W3C home > Mailing lists > Public > public-device-apis@w3.org > February 2013

Re: [discovery-api] Consolidated comments and questions

From: Jean-Claude Dufourd <jean-claude.dufourd@telecom-paristech.fr>
Date: Fri, 08 Feb 2013 17:02:27 +0100
Message-ID: <51152193.8040301@telecom-paristech.fr>
To: public-device-apis@w3.org
Le 8/2/13 16:21 , Rich Tibbett a écrit :
> Jean-Claude Dufourd wrote:
>> Le 8/2/13 15:27 , Rich Tibbett a écrit :
>>>> And also, my question about polling boils down to: what is the 
>>>> practical
>>>> difference between "available" and "online" ?
>>> 'available' means that the service is present on the local network but
>>> not yet shared with any web page.
>>> 'online' means the the service is present on the local network and
>>> shared with a web page.
>>> We settled on these terms for lack of a better naming suggestion.
>>>> What is "online" ? A response to ping is characteristic of the device,
>>>> not of the service.
>>>> Is there anything standard in Bonjour and UPnP that can be used as a
>>>> test of online-ness ?
>>> 'online' is a concept specific to this specification when a
>>> NetworkService object is provided to a web page. At that point the
>>> service is either online or offline for that web page to interact with.
>>>> Should the implementation poll for that ?
>>>> Should the online attribute and events not be optional (SHOULD or MAY,
>>>> rather that MUST now) ?
>>> There is some use to receiving these events for a web page. There is
>>> the option for the web page to check the online-ness of a shared
>>> NetworkService object by querying its .url attribute via an e.g. XHR
>>> call. If that fails with a 4XX then they may be able to assume that
>>> the service is no longer responding and hence offline.
>>> Since we're aware of general registration/deregitration/expiry of
>>> Local-networked Services in the network at the underlying
>>> implementation level we provide any status updates we can through the
>>> 'online' attribute of the corresponding NetworkService object as a
>>> convenience feature.
>>>> If it is too similar to "available", should it not be removed
>>>> altogether ?
>>> It seems there's enough of a difference to warrant both events. One is
>>> a generic notifier of the state of the network relating the requested
>>> service types fired against NetworkServices. The other is a specific
>>> notifier of the state of the service fired against its corresponding
>>> NetworkService object.
>> JCD: I still do not understand the difference.
>> You write:
>> /'available' means that the service is present on the local network but
>> not yet shared with any web page. //
>> ////
>> //'online' means the the service is present on the local network and
>> shared with a web page. //
>> /If "I" see a NetworkService object, then it has been provided to the
>> web page "I" am in.
> Yes.
>> A NetworkService object has no existence (for the purpose of this
>> standard) until it is provided to a web page.
>> So if "I" see it, then online and available must have the same value.
>> Best regards
> Yes, in this particular case that will be true.
> If we take another scenario, where I don't have access to a 
> NetworkService object but where a new service is detected on the 
> network, then the user agent is still going to fire a serviceavailable 
> event on the NetworkServices object (since I don't have any binding of 
> that service to a NetworkService object there can be no serviceonline 
> event fired). This is still useful because it is an indicator that I 
> can re-invoke getNetworkServices if I wish and more devices will be 
> available in the list presented to the user.
> When you're working with a NetworkService object it's quite important 
> to know whether _that particular service_ is available or not. In that 
> sense, 'online' is for tracking a shared service's availability and 
> 'servicesAvailable' is for tracking the general number of services 
> available on the current network(s).
> Like I said previously, we can change the naming here if there are any 
> better suggestions but I feel the need for both events is still 
> justified.
JCD: I get it at last.
It is the same thing.
NetworkServices.onserviceavailable is fired when *a* new service becomes 
NetworkService.onserviceonline is fired when *this* service becomes 

Maybe better names could be NetworkServices.onnewserviceavailable and 
NetworkService.onavailable but I am not positive.


JC Dufourd
Directeur d'Etudes/Professor
Groupe Multimedia/Multimedia Group
Traitement du Signal et Images/Signal and Image Processing
Telecom ParisTech, 37-39 rue Dareau, 75014 Paris, France
Tel: +33145817733 - Mob: +33677843843 - Fax: +33145817144
Received on Friday, 8 February 2013 16:02:57 UTC

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