have a wizard or some small cases where
> > the UI should look different from the general case. Or unregister and
> > then re-register agents. For example for multi user scenarios.
>
> There's already a split between the UI bits and the configuration  
> bits;
> I think what you'd like to see is the User Settings service grab  
> unique
> bus names so that each user could provide their own user settings at  
> the
> same time to NetworkManager.  I'd like to go there in the future, but
> then we need to figure out things that ConsoleKit will help with, but
> which are still very much undefined, stuff like:
>
> - If user A has activated a connection on eth0, is user B allowed to
> control eth0 and potentially tear down user A's connection?
>
> - If user A has activated eth0, is user B allowed to start a VPN
> connection on eth0?
>
> Stuff like that.  There might be uses for this sort of thing in
> virtualization actually, but it's kind of hard to see networking as  
> the
> same sort of thing as Multi-User X, for example, where different users
> get exclusive access to individual mice, keyboards, displays, and USB
> flash drives.

I wasn't talking on these kind of things. They are totally different.

> Right now the only thing that acquires a well-known bus name is the
> settings service, which doesn't have to have an UI at all.  Up until  
> NM
> 0.5 it actually was a completely separate process from the applet.    
> If
> your user has permissions to do so at the D-Bus level (ie,  
> at_console or
> whatever), then any process can call
> ActivateConnection/DeactivateConnection right now.  But only 1 process
> can provide the configuration data, which limits the set of what
> ActivateConnection can actually connect to.

I meant the settings service. Still need to get deeper into the  
Network Manager terminology. So having a "static" settings service is  
a bad design (from my perspective). Turning the settings service into  
agent would be a good thing. From the logical point of view it is not  
really different, but from a technical point of view it makes all the  
difference.

So I see the UI as something totally "stupid" that the daemon tells it  
what to do and the user can use it to influence settings inside the  
daemon. Besides that both are loosely coupled.

An example would be that the daemon needs a passphrase for a network.  
So it calls a callback inside the UI agent. This call should always go  
to the logged in user and thus to the applet run by that user. Any  
other callback into the UI (or settings service) should be like this.

So big technical advantage here is that the daemon always knows if an  
agent is present or not. And if not, it can do a proper fallback.  
Calling a D-Bus method that fails is a bad thing. Also the fact that  
you don't have to agree on a well known name and object path makes  
this a lot cleaner. The D-Bus interface for the agent should be the  
only contract between the UI and the daemon. The rest a variable  
details.

> Could you describe some of the scenarios that would involve stacking  
> UI
> agents?  I'm not sure I follow the use-cases here.

So you have the applet handling all security request (passphrase  
etc.), but then you wanna do a wizard to setup a new network  
connection. In that case you could re-direct the passphrase request to  
the wizard (which is a different application from the applet). While  
for WiFi this might be still limited in its usefulness. For all the  
other wireless technologies like Bluetooth, WiMAX, UWB etc. it is  
different.

Regards

Marcel