Building a Chat Room with Lambda and Websocket in AWS API Gateway

WebSocket Overview #

• WebSocket Characteristics
  • Bi-directional, persistent TCP connection between client and server
• Scaling Limitations
  • WebSocket is stateful, cannot horizontally scale without a backend to store state (eg: Redis)

API Gateway with Websocket #

• Benefits

  • Aids scaling by maintaining WebSocket connections
  • Serverless - Client and Lambda interact with API Gateway
  • Lambda uses APIGatewayConnection API to send data to open connections
  • Client sends data to gateway directly

• Limitations

  • WebSocket connection lasts for 2 hours
  • Idle timeout is 10 minutes

Lab: Building a Chat Room #

Setup #

I watched this tutorial on how to build a chat using Lambda, WebSocket, and API Gateway with Node.js

Access the code in this GitHub Gist

• Server Setup

  • server.py is a simple HTTP server with Python to serve the client-side code

• Lambda Function

  • index.mjs is the Lambda code for the chat room
  • Implements $connect, $disconnect, join, leave, and sendAll functions
  • Uses DynamoDB to store connectionIds and roomId with the connectionIds that joined

Common Errors and Solutions #

Global Variable Persistence #

• The video stores the connections and rooms data in global variables in lambda.
• Initially, it was thought that it would not work because each invocation is stateless.
• However, variables in the global scope can be shared between Lambda invocations.
• This is mentioned in the Comparing the effect of global scope.
• However, it is more reliable to use a database for storing data.
• The use of global variables should be limited to demo purposes only and should never be used in the real world.

Connection Endpoint #

• The endpoints can be found in the AWS Console under “API Gateway” > “Stages”.
• One is WebSocket, starting with wss://, and another is HTTPS, starting with https://.
• When posting the data back to the client from Lambda, use the HTTPS endpoint but remove @connections at the end.
• For example, if the endpoint on the console is https://d6sq1c52q5.execute-api.us-east-1.amazonaws.com/production/@connections, we should omit the @connection when using PostToConnectionCommand.

Lambda Permissions #

• Lambda needs permission to call invoke and manage connection
• Use AmazonAPIGatewayInvokeFullAccess or create a custom policy

Custom policy example:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "execute-api:Invoke",
                "execute-api:ManageConnections"
            ],
            "Resource": "arn:aws:execute-api:us-east-1:059035646743:d8ck9i69wa/production/*""
        }
    ]
}

Refer to the AWS documentation for more information.

“Going Away” Exception #

• You might encounter a “Going away” exception if you try to post data to a connection in $connect
• This is not supported, as mentioned in this Stack Overflow post

Using Node.js 18 in Lambda Function #

• If you use Node.js 18, require('aws-sdk') will not work due to the use of ES module style
• When connecting to the WebSocket, it may return a 502 error, which is also logged in the API Gateway access log but not useful for debugging purposes.
• To diagnose the issue, check the Lambda error log on CloudWatch

To tail the CloudWatch log, use the following command:

aws logs tail <log_group> --follow

Refer to the AWS CLI documentation for more information.