[Ur] Ur/Web & RDP

[Adam Chlipala]

David Barbour wrote:
> On Sun, Feb 26, 2012 at 8:17 AM, Adam Chlipala 
> <adamc at impredicative.com <mailto:adamc at impredicative.com>> wrote:
>
>            It's easy to start a server process with an arbitrary
>         number of OS
>            threads to handle client connections, and they all run in
>            parallel.  There is also support for periodic tasks, which
>         run in
>            their own threads, alternating between sleeping N seconds and
>            running single transactions.
>
>
>         That would probably be sufficient for my purposes, if not
>         ideal. But I think it wouldn't take much extra effort to add
>         some way for client code to signal the periodic tasks, to
>         avoid polling, if I did take that route. Via channel, perhaps.
>
>
>     I don't understand what deficiency you're pointing to, but the
>     usual combination of RPCs and message-passing might already do
>     more than you're thinking.
>
>
> I need latencies in milliseconds, not `N seconds`, for most of the web 
> apps I want to build (e.g. teleop of robots). Polling is among the 
> very worst software engineering tactics for low latency operations 
> that effect low-frequency changes. There might be some clever way to 
> combine RPCs and message passing to achieve what I need, but will it 
> meet my timing concerns?

I don't think you need to be very clever to do what you want with RPCs 
and message-passing.  Don't worry; there is no per-event polling 
involved.  (I'm not suggesting you use periodic tasks at all for this 
sort of thing.)  If you look at the demos of message-passing (these are 
the last few in the main demo site), either (a) it should quickly become 
very clear how it can work for you or (b) you can point out 
documentation problems that prevent it from becoming clear. :)

> Something like http://jsfiddle.net/jashkenas/CGSd5/, with a tunable 
> number of elements in a page, might be useful to demonstrate animation 
> of your reactive system. The latency question could be answered by 
> animating, say, the motion of a mouse pointer through the server 
> across frames.

In the design of Ur/Web, I haven't focused on "animation" at all, but 
rather on conventional page content trees that evolve in response to 
events.  I don't think the current Ur/Web supports "animating mouse 
pointer motion" in any way that is at all pleasant without using the 
JavaScript FFI.