Remote Tag Provider subscriptions created by Tag References when source tag value never changes

Ignition
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1

Hi all,

I'm trying to better understand how subscriptions work between gateways when using a Remote Tag Provider in Ignition 8.1.

Let's assume the following architecture:

  • Gateway A exposes tags through a Remote Tag Provider.
  • Gateway B contains Tag References (or tag bindings that effectively reference remote tags) pointing to tags in Gateway A.
  • These Tag References are enabled, but:
    • are not historized,
    • are not always displayed in any Vision/Perspective view only when the view containing them is showed,
    • are not used by alarms,
    • are not otherwise consumed by clients.

My question is whether the mere existence of the Tag Reference creates and maintains an active subscription to the source tag on Gateway A.

More specifically, I would like to distinguish between two scenarios:

Scenario 1: Source tag changes regularly

The source tag in Gateway A changes value frequently.

In this case, I would expect an active subscription and value updates to be propagated through the Gateway Network.

Scenario 2: Source tag never changes

The source tag in Gateway A remains constant for a long period of time:

  • Value does not change.
  • Quality does not change.
  • Timestamp does not change.

In this scenario:

  1. Does the Tag Reference in Gateway B still maintain an active subscription on Gateway A?
  2. Does this subscription generate any Gateway Network traffic besides connection keep-alives?
  3. More importantly, can the existence of that subscription cause the source OPC tag to continue being scanned/read cyclically on Gateway A, even though no value, quality, or timestamp updates are being generated?

My main goal is to understand whether a large number of remote references to mostly static tags can still have a performance impact due to maintained subscriptions and/or continued field device polling at the source gateway.

Has anyone tested this behaviour or can confirm how Ignition 8.1 handles these subscriptions internally?

Thanks!

2

Yes, No, and No.

Subscriptions themselves do not cause any polling. Ignition's entire tag subscription architecture reports changes only. Only OPC drivers (not OPC connections) typically require polling.

Not sure what happens with Reference Tags or Expression Tags set to execute periodically on group mode. Worth testing that case.

3

Thanks, that's helpful.

I think my remaining doubt is slightly different from the polling question.

I understand that subscriptions themselves do not cause polling of the source OPC tag. However, I'm still trying to understand the behaviour of the Gateway Network subscription itself.

Let's assume:

  • Gateway B has a Tag Reference pointing to a tag in a Remote Tag Provider on Gateway A.
  • The source tag on Gateway A remains completely unchanged (value, quality and timestamp stay the same).

In that situation, does Gateway A send any periodic updates across the Gateway Network to keep the reference on Gateway B up to date, or is the subscription entirely event-driven and therefore completely silent until an actual change occurs?

In other words, once the subscription is established, should I expect:

  1. Only connection/subscription maintenance traffic (keep-alives, session management, etc.), with no tag value updates being transmitted?
  2. Or are there periodic tag value refreshes sent to Gateway B even when the value, quality and timestamp have not changed?

My concern is not the OPC polling side anymore, but whether a large number of remote references to mostly static tags could still generate noticeable Gateway Network traffic simply to keep the remote references synchronized.

Kind Regards.

4

Entirely event-driven. IIRC, the only keep-alives are for the gateway network itself, not for any provider connection within.