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 connected 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 ws://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 sales@iotready.com