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Session 3: Live updates on your frontend using Websockets

First, checkout session-3 branch, and deploy the resources for session-3.

git checkout session-3-v2
cd backend
sls deploy

You can then open:

Now, it's your turn 💪

1. Store the connections

  • The lambdas connect.ts and disconnect.ts must be triggered when a new websocket connection is established and when a connection is closed. Register them in the serverless.ts (useful link)
  • Implement createConnection to store a connection in the DynamoDB table when connect.ts is triggered
  • Implement deleteConnection to delete the connection when disconnect.ts is triggered
  • Deploy your stack
  • Check on the API Gateway interface that you have a new API Gateway

2. Connect the frontend and add viruses on websocket messages

  • Execute yarn intall to get the new dependency: reconnecting-websocket
  • Add your websocket API url in REACT_APP_WEBSOCKET_URL of .env.development.local
  • Uncomment the code in Home.tsx to connect the frontend to the websocket API and add viruses when a message is received
  • Check on the DynamoDB interface that a connection is stored in the table when the frontend is started

3. Send a virus to your frontend each minute

  • Trigger the createVirus lambda every minute (useful link)
  • Modify create.ts to send the created virus to the front through the Websocket API. You will need to:
    • Fetch all active connections
    • Send to each one a message with the new virus id (useful link)
  • To avoid the double display of a virus created by clicking on the +. Remove the code which display a new virus after the http call.

Bonus 1: Use DynamoDB streams to separate createVirus from sendMessageToClient

The lambda createVirus has two functions: create the virus and send a message to each client. It's a better practise to split it in two lambdas.

  • Uncomment the code in dynamodb.ts to enable the stream of the DynamoDB. Lambdas could subscribe to this stream and be trigger at every action on the table
  • Uncomment the code in serverless.ts to give the right to your lambda to listen the streams
  • Uncomment the code in serverless.ts to trigger sendMessageToClient on DynamoDB stream
  • Move the code which send the message to the clients from create.ts to sendMessageToClient.ts
  • On every insert of Virus, send a message to the clients:
    • SendMessageToClient will receive events from the DynamoDB batched. You will need to loop over the batch to handle each event (useful link)
    • The object NewImage of a streamRecord must be parsed to be used: use Converter.unmarshall to parse it. (useful link)
    • Don't forget to filter the record on eventName === 'INSERT' and primaryKey === 'Virus'

Bonus 2: Create a fully real time application

  • If you open a second frontend in another windows, you will see the viruses appear in real time, but if you destroy a virus it will not disappear in the other windows. Make it happens!

Pro tips:

  • If you want to deploy only one function, use serverless deploy -f <your-function-name>. It is much faster (~10s) than deploying a stack (~30s) but will only deploy the function's code, not any config change in your serverless.ts.
  • To call locally one function, use serverless invoke -f <your-function-name> --path path/to/mocked-event.json

Done ? Nice work ! Don't forget to kill your stack by running serverless remove in the backend folder !

Have you killed your stack ? You just have created an app which trigger a lambda every minute! Don't forget it or it could be expensive ;)

To see final result, checkout the start of session 4.