Asynchronous communication, Web Sockets, MQTT

Synchronous communication

Example of synchronous communication

• A user clicks a button on a webpage
• The browser makes a HTTP request to the server
• The browser waits until the server repsonds and renders and renders the output on the reponse

JavaScript brings asynchronous nature to the UI. exmaple

• A user clicks a button on a web page. This is intercepted by the JS layer.
• The JS layer makes a HTTP request to the server and binds an event handler to execute when the response arrives. Other activities can continue meanwhile.
• When the server responds, the event handler code is executed

Asynchronous Communication

• Asycnronous commuicnation is often neccessary on the web
  • HTTP requests have a time limit
  • Activities may actually take long(e.g hours)
  • We do not have control of the other end or its load
• Three common ways to implement asynchronous communication using HTTP
  • Polling
    • Sending requets at a consistent rate and server returns updates accordingly
  • Long-polling
    • the client sends a request which is immediately responded ,followed by another request for which the server returns a response when something new is available
  • Push
    • after a request is being sent, the repsonse by the server kept open.

Web Socket

• HTTP is not a bi-directonal protocol
• Web Sockets provdes full-duplex communication channel over a single TCP connection
  • Allows interaction between a web browser or any other client application and the web server with lower overhead
  • No need to make requests, each party can send content and pass messages back and forth
  • Designed for standard ports 80 and 443 to be usable with firewall

Client request:

GET / HTTP/1.1
Upgrade: websocket
Connection: Upgrade
Host: example.com
Origin: http://example.com
Sec-WebSocket-Key: sN9cRrP/n9NdMgdcy2VJFQ==
Sec-WebSocket-Version: 13

Server Respose:

HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: fFBooB7FAkLlXgRSz0BT3v4hq5s=
Sec-WebSocket-Location: ws://example.com/

Message Brokers

• can act as an intermediary between two communicating parties
• Asynchronous by nature
• Pattern
  • Streams/queues
  • Publish-subscribe
• Many support HTTP, REST,SOAP or any other means for client/application components to connect

MQTT(OASIS)

• Message Queuing Telemetry Transport

• Lightweight IoT or M2M protocol

• Based on publish/subscibe communication pattern

  • event-driven and enbales messages to be pushed to clients

• Message exchanged based on topics

  • my house/bedroom2/temperature
  • house/+/temperature
  • house/#

• JSON/XML or whatever content

  

MQTT messages and QoS

• Messages

  • Connect
  • Disconnet
  • Publich(TopicName, QoS, RetainFlag, Payload, Packet identifier, …)
  • Subscribe/SubAck
  • Unsubscrible /UnsubAck

• Each connection has QoS measure

  • at most once (QoS 0)

    • the message is sent once and the client and broker take no additional steps to acknowledge delivery (fire and forget)

• At least once (QoS1)

  • the sender re-sends the message multiple times until acknowledgement is received (acknowledged delivery)
  • Exactly once (QoS2)
    • the sender and receiver engage in a handshake to ensure only one copy of the message is received (Assured Delivery)

AMQP

• Adanced Message Queuing Protocol is an open standard protocol for message oriented middleware

• • Message Oriented Middleware is a concept that involves the passing of data between applications using a communication channel that carries self-contained units of information (messages).
• AMQP was designed to efficiently support a wide variety of messaging applications and communication patterns.

• Typically AMQP is used as a message broker and it has much more complex and versatile communication patterns than, for example, MQTT

• Two versions currently
  • Pre 1.0 (e.g. 0-9-1 RabbitMQ, …)
  • 1.0 (ISO/IEC 19464) (fewer implementations, more complex)
• Can be scaled and chained using multiple message broker instances for extreme scalability (fairly easy above 1M+ message per second)

Exchange

• Exchanges are AMQP 0-9-1 entities where messages are sent
• Exchanges take a message and route it into zero or more queues.
• The routing algorithm used depends on the exchange type and rules called bindings

Illustration of the
	It has one special property that makes it very useful for simple applications: every queue that is created is automatically bound to it with a routing key which is the same as the queue name.
	A fanout exchange routes messages to all of the queues that are bound to it and the routing key is ignored. Fanout exchanges are ideal for the broadcast routing of messages.

DDS - Data Distribution Service

• A real-time machine to machine communication standard aiming to enable data exchange in a publish-subscribe pattern to be
  • scalable
  • real-time
  • dependable
  • high-performance
  • interoperable
• Intended application areas includes
  • Air-traffic control
  • Smart-grid management
  • Autonomous vehicles
  • Robotics
  • Transportation systems
  • Aerospace
• Characteristics
  • Empahasis on QoS parameters and configuration up-front
  • Anonymous transmission of messages
  • Automatic failover and handling of-redundant information sources

OPC UA

• Communication Standard originally designed for industrial automation and control purposes but nowadays has application in many other application areas as well

• Designed to unify communication and facilitate integration of components from different vendors

OPC UA Publish/Subscribe (2018)

• Message Bus based publish/subscribe communication
  • Enables using the OPC UA information model in other means of communication and standard exchange such as MQTT and AMQP