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Multiple monitored items in a single subscription vs multiple subscriptions with single item
March 7, 2014
9:26, EET
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janwe
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Hi,

I’m working on a system where we are currently creating one subscription per monitored item. Looking at the API and some other examples it seems to me we could have added monitored items for all the events and data items we are interested in to a single subscription instead.

Are there any recommended practices regarding this? E.g. how many items you should add to a subscription, whether there are any downsides/upsides to multiple items per subscription vs multiple subscriptions with a single item?

March 7, 2014
9:58, EET
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Jouni Aro
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Yes, in practice, I would say that it’s better to minimise the number of subscriptions, since every one of them needs more infrastructure and checking in the server, especially. Also if you have several items in one subscriptions, their data changes will be packaged into the same data change notification message, which takes less communication bandwidth. Of course, very big subscriptions may lead to too big messages (but that means thousands of items). We have not really measured any performance differences, though. I suppose some balanced design would give the best overall performance.

Anyway, I would suggest that you design the client application according to the most suitable application logic and group items that are logically connected to the same subscription. If you notice any performance issues, then you can start optimising the design from that.

In general, the communication is very fast locally, and the optimisation may be more necessary, if you have a slow network connection between the applications.

March 10, 2014
12:14, EET
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janwe
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Thanks. This confirms some of our theories regarding server constraints and bandwidth problems we are already experiencing. We’ll look into grouping items into fewer subscriptions very soon.

March 10, 2014
12:32, EET
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Jouni Aro
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Fine. And please, if possible, report back here if you manage to improve your communication with this.

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