Re: PV's on the www

[Andrew Johnson <anj@aps.anl.gov>]

Paul Sichta wrote:
> 
> I would like to make a few PV's available (read-only) to our collaborators
> through a web browser, strictly low performance.  What approach has been
> used for this?  Do I need to program in Java and use JCA ?  Or,  are there
> unix/scripting  tools that can be applied?

The obvious and simplistic method is to write a cgi script that uses the
caget & caGet script level commands.  However there are performance
reasons to avoid doing this - every time anyone requests the relevent
information the web server calls this which then does a CA broadcast name
request, makes the connection, gets the value and then drops it again. 
For low performance and low usage this will provide up-to-date
information, but it does load your IOCs and it requires that the
web-server be able to make direct CA connections to them.

It is probably better to write a CA client which monitors the relevent PVs
and can either be contacted by the cgi script somehow at request time, or
which periodically updates a file which the web-server makes available. 
We use this approach at APS for graphical information about the current
machine status, beam history etc, which is refreshed at 60 second
intervals.  It has the advantage that the web-server doesn't need direct
access to your controls network, the updates can be pushed out through a
firewall to the server outside, but does add a slight delay to the data.

- Andrew
-- 
Complexity comes for free, Simplicity you have to work for.