Websockets for Hardware Engineers - Part 1

Before we start, what is HTTP?

This is what HTTP REST looks like…

We noticed some issues…

• Each request is preceded by an SSL handshake (if HTTPS) and authentication
• This can consume significant resources (CPU, time, memory) each time
• The server can never send data to the client
  • The client needs to implement polling to request data any time it is needed.

What are websockets?

Websockets are a communication protocol that allow the creation of persistent, bidirectional communication channels between client and server.

Persistent

• Once established, the connection between client and server is kept-alive for a long period
  • No theoretical limit
  • Connection can be kept alive through regular pings
• Disconnections are events that can be monitored to trigger reconnection

Bidirectional

• Once the connection is established, the server can send information to the client at any time.
  • No need for the client to initiate the request
• Client can also send requests to the server

Channels

• Each websocket connection is actually a channel
  • Imagine a long pipe where information can flow both ways
• Each channel is opened on a specific address, and typically, has a specific purpose
• Each client can open multiple channels with the same server
• Multiple clients can open channels with the same server

Points to Note

• Websockets is based on TCP so the server cannot broadcast messages to all clients
  • Instead, on the server, maintain a list of connnected clients and send them messages in a loop
• Each websocket connection consumes memory so close unused connections
  • Enforce ping based check of alive clients - if no response, close the connection!
• Websockets only concerns itself with the transport layer (on top of TCP) and makes no recommendation about the encoding of the data being transmitted.
  • You can transmit text
  • Or JSON
  • Or even binary files

Exercises

Tools we need

• websocat or similar command line websockets client for Exercise 1
• NodeJS or Python 3 for Exercise 2
• ESP32 and ESP-IDF for Exercise 3

1. Public Websocket Server

• Use websocat from a terminal to connect to the Binance websocket streams
  • These endpoints are known to work:
    • wss://stream.binance.com:9443/ws/!miniTicker@arr
    • wss://stream.binance.com:9443/ws/btcusdt@depth
    • wss://stream.binance.com:9443/ws/bnbbtc@depth
• Open another terminal/tab and connect to a different stream from Binance
• For the really patient, check how long the connection remains open for with:

  date; websocat wss://stream.binance.com:9443/ws/bnbbtc@depth; date
  

2. Local Websocket Server

NodeJS

• Follow this excellent tutorial to set up a local websocket client and server

Python

• Follow this tutorial to set up a local websocket client and server

3. ESP32 as a Websocket Server

• Set up ESP-IDF on your computer
• Build and flash the Websocket Echo Server example from ESP-IDF on the ESP32
  • You will need to configure your WiFi settings with idf.py menuconfig
  • Once booted, the ESP32 will print its ip_address. You will need this.
• Using websocat, connect to http://ip_address/ws/
• Once connected, type and send any string message, e.g. hello
• Open a new terminal and repeat the websocat messaging.
  • What do you notice across the two clients?

More advanced example

• Try this slightly more advanced example

Questions

• What are the similarities and differences between HTTP and BLE?
• What are the similarities and differences between Websockets and BLE?
• How would you implement an API for client to request changes (e.g. switch on lights)?

Ideas, questions or corrections?

Write to us at hello@iotready.co