Re: Storing data across webserver calls

On 05/28/2014 12:34 PM, Parker Jones wrote:
> [Hope this list address still works, I haven't got around to moving across to google yet]

Send an empty message to [email protected] and you 
are done.  This still works, but many active members have gone.  Cc'ed
to the new place.

> I am converting a text-based Prolog application to have a GUI via a webserver.  In the code I have a predicate where data is threaded through the body.
>
> p(DataOut) :-
>      q1(Data1),          % q1(-Data)
>      q2(Data1, Data2),   % q2(+Data, -Data)
>      q3(Data2, DataOut). % q3(+Data, -Data)
>
> As part of the conversion to a UI application p/1 will be replaced
> by
button presses for each of q1, q2, and q3 and a handler for each button
press.
>
> % for webserver
> handler(q1_button, q1).
> handler(q2_button, q2).
> handler(q3_button, q3).
>
> The question then arises, how should data be threaded across
> handlers
q1, q2 and q3.
>
> Option 1: use the clause database, asserting and retracting
intermediate data. Each handler retrieves data, performs a calculation
and then stores the data again.
> Option 2: use http session data. This moves the intermediate data
> back
and forth to the client. Not so good if the data is substantial.

Nope.  http session data is just a layer around assert/retract that
makes sure that data is registered with the session.  This fixes the
issue with 1: you can get concurrent requests from different clients,
messing up your single global state.  It also ensures that the data
is destroyed if the session gets a timeout.

> My question is are these the only options or are there any other
> ways
to thread data across web handlers?

Yes.  You can create a thread and associate that with the session
and make the thread wait for a message from an HTTP handler.
You can find how that works in an old talk, which is still online at
http://www.swi-prolog.org/download/publications/ICLP09-Tutorial.ppt,
starting from slide 58.

The Pengines work provides another way to do this, although this expects
you to do the client work in JavaScript.  It is a lot easier though. 
See also some of the examples on the scratchpad of 
http://pengines.swi-prolog.org.  Use SWI-Prolog 7.1.16 (to appear very 
soon).

	Cheers --- Jan

>
> Many thanks for any suggestions,
> Parker
>   		 	   		
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