Re: polling through the gateway

[Dirk Zimoch <dirk.zimoch@psi.ch>]

Hi Geoff,

In the current version, the gateway has the option -no_cache which allows a 
client to poll through the gateway. The previous version (or when started 
without -no_cache) has a very strange behavior: Only if no client monitors the 
channel a poll is passed through to the IOC. If any client monitors a poll does 
not pass the gateway but instead delivers the last value received by monitor. 
The problem is that you can hardly know when there is any other client with a 
monitor.
That is very annoying if you have values that don't cause monitors. For example 
if HOPR changes dynamically and you do a caget with DBR_CTRL_DOUBLE on the VAL 
field. Then you would always get the HOPR value at the time when the first 
client started a monitor on the VAL field. This is because the gateway uses 
DBR_STS_* for monitors (as most CA clients do). So the HOPR information in the 
gateway is never updated.
We user the -no_cache option everywhere. The additional overhead is moderate. It 
is only TCP traffic between gateway and IOC. The gateway still reduces the UDP 
traffic, so that even a shell script doing caget in a loop does not flood the 
IOC network with broadcasts.
Of course, polling over the network is generally a bad idea.

Dirk

Geoff Savage wrote:
> Hi,
>
> Is it possible to ask the gateway to poll PVs?  Currently the gateway 
> uses monitors when it connects to the PVs on the client side.  I would 
> like the gateway to have the equivalent of a SCAN field for a PV name it 
> is reading.
> Thanks,  Geoff
--
Dr. Dirk Zimoch
Paul Scherrer Institut, WBGB/006
5232 Villigen PSI, Switzerland
Phone +41 56 310 5182