Experimental Physics and
Industrial Control System

Ralph Lange <[email protected]> · Thu, 14 Jan 2016 13:23:08 +0100

The issue with subscriptions and first updates
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This issue applies to monitor subscriptions to ANALOG values that use a

DEADBAND technique to limit the update rate, e.g. CA monitors to EPICS

database records of analog type that use MDEL.

The specification (or documentation) says a client gets the current

value on connection, then updates whenever the value leaves a configured

deadband around the value last sent.

The EPICS database uses the cache of the "value last sent" once per

record instance. The first client will correctly get the current value,

then updates when the values leaves the deadband. Additional clients

will get the current value, then updates when the values leaves the

deadband of the first client's last update. This effectively changes the

deadband for the second update of additional clients to be anywhere

between zero and twice the configured deadband.

In most cases, the effects are not prominent. For any values that change

frequently, there's a short oddity right after connecting, then

everything is in sync across all clients.

For slowly changing channels, however, that first update interval of

additional clients can last minutes or longer. During that time (i.e.

until the first client gets a new update), any additional newly

connecting client will show a different value.

I have seen situations (special synchrotron operation mode with

extremely good lifetime) where a handful of panels on screens in the

main control room where all showing different values for the same

prominent PV (beam current).

Conceptually, there are two ways out of this:

Either, the deadband algorithm (including the last value cache) is done

per client - this is what happens when you use the 3.15 server-side

plugin that does deadband limitation. This guarantees correct deadband

behavior for each client, but makes the across-client situation worse:

different panels showing the same PV will *always* show different values.

Or - and that would be my suggestion - the first update that clients

receive is redefined as being the cached last value that the first

client of the same subscription received. That way, all additional

panels that you open will always show the same value as the first

client, even across Gateways. Also, the effective deadband for second

updates to additional clients will be between zero and the configured

value, and never larger than configured as in the current situation.

Would that be something to change in 3.16, and follow in V4?

Cheers,
~Ralph

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System