Websocket on invoked lambda ignore message and send the last message received instead

The issue is the way Node.js works. You are calling postToConnection, but what actually happens is that an event is added to the Node.js event loop. It is not performed immediately. You get to the end of the function and the run-time is frozen. At the point we do not handle the rest of the events until you invoke the function again. We then unfreeze the run-time and handle the next event.

To fix it, you should use await when calling postToConnection.

Hello @Uri, thank you for answering.

Unfortunately, this is already the case, I am already using async/await :

const AWS = require("aws-sdk");
const apigwManagementApi = new AWS.ApiGatewayManagementApi({
  endpoint: process.env.WEBSOCKET_DOMAIN_NAME
});
const dynamo = new AWS.DynamoDB.DocumentClient({ region: process.env.AWS_REGION });

/**
 * Send Websocket Message
 */
const websocketSendMessage = async (connectionId, data) => {
  const params = {
    ConnectionId: connectionId,
    Data: JSON.stringify(data)
  };
  await apigwManagementApi.postToConnection(params).promise();
  return;
};

/**
 * Handler
 */
const handler = async (event) => {
  const { userId, data } = event;

  // Retrieve all connectionId of user from DynamoDB
  let items;
  const dynamoItem = await dynamo.scan({
    TableName: process.env.DYNAMO_TABLE_NAME,
    FilterExpression: "userId = :userId",
    ExpressionAttributeValues: { ":userId": userId }
  }).promise();
  items = dynamoItem.Items;

  // Send websocket message for each websocket client of user
  items?.forEach(async (item) => {
    await websocketSendMessage(item.connectionId, data);
  });

  return { statusCode: 200 };
};