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Re: [discovery-api] Supporting UPnP devices

From: Rich Tibbett <richt@opera.com>
Date: Tue, 19 Mar 2013 05:53:28 +0100
Message-ID: <5147EF48.9060004@opera.com>
To: Cathy.Chan@nokia.com
CC: public-device-apis@w3.org
Cathy.Chan@nokia.com wrote:
> There has been on and off discussion on whether the discovery API should
> support UPnP device discovery by device type. See e.g. [1], [2], [3]. At the
> March 6 DAP call we had consensus to explore supporting UPnP devices in the
> discovery API, and I agreed to take a deeper look at the implications of such
> a change and make a proposal. Below are some key findings that I think are
> worth discussing.
> The first thing is I'd assume that we want to continue supporting searching
> for individual UPnP services, in addition to searching for UPnP devices which
> contain UPnP services. By this I mean that a web app would continue to be able
> to search for say a ContentDirectory service and obtain a NetworkService
> object that represents the service, regardless of what UPnP device that
> service resides in. In addition to that, a web app would also be able to
> search for a say MediaServer device and obtain an object that represents the
> UPnP device, which in turn contains a ContentDirectory service. I believe this
> is important as there are "add-on" UPnP services that are not tied to any
> particular device types. Besides, the flexibility may come in handy for some
> web app developers.
> The biggest impact in adding UPnP device level support is on the object model.
> The NetworkService interface currently consists of the following attributes:
> * id
> * name
> * type
> * url
> * config
> * online
> and the following event handlers
> * onserviceonline
> * onserviceoffline
> * onnotify
> While this works well for both mDNS and individual UPnP services (and DIAL,
> which is a special case of UPnP), it isn't particularly suited for
> representing a UPnP device. For one thing, a UPnP device does not have a
> single url associated with it to send messages. Instead, each of the services
> inside it would have one such url. Similarly, a UPnP device itself does not
> receive event messages. The underlying services do. Thus, a UPnP device does
> not need/utilize the url attribute and onnotify handler. Instead, it needs an
> array of objects that represents the underlying services. So, the difference
> between the desired interface to represent a UPnP device and the current
> NetworkService interface would be
> - url
> - onnotify
> + services[]
> Now, if we look at the services objects to be included in a UPnP device
> object, they need a little less than what is provided by the NetworkService
> interface, as some of those would now belong to the parent device object.
> Having them also at the service level would only make it more confusing. Most
> notably, the online attribute and the associated online/offline events belong
> to the parent device. The difference between the desired interface to
> represent a UPnP service *residing under a UPnP device object* and the current
> NetworkService interface would be
> - id
> - name
> - config
> - online
> - onserviceonline
> - onserviceoffline
> (In other words, the desired interface for a UPnP service needs only type, url
> and onnotify.)

Interesting analysis.

> My proposal would be to expand the NetworkService interface to add an optional
> attribute for the services array and allow the url attribute to be
> optional/nullable, with the caveat that the onnotify handler would be entirely
> unused when the object represents a UPnP device. For representing the UPnP
> service objects, I would propose introducing a separate interface with only
> the necessary attributes/event handlers instead of reusing the NetworkService
> interface to minimize confusion. The name of the interface would need some
> serious thinking/bikeshedding though.

We could detach the url, type and onnotify attributes completely from 
NetworkService objects and supply all 'access point'-like info (type, 
url and onnotify) via a set of indexed properties on each NetworkService 

So a NetworkService object itself can represent zero or n number of 
indexed properties each providing some separate access point info that 
could have been obtained from either a UPnP or Zeroconf service or 
device. We would also likely add a few helper methods to the 
NetworkService interface e.g. length, getServiceByType.

function(services) {
   for(var i = 0; i < services.length; i++) {
     var service = services[i];
     for(var j = 0; j < service.length; j++) {
       var accessInfo = service[j];
       console.log(accessInfo.type + ": " + accessInfo.url);

       // Do something with the access point info
       // ... e.g. ...
       var xhr = new XMLHttpRequest();
       xhr.open('POST', accessInfo.url, false);
       // ...
       // xhr.send(someRequestBody);
}, function(err) { console.dir(err); });

This makes the design of the NetworkService interface similar to how the 
NetworkServices interface currently works in the spec. That's probably a 
Good Thing.

> Once we agree on the object model, most of the changes to add UPnP device
> support should be rather straightforward. However, I do expect to see some
> sub-steps in various algorithms that would look significantly different for
> UPnP
> devices compared with the existing services.

I expect we're looking at a significant amount of effort to rewrite the 
UPnP related algorithms but I think this change gives us more 
flexibility in how we ultimately implement the specification.

Each 'service' object (if that is still the correct terminology) can 
have 0, 1 or n access points. That means we may also be able to provide 
access point information for things in the device description file 
(assuming we can bind them to 'type' names that make sense).

Thanks for kicking this off, Cathy. It is always great to discuss around 
the spec itself.

- Rich

> Regards, Cathy.
> [1] http://lists.w3.org/Archives/Public/public-device-apis/2012Nov/0101.html
> [2] http://lists.w3.org/Archives/Public/public-device-apis/2013Feb/0012.html
> [3] http://lists.w3.org/Archives/Public/public-device-apis/2013Mar/0003.html
Received on Tuesday, 19 March 2013 04:54:01 UTC

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