Experimental Physics and
Industrial Control System

"Kasemir, Kay" <[email protected]> · Wed, 26 May 2010 11:14:06 -0400

Hello:

On 5/26/10 09:30 , "Pelaia II, Tom" <[email protected]> wrote:
> ... I want to abstract the CA source so it can be from a plugin or a web
> service. In either case, it will get data packaged in JSON. This should work
> fine for any modern browser ...

I think that is a web service is a great idea. Not for the highest
performance needs, but for the other 98% of the use cases.

Does anybody have a good Channel Access web service?

We've developed the very basics of one for a general status web page that
uses JSP. It's similar to something that Matthias Clausen described to me at
EclipseCon 2010:

A PV pool subscribes to Channel Access, so the client code always gets the
"latest" value right away.
PVs are added to the pool on first access.
Since we write the JSP code, we control how many PVs are in the pool, and
our code indeed fetches their value frequently.

In principle it would be easy to turn that into a web service that can be
used by any other web page, Javascript, web client, somewhat like this:

http://your.site/livedata/get?name=SomePVName
Get current value with time, status, units as string for display to user.

http://your.site/livedata/get?name=SomePVName&format=value
Get only the value, no time stamp etc.

http://your.site/livedata/get?name=SomePVName&format=json
Get current value with meta data in JSON notation.

.. except that would need a lot more though to prevent an explosion of the
PV pool:

Who's allowed to call the service?

Should any requested PV that's not in the pool be added to the pool, or is
that limited to some PVs requested from a certain number of hosts/users?

How long should PVs stay in the pool when nobody asks for them any more?

----

Does anybody have a good Channel Access web service?
Do you have answers to those questions and you're interested in creating a
good web service for that?

Thanks,
Kay

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System