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05/24/2017
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Hi,
I have a questions about the use case of subscriptions and monitored items of the DataChange subscription feature. Lets say I have a device implementing the Micro Embedded Device Server profile that supports 2 parallel sessions (minimum requirement of the profile) and has about 50 variables that is of interest to monitor. Is the normal use case to monitor all variables using a single subscription or is there any advantages to spread out the monitored items over several subscriptions?
Br, Jonas.
10/02/2014
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You would only use different subscriptions if you wanted different Publish intervals. Say you had 10 out of the 50 items that were process variables changing every 1000 msec. You might have a subscription that published every 1000 msec but the items are sampled at 500 msec. Perhaps the other 40 items are set points and range values. You probably do not need to publish or sample them as fast so you would put them in a separate subscription.
With that said in a real world manufacturing process environment we see most people do not need data master then every 1000msec nor does that data change much faster than that.
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02/24/2014
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You might also group subscriptions based on other action – for example an HMI might create a subscription for each screen that is open, than when the screen is closed the subscription for all items on the screen can be easily eliminated. A historian may group all data in specific collection frequency into there own subscription. It might also just subscribe for all data changes in a single subscription and then sort the values once they are received. The key point is that the choice in Client/ Server is that it is up to the Client. The Server just needs to be able to support what the client requests.
In the process industry 1000 msec data or slower is very common, in manufacturing it can be much faster depending on what is being done. Controller to robot may very well required 10 MSec or faster. This is part of the reason Pub/Sub was added.
Paul
Paul Hunkar - DSInteroperability
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