Introduction

Greater interoperability and seamless communication are the prime motives behind the emergence of OPC UA. While client-server model is the historical way OPC UA applications have been communicating with each other, it is considered more appropriate for point to point communication.

The client and server are directly connected to each other, where a PLC or a host workstation can request for an information, which will be fetched by the sensors, valves, or other field devices. However, as the number of devices on both the ends increase, the number of data requests increase, and in turn the network throughput decreases. Additionally, in client server Model, the response rate for data was always the least of what both components could support.

With a swift proliferation of IIoT devices, a more robust communication network that was independent of client server capabilities and worked in a connection less environment was essential. In order to meet this pressing need, OPC UA Part 14 Specification has emerged, that discusses a PubSub model, a perfect solution to enable communication in modern IIoT scenarios. OPC UA PubSub is the established standard for the IIoT platforms for all the cloud vendors.

In contrast to client-server communication, OPC UA PubSub Bridge enables scalable fast communication which is independent of client or server limitations.

In this model, the publishers send the messages to Message Oriented Middleware without knowing about the Subscriber(s). In the similar fashion, the subscribers show interest in specific messages without knowing about the publisher(s). These publishers are OPC UA servers and the subscribers are OPC UA clients.

Why Industries Need OPC UA PubSub?

In OPC UA Client-Server based communication, the servers wait for the client(s) to make the communication requests. Once the server answers to the request(s), it goes back into the wait state again, till another communication request arrives. So the OPC UA server can not publish data faster than what OPC UA client demands.

In a typical client-server communication, the service-oriented architecture is followed, while it provides compatibility and interoperability, it has some limitations which are:

  • Limited Scalability : As the client(s) and server(s) are directly connected to each other, this limits the capabilities of both the ends with increasing load. An OPC UA server cannot scale up supporting many OPC clients while keeping the update rates in seconds or milliseconds.
  • Publish Cycle Dependency : The data samples which have missed a particular publish cycle will only be picked up in the next Publish cycle.
  • Requirement for Larger Footprint : With increased number of clients, the server’s memory and processing requirements increase to support multiple client messaging sessions, data storage, and messaging requirements with separate clients.

OPC UA PubSub Bridge Architecture

OPC UA PubSub addresses the above challenges in a more secure and streamlined fashion. Here the OPC Applications can take the role of publishers and subscribers with messages being exchanged via a middleware. The Message Oriented Middleware supports connecting, sending, and receiving messages between distributed applications with the help of protocols.



The concept of PubSub allows using OPC UA directly over the internet or the cloud using middleware like MQTT (Message Queue Telemetry Transport) and AMQP (Advanced Message Queuing Protocol). Therefore, none of the applications communicate to each other directly. The middleware acts as a broker that directs messages.

Every DataSet is transmitted as a DataSetMessage in a specific format specified by the DataSetWriter. There are two formats in which the DataSets are written: JSON and UADP.

OPC UA PubSub resolves the challenges posed by Client-Server in the following ways:

  • Easy to Scale: As the client(s) and Server(s) are not directly connected to each other, instead communicate to a broker middleware, increasing the number of devices in either side does not impact the performance. This way, the architecture becomes more scalable.
  • No Data Response Rate Constraints: In contrast to client-server model, both the parties need not agree upon a messaging interval. The server can publish the datasets whenever it wants irrespective of the client’s data request.
  • IIoT Enabler: OPC UA PubSub is the established standard for the IIoT platforms for all the cloud vendors. With an ability to enable distributed intelligence, PubSub balances the network loads and distributed messages to the peers in an effective manner, which is the foundation for IIoT.
  • Why Utthunga for OPC UA PubSub Bridge?

    Utthunga is one of early adopters in the OPC PubSub products. We are coming up with an OPC PubSub Bridge that is built to be compliant with the OPC UA specification. The reasons why Utthunga should be your first choice for OPC UA PubSub Bridge implementations are:

    1. Multiple Scenarios for Deployment: Utthunga’s top-notch skills in OPC allow deploying PubSub Bridge in multiple variations, two of these cases are explained below.
      • One-to-One: One publisher sends the message to one middleware through which one subscriber receives the intended message.
      • Many-to-Many: Multiple publishers send the data to many middleware, thereafter many subscribers receive the message.

    2. Support for Classic Framework: Utthunga’ s PubSub architecture supports the non-OPC UA data sources like classic servers, databases or other message queues.
    3. Near Real-time Responsiveness: Utthunga lets you experience near real-time message (microseconds) transmission and receiving large chunks of messages.
    4. End-to-End Security: By deploying Utthunga’ s PubSub model, you will equip your network with end-to-end security regardless of the transport protocol implemented by you. With JSON and UADP, whether it is on-premise, or on cloud, your network remains secure.

    OPC UA PubSub Bridge is the future of industrial automation. With ability to be deployed on the embedded devices deployed on the shop floor and integrated on the scalable cloud-based applications, OPC UA PubSub is going to make production smarter and more optimized. Though it is in early stages of development, but it has a wide range of industrial applications.

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