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<== Date ==> <== Thread ==>

Subject: Re: fallback CA servers?
From: Andrew Johnson <anj@ast.cam.ac.uk>
To: tech-talk@aps.anl.gov
Date: Fri, 3 May 1996 18:36:31 +0100 (GMT)
William Lupton wrote:
> >   We are discussing how to handle simulation both when IOCs are present and
> > when they are absent. For simulation at other sites we are writing a Unix
> > CA server which emulates part of our VME database. For simulation at the
> > telescope we are toying with the idea that we might run the Unix CA server
> > all the time but that connections to it would only be used when the vxWorks
> > CA server was not running.

If you know that the Unix CA server will always be running, you could 
always have the Unix server reflect PVs from the IOC into a different 
namespace, and have all your clients attach to the Unix names and never to 
the original IOC names.  This way the Unix server knows exactly when it 
needs to be a simulator (because it gets connect and disconnect messages 
from the IOC), and the clients couldn't care less if an IOC gets turned 
off because they're never talking directly to it. There will be a 
performance hit here of course, but for slow records it shouldn't matter.

If you were really clever you might even be able to automate it, so that 
if a client requests a PV from the Unix server which the Unix system 
doesn't actually have a value for, it could do a name translation and send 
the request out to the real IOC, waiting for a response from the IOC 
before it responds to the client connection request.  Don't know if this 
would work or not, but it's an interesting idea.

I would agree with Steven Beard's comment though - you probably ought to 
have to do something explicit to enable a simulator, or at least have a 
big switch somewhere which disables all simulation for use during the 
night.

- Andrew

  O o ._    Andrew Johnson, Royal Greenwich Observatory
     __\\_	Madingley Road, Cambridge, CB3 0EZ
  __|ooooo|__	Tel +44 (0)1223 374823  Fax 374700
~~\_________/~~	WWW http://www.ast.cam.ac.uk/~anj/




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