RE: CA gateway question

["Jeff Hill" <johill@lanl.gov>]

Hello Artem,

> As far as I understand gateway does not have any "internal state", which
> is needed to be synchronized.

The gateway maintains shadow copies of the PV's that its clients are
watching. These shadow PVs are used to propagate publish and subscribe
semantics through the gateway. As I recall it's a GW configurable option
whether gets are serviced from these cache PVs, or alternatively, if they
are to be forwarded directly to the IOCs.

In principal, the net functional impact from introducing a gateway might
only be some propagation delay between the client and the IOC. There might
also be, only for PVs that change at a very high frequency, an increased
probability for certain transitional subscription updates to be discarded.
All of this depends on how much load the gateway experiences which leads us
to another functional difference of a gateway based EPICS system; the
gateways tends to introduce bottle necks from a loading perspective. Also,
the gateway has a less robust implementation compared the rest of EPICS.
This is maybe in large part due to its being based on the "portable CA
server" which is a less commonly used component of EPICS. Nevertheless, the
gateway is used at many sites in production installations, and the gateway
is being continually refined as fringe problems are identified and fixed.
The mantis bug tracker system on the EPICS home page can be used a general
guide to the stability of the software.

> I wonder, what problems could occur if we have multiple CA-servers on the
> same network? Are there any CA related problems ?
> In our case those CA-servers are gateways with the same configuration.

Of course setting up multiple servers (IOCs) all sharing the same network is
a very normal configuration with EPICS. However, with what I will call
bi-directional gateways, one has to be careful about introducing PV
resolution infinite loops. You might end up with what I will call
bi-directional gateways if you have a gateway granting access into subnet A
from subnet B, and also a gateway granting access into subnet B from subnet
A. In that situation you will need to be very careful to configure the
gateways so that they will ignore name resolution attempts coming from a
gateway going in the opposite direction. Of course, more complex PV
resolution looping situations are possible if there are complex CA gateway
based paths between subnets. Hopefully Ken or Ralph might have included a
brief explanation of how to avoid these situations in the gateway
documentation?

Best Regards,

Jeff

> -----Original Message-----
> From: Artem Kazakov [mailto:kazakov@gmail.com]
> Sent: Thursday, November 23, 2006 8:56 PM
> To: johill@lanl.gov
> Subject: CA question
> 
> Hello Jeff!
> 
> Could you please help me understand one particular thing about CA.
> Here at KEK we are thinking about implementing redundant gateway.
> As far as I understand gateway does not have any "internal state", which
> is needed to be synchronized.
> 
> I wonder, what problems could occur if we have multiple CA-servers on the
> same network? Are there any CA related problems ?
> In our case those CA-servers are gateways with the same configuration.
> 
> Thanks in advance and regards,
> Artem Kazakov.