David Barbour wrote:
On Sun, Feb 26, 2012 at 8:17 AM, Adam Chlipala
<[email protected] <mailto:[email protected]>> 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.
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