pendulums

Simple pendulum simulation

Take Home Exercise

In node.js, implement a "simple pendulum" in 1 dimension. A REST interface will allow to set up an initial angular offset, a mass, a string length (or other settings of your choice). It will also allow you to read the last coordinates, such as to be able to represent the pendulum in a UI, in an external process. The rate of simulation may be higher than the rate of visualization. Let's run 5 instances of the simple pendulum node.js process, each on its own tcp port. During configuration, also tell each node who are their immediate neighbours. Let's have one UI process, displaying in the web browser the five pendulums. It would allow the user to configure them (starting angle, mass, length, or anything else you have chosen) in an intuitive way, conveying an easy-to-use user experience. The UI should expose a canvas where the user could drag and drop the pendulum in their desired start position. All the resulting parameters of that "configuration" would be transferred in JSON format to the REST API.

The UI would also expose some simulation controls (start, pause, stop), and would poll periodically the 5 pendulum instances (using a HTTP client) to display them. A refresh rate of a few frames per second is fine. During the simulation, each pendulum node should watch the position of its immediate neighbours periodically (via REST). Whenever they come "too close" (threshold is at your discretion), send a STOP message using a "guaranteed delivery" communication channel like MQTT, to all 5 instances. Upon reception of STOP, each instance stops moving immediately, and waits for 5 seconds before sending RESTART on the same channel. Once all 5 instances have received all 5 distinct RESTART messages, all will restart moving, jumping back to their original position. Document briefly the resulting REST interface.

To Run

Run both the React frontend and Node.js backend in seperate terminals.

Run the backend

1. cd server
2. npm run dev ( this will start 5 instance, one for each pendulum )

Run the frontend

1. cd client
2. npm run start

Open browser to http://localhost:3000/ to see animation.

Usage

• Drag pendulums from the center to move
• Drag pendulums from the edges to size
• Pendulum editing is only allowed when the simulation is stopped (no editing a paused simulation)

Backend REST Endpoints

The REST API for the Pendulum Simulation Backend is available here Pendulum API Docs

Implementation Plan

Day 1

1. Setup React client and Node.js server folders and projects
   • npx create-react-app my-app --template typescript
2. Get editor working in client..
3. Create initial setup
   • 5 pendulums always (backend ports will be 5001-5005)
   • colors will be fixed (blue, orange, yellow, green, blue)
   • all pendulums have a default size
4. Create edit pendulum feature
   • drag centrer of pendulum to move pendulum location
   • drag edge of pendulm to change size
5. Get simulation working in client
   • create [play] button to start simulation
   • create [stop/edit] button to reset simulation and enter edit mode
   • create [pause] button to pause simulation

Day 2

• create 5 pendulum servers on ports 5001-5005
• during configuration tell each node who are their immediate neighbors.
• during configuration set the "too close" value
• configure clients to use mqtt https://www.npmjs.com/package/mqtt listening to test.mosquitto.org on private channel
• create PUT pendulum endpoint to replace / edit a pendulum
• create GET pendulum endpoint to return the pendulum's data
• create PUT simulation/status endpoint to start/pause/stop simulation (stop resets)
• update GET pendulum endpoint to return the current simulation state of the pendulum (coordinates)
• during simulation each pendulum node should watch the position of its immediate neighbours periodically (via REST). Whenever they come "too close" send a STOP message using a "guaranteed delivery" communication channel like MQTT, to all 5 instances. Upon reception of STOP, each instance stops moving immediately, and waits for 5 seconds before sending RESTART on the same channel. Once all 5 instances have received all 5 distinct RESTART messages, all will restart moving, jumping back to their original position.
• add instructions to run demo
• document the endpoints