Websocket message sended twice by a lambda triggered by DynamoDB Stream and with a buffer offset

It looks as if your Lambda function execution environment freezes before actually sending the data. Can it be that you are not waiting on the right thing so it only gets into the Node.js event loop but only gets performed in the next invocation?

This is the code of my lambda triggerred by dynamoDB Stream :

'use strict';

var AWS = require("aws-sdk");
const ENDPOINT = 'https://8tfhywx7dk.execute-api.eu-west-2.amazonaws.com/dev/';
const DB_TABLE = 'ConnectionDB';

let client = new AWS.ApiGatewayManagementApi({ endpoint: ENDPOINT });
let dynamo = new AWS.DynamoDB.DocumentClient();

exports.handler = async (event, context, callback) => {
    // on récupère les connexions clients
    const connections = await getAllConnections();

    event.Records.forEach((record) => { 
        // console.log('Stream enregistrement: ', record)
        if (record.eventName == 'INSERT') { 
            var id = JSON.stringify(record.dynamodb.NewImage.id.S);
            var nom = JSON.stringify(record.dynamodb.NewImage.nom.S);
            const message = {
                action: 'insert',
                message: `{"id":${id},"nom":${nom}}`
            }
            for (var connectID of connections) {
                sendMessage(connectID, message);
            }
        }
    });
};

async function getAllConnections() {
    const { Items } = await dynamo.scan({
        TableName: DB_TABLE,
        AttributesToGet: ['connectionID']
    }).promise();

    const connections = Items.map(({ connectionID }) => connectionID)
    return connections;
}

async function sendMessage(clientId, message) {
    const params = {
        ConnectionId: clientId,
        Data: JSON.stringify(message) // message envoyé au client 
    };
    console.log('postToConnection: ', clientId, " message : ", message)
    await client.postToConnection(params).promise()
    console.log('Message émis')
}

Now if I add a new data in DynamoDB :

{ "id": {  "S": "UC0005"  },  "nom": {  "S": "mon mac5" }}

1) The value is added in the DB.

2) CloudWatch for this lambda see the POST at the right hour without any error :

| 1646755297533 | 2022-03-08T16:01:37.533Z 99bd4d1d-a40f-4357-9979-317711de0d74 INFO postToConnection:  OqyaRcjprPEAcvw=  message :  { action: 'insert', message: '{"id":"UC0005","nom":"mon mac5"}' } 
| 1646755297595 | END RequestId: 99bd4d1d-a40f-4357-9979-317711de0d74
etc.

3) CloudWatch for the API Getway (/aws/apigateway/8tfhywx7dk/dev) DON'T show any log !!! 4) And of course the front-end don't received anything !

Now I add a new value:

{ "id": {  "S": "UC0006"  },  "nom": {  "S": "mon mac6" }}

1) The value is added in the DB.

2) CloudWatch for this lambda see the POST at the right hour :

2022-03-08T16:04:41.894Z	a43ee8ba-ecad-4d16-956d-496f89e8b41e	INFO	postToConnection:  OqyaRcjprPEAcvw=  message :  { action: 'insert', message: '{"id":"UC0006","nom":"mon mac6"}' }
END RequestId: a43ee8ba-ecad-4d16-956d-496f89e8b41e

3) CloudWatch for the API Getway (/aws/apigateway/xxxxxxxxx/dev) shows now a new log :

...                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
| 1646755480663 | (b6478f03-36ad-4bb3-b27b-19ea70ab7254) WebSocket Request Route: [POST /@connections/{connectionId}]                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
| 1646755480663 | (b6478f03-36ad-4bb3-b27b-19ea70ab7254) Method request path: {connectionId={connectionId}}   
(b6478f03-36ad-4bb3-b27b-19ea70ab7254) Method request body before transformations: {
    "action": "insert",
    "message": "{\"id\":\"UC0005\",\"nom\":\"mon mac5\"}"
}
etc...                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     

4) And of course the front-end (I used https://www.piesocket.com/) received the old value ! But two times !

- RECEIVED: {"action":"insert","message":"{\"id\":\"UC0005\",\"nom\":\"mon mac5\"}"}
- RECEIVED: {"action":"insert","message":"{\"id\":\"UC0005\",\"nom\":\"mon mac5\"}"}

I AM LOST !!!