Re: PV's on the www

[Alan Biocca <AKBiocca@swrlnx.als.lbl.gov>]

We have a solution for the requirement for low performance readouts that we 
use for this exact problem. In fact it is just about exactly what Andrew 
outlined, except it is not specific for the ALS but allows the cgi-bin 
client to define the channel names to be read.

It consists of a server that makes the necessary CA connections, and a 
native perl-based client library that the cgi-bin includes to get data from 
the server. CA Connections are managed (shared, timed out, etc) for all 
clients. The perl (and soon a C library) clients don't have to be in 
"CA-range", only the server does. There is no writing, this is a readonly 
service.

The client api for data collection is a group based. It has calls for add 
channel to group, get group (to private cache), and read channel 
data/status from cache. The network activity is two-phase - a 'define 
group' that occurs the first time the group data is requested, and a 'get 
group' that gathers the whole dataset across the wire. It is udp based so 
the cost and maintenance of connections is not problematic. There are 
retries and timeouts to make the service reliable.

We are currently running the server (solaris unix) from inetd, so it starts 
up when clients are active. If no clients are active for awhile it times 
out the CA connections and eventually shuts down itself.

This system is fairly new but has been in test/production at the ALS for a 
few months. Loren Shalz (llshalz@lbl.gov) has done the programming for me.

At this point we don't have a distribution setup, but if other sites are 
interested we can possibly set up an alpha-beta release in a week or so.

Alan K Biocca
Advanced Light Source
akbiocca@lbl.gov


At 02:36 PM 11/24/99 -0600, you wrote:
> 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.