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How does the MonitoredDataItem react to communication breaks?
June 22, 2016
12:28, EEST
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Gerd
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Dear people,
after reading the JavaDoc and the book “OPC Unified Architecture”, I got the impression, that the value in MonitoredDataItem will just freeze after loosing the connection.
Then on reconnect it will resume with whatever the server has in the queue.
Is this correct?

BR,
Gerd

June 22, 2016
15:47, EEST
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Heikki Tahvanainen
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Hello,

This is indeed pretty much what happens in the communication error scenario. The size of the server RetransmissionQueue and the size of queues for individual monitored items will determine whether data gets lost during the communication break. Pictures in pages 159 and 161 in the mentioned OPC Unified Architecture book contain an overview of the subscription architecture.

June 27, 2016
10:26, EEST
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Gerd
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Thank you, Heikki.

So I have to combine the MonitoredDataItem with the ServerStatusListener to get the current state of the Item displayed as BAD if there is a communication break, right?
This leads up to the next question: If the connection is reestablished, can I rely on the server to send an update to the item even if there was no change in the data?

June 27, 2016
17:57, EEST
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Heikki Tahvanainen
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Hi,

Yes, you are correct, you have to combine MonitoredDataItem with the ServerStatusListener.

When the connection is re-established, you cannot rely on the server to send updates but you may use the Subscription.readAll method to refresh the current values of the items in a subscription.

June 28, 2016
13:18, EEST
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Gerd
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Thank you very much, Heikki.
I will try that out.

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