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Re: data streams

From: Pablo Chacin <pchacin@sensefields.com>
Date: Wed, 3 Jun 2015 11:36:27 +0200
Message-ID: <CABbgcNbkBhNGaM8R-Nf+5WTLh0HKhMgOPOYps02uZpWjt=4kGA@mail.gmail.com>
To: Dave Raggett <dsr@w3.org>
Cc: Magnus Olsson <magnus.olsson@ericsson.com>, public-wot-ig@w3.org

>Script developers ony need to know the object model for the sensor and
display. The details of the underlying protocols are dealt with
>automatically by the server platform. For this to work, we need a standard
way to describe data streams as part of the data model for "things".

What would be the scope of this definition? Only a model of the stream
(metadata), or will also propose an api to handle streams. In this later
case, I would suggest to take a look at what the reactive programming
community has been developing, for example in RxJS (which is very similar
to how java 8 handles streams)

Pablo Chacin
SenseFields SL
Tlf (+34) 93 250 45 98
Gran Via 674, principal 1º
08010 Barcelona, Spain

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On Wed, Jun 3, 2015 at 11:25 AM, Dave Raggett <dsr@w3.org> wrote:

> On 3 Jun 2015, at 09:04, Magnus Olsson <magnus.olsson@ericsson.com> wrote:
> Streams sounds like a very useful thing to support :-)
> - For the multiple sinks, why not use a pub sub broker as the identified
> source of the stream that way any number of sinks can attach to that
> pub-source. Seems to me that setting a proxy output value to point to a
> single output is a limitation that might result in someone creating an
> upstream proxy to split to multiple sinks.
> This is a question of the underlying API and data structures. For files,
> for example, an open file handle can only be read by a single client. If
> you want several clients to read the stream, then they each need to open it
> individually so that they have their own stream handles/stream objects.
> I completely agree that pub-sub protocols provide a good solution to
> streaming to multiple clients, which is why I advocate support for bindings
> to MQTT and XMPP (as examples of such protocols), but this is at a
> different level of abstraction.  We need to distinguish between the data
> model exposed to scripts and the bindings to the transport protocols, as it
> is important to decouple these to simplify scripting amongst other reasons.
> Any thing should be able to redirect any aspect of their source (defined
> by its meta description) to a nearby pub-sub proxy service. Still that
> could be an "on device" (localhost) broker, a home broker, a service
> provider broker or a public broker etc.
> The Web of Things Framework allows you to have multiple proxies for the
> same thing, and you can define new things that transform streams etc.
> It seems reasonable to me to have use case such as one part of the stream
> for the snapshot, another for the live view a third for data analysis, a
> fourth for a remote tele medicine, a fifth for a emergency monitoring
> function etc.
> - Even discrete value type of sensors could be handled as a stream if only
> given long enough time to observe
> To keep simple use cases simple to script, we need a data model where
> values are simple object properties, e.g. a floating point number
> corresponding to a temperature reading. This will change over time as the
> corresponding physical value changes.  This is applicable where you are
> only interested in the current reading.
> Note that you can define a data model where an event is generated to
> signal a change to a value. This raises the question of what constitutes a
> significant change, but that is something for applications to determine.
> Such events enable applications to implement their own history buffers.
> A data logging application has different requirements, in that you want to
> record the history of changes along with the times that they occurred. This
> involves a different data model where you can ask for a the value at a
> specific time or for the sequence of changes in a given time interval. This
> is a common enough meme that it deserves to be supported by the server
> platform rather than requiring apps to implement logging and querying
> themselves.
> —
>    Dave Raggett <dsr@w3.org>
Received on Wednesday, 3 June 2015 09:36:56 UTC

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