Experimental Physics and
Industrial Control System

"Ernest L. Williams Jr." <[email protected]> · Wed, 05 Aug 2009 08:06:01 -0700

Ralph Lange wrote:

Hi Steve,

Stephen Lewis wrote:

I consider forwarding a ca_put from a client into a ca_put-notify a

design flaw: behavior would be very different between a local IOC on

the client LAN and a distant IOC behind a gateway.

Correct.

Keep in mind, though, that until Jeff's very recent patch to PCAS the

GW simply couldn't distinguish between a client doing a ca_put and a

client doing a ca_put_notify.

In that situation, forwarding a simple ca_put as an acknowledged

ca_put_notify is definitely better than forwarding a ca_put_notify as

a fire-and-forget ca_put, faking the acknowledgment. So, from the two

possible options which were both wrong, the gateway was using the better.

Caching/no-caching:

Among the GW's objectives are keeping load off the IOCs and acting

transparently. With certain aspects of Channel Access, e.g. answering

get requests, forwarding IOC-side beacons to the clients, and

forwarding client side name-resolution-requests to the IOCs, these two

objectives are contrary. If the GW acts 100% transparent, it does not

offload the IOCs (in some cases, it would even put more load onto the

IOCs). If the GW tries to shield the IOCs, it will not be able to act

completely transparent. While the original approach was trying to

implement a reasonable compromise, the direction that Dirk is pursuing

is certainly the right one: when the level of compromise is made

configurable for these aspects, the user can select the appropriate

behavior for each GW instance.

Ralph do you agree that with respect to TCP-circuit load the IOC does

get shielded?

So, yes the IOCs will get hit with search requests (UDP, BCAST) but this

is as you say a trade-off.

Also, statistically most connections to PVs are monitored as opposed to

cagets.

Thanks,
Ernest

Cheers,
Ralph

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System