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Subscription to nodes in OPC UA
May 21, 2013
5:00, EEST
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Hi ,

I am using OPC UA java client evaluation version . It is very powerful. I have used the subscription facilities to subscribe to certain nodes and fetch data at regular intervals. I am developing an application. I start the subscription. The node got subscribed. Now I logged out from the application. Then again I want to connect to the OPC UA server and know what items are already subscribed using the java client sdk?Is it possible?Is there any API. Can I get a sample java code snippet?

Thanks in advance.

May 21, 2013
7:28, EEST
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Jouni Aro
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I am not quite sure what you are looking for.

The Subscriptions are available from the UaClient and the subscription has a list of Items, which define what the subscription contains.

Or what did you mean with logging out of the application?

May 24, 2013
10:54, EEST
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Jouni Aro said

I am not quite sure what you are looking for.

The Subscriptions are available from the UaClient and the subscription has a list of Items, which define what the subscription contains.

Or what did you mean with logging out of the application?

Sorry I just confused you.
When I subscribe to a node using Prosys OPC UA java client , then if I notice the console , I observe that subscribed node’s data are coming. Is there a thread running at the back end? How is this continuous data coming? I just wanted to know if I can get any identifier / id of this service running at the back end? Can you clarify the back end service! Is this any worker thread? Any job?

Thanks in advance.

May 24, 2013
12:25, EEST
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Jouni Aro
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Yes, the UaClient has a PublishTask, which is run inside a worker thread. It is dispatching PublishRequests to the server and also handling of the PublishResponses that have been received. The PublishResponses come in asynchronously and are just put in a queue, which is cleared by the PublishTask.

Which kind of ID are you looking for?

May 28, 2013
9:01, EEST
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Jouni Aro said

Yes, the UaClient has a PublishTask, which is run inside a worker thread. It is dispatching PublishRequests to the server and also handling of the PublishResponses that have been received. The PublishResponses come in asynchronously and are just put in a queue, which is cleared by the PublishTask.

Which kind of ID are you looking for?

Thanks. Just wanted to know that suppose my ua client object subscribe to a node for any data item listener / event listener. Then does the client sends request every time to get a response or it is like one time subscription ? The client just subscribes to the node and the server asynchronously sends response.

Thanks in advance.

May 28, 2013
11:28, EEST
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Jouni Aro
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Yes that’s the case. In OPC UA, the client frequently “polls” the server, which is sampling the data and keeps a notification queue from which it sends new notification messages as responses to client polls.

So in practice, you just subscribe to data in the client and you get all the changes from the server in order. The properties of the Subscription and MonitoredItems define the exact details of sampling rates and publishing intervals that define how often the notifications are transferred and how many samples they may contain each.

The various parameters are explained in the blog post

February 21, 2014
5:12, EET
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Jouni Aro said

Yes that’s the case. In OPC UA, the client frequently “polls” the server, which is sampling the data and keeps a notification queue from which it sends new notification messages as responses to client polls.

So in practice, you just subscribe to data in the client and you get all the changes from the server in order. The properties of the Subscription and MonitoredItems define the exact details of sampling rates and publishing intervals that define how often the notifications are transferred and how many samples they may contain each.

The various parameters are explained in the blog post

Hi,

Could you please let me know how can the queue size be maintained from the opc ua client side and get bulk data from the queue at regular intervals . Also where is the queue is being maintained , is it at the client side or server side ? Using queues , can the performance of data fetching from OPC UA server be controlled. ?

February 21, 2014
9:22, EET
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Jouni Aro
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In the client, you can define Subscription.PublishingInterval, which defines how often the server “sends” new notification messages. You cannot configure the notification message queue from the client and in practice, the queue is only used for sent packages to be able to resend in case of communication problems.

If the monitored items may change faster than the PublishingInterval, you can define a smaller SamplingInterval and a QueueSize for the item. This is a bit different, but enables fast sampling, even though the samples would not be transferred to the client so fast.

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