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Question on Local Directory Service Architecture
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Jacky Bek
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04/23/2021 - 04:30
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in part 14 of the OPCUA Specifications, figure shows a single instance of the LDS residing between a publisher (left hand side) and a subscriber (right hand side), separated by a Message Oriented middleware.

This is my setup tweaked according to the above reference architecture:

OPCUA Servers (publisher) -> MQTT broker1    <-------------------------------> MQTT broker2 <- OPCUA Servers (subscriber)

Question

Can I add 1 LDS-A server to the network (network-1) of the publisher, and another LDS-B server to the network (network-2) of the subscriber in the DMZ.

All the OPCServers at network-1 will announce itself to LDS-A and all the OPCServers at the network-2 will announce itself to LDS-B.

Between LDS-A and LDS-B, it will automatically announce each own group of OPCServers to each other.

In this way, if i add another network (network-C) of OPCServers, I only need to configure the comms between network-1, network-2 and network-3.

Is my understanding correct?

Revised setup according to my description above:

OPCUA Servers (publisher) -> MQTT broker1    <-----------------------> MQTT broker2 <- OPCUA Servers (subscriber)

In the DMZ at both ends, LDS-A@DMZ-A <-> LDS-B@DMZ-B

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zbynekz
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04/25/2021 - 02:43
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Can you identify the figure in Part 14 which shows this? I cannot find it.

Thank you

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Jacky Bek
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04/25/2021 - 16:37
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It is in chapter 5 Pubsub Concepts

5.1 Introduction

Figure 2 Publisher and Subscriber entities

https://reference.opcfoundatio.....art14/5.1/

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zbynekz
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04/25/2021 - 22:58
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Unless I am looking at wrong place or I am effectively blind, there is no LDS on that figure.

You might be referring to the "Directory" box on Figure 2. In my understanding, that does not refer to any specific implementation or technology - it just shows a concept that publishers can register themselves into some kind of directory service, and other applications can query the directory to find the publishers.

I can imagine that on a single subnet, the "Directory" can be LDS-ME. But in your case you are likely to have multiple subnets, and the following statement:

"Between LDS-A and LDS-B, it will automatically announce each own group of OPCServers to each other."

is not true when LDS-A and LDS-B are on different subnets.

I would say that in your case, the role of "Directory" (if any) is more likely to be fulfilled by a GDS.

Best regards

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Jacky Bek
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04/26/2021 - 04:41
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sorry.. you are right.. i am referring to the blue object labelled as 'Directory'.

I assume this refers to the UA Local Directory Services.

It is better for me to show you a diagram illustrating what i meant.

MQTTPubSub.png

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zbynekz
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04/26/2021 - 10:31
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Somehow, the attached image is not visible/accessible to me.

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Jacky Bek
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04/27/2021 - 09:06
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