AWS API Gatewat WebSocket：集成后端中缺少转换后请求的请求模板正文

Question

I am building my app on AWS and yy app uses websocket like this:

Frontend WebSocket client ---> AWS API Gateway Websocket API ----> Backend in EC2 instance.

Now, in order to let my backend Express code know how to send message to a particular client, I have let it know the connectionId of a websocket client / a user. I am following these two answers: https://stackoverflow.com/a/59220644/3703783

https://stackoverflow.com/a/65112135/3703783

which have explained very clearly.

Below is my configuration in AWS API Gateway WebSocket API:

What I did was using Request Template, matching all incoming requests, to transform the content which will be sent to my integration endpoint as the body of the request. Eventually I want to keep the original request, while adding attributes (like Connection Id) on top of it. For testing purpose, I set the following template:

{
    "myConnectionId": "$context.connectionId",
    "body": "$context"
}

This worked and I can check in AWS CloudWatch that the request body after transform is

{
   "myConnectionId":"MFEdof95tjMCK0w=",
   "body":"{routeKey=$connect, disconnectStatusCode=null, messageId=null, eventType=CONNECT, extendedRequestId=MFEdoFJPtjMF64Q=, requestTime=17/Jan/2022:07:29:01 +0000, messageDirection=IN, disconnectReason=null, stage=production, connectedAt=1642404541417, requestTimeEpoch=1642404541418, identity={cognitoIdentityPoolId=null, cognitoIdentityId=null, principalOrgId=null, cognitoAuthenticationType=null, userArn=null, userAgent=Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/97.0.4692.71 Safari/537.36, accountId=null, caller=null, sourceIp=219.102.102.145, accessKey=null, cognitoAuthenticationProvider=null, user=null}, requestId=MFEdoFJPtjMF64Q=, domainName=123.execute-api.ap-northeast-1.amazonaws.com, connectionId=MFEdof95tjMCK0w=, apiId=hd5zymklr8}"
}

However, in my Express backend running in EC2 instance, even though the endpoint is triggered, the request body is empty and there is no place to find myConnectionId in it.

Backend: NodeJS / ExpressJS, and in index.ts :

  app.get('/connect', function(_req, res) {
    logger.info(`/connect _req: ${Object.keys(_req)}`);
    logger.info(`/connect _req.query: ${JSON.stringify(_req.query)}`);
    logger.info(`/connect _req.params: ${JSON.stringify(_req.params)}`);
    logger.info(`/connect _req.body: ${JSON.stringify(_req.body)}`);
    logger.info(`/connect _req.headers: ${JSON.stringify(_req.headers)}`);
    res.send('/connect hahaha success');
  });

Log output:

2022-Jan-17 05:05:00:50  info: /connect _req: _readableState,_events,_eventsCount,_maxListeners,socket,httpVersionMajor,httpVersionMinor,httpVersion,complete,rawHeaders,rawTrailers,aborted,upgrade,url,method,statusCode,statusMessage,client,_consuming,_dumped,next,baseUrl,originalUrl,_parsedUrl,params,query,res,_startAt,_startTime,_remoteAddress,body,_parsedOriginalUrl,route
2022-Jan-17 05:05:00:50  info: /connect _req.query: {}
2022-Jan-17 05:05:00:50  info: /connect _req.params: {}
2022-Jan-17 05:05:00:50  info: /connect _req.body: {}
2022-Jan-17 05:05:00:50  info: /connect _req.headers: {"x-amzn-apigateway-api-id":"hd5zymklr8","x-amzn-trace-id":"Root=1-61e5232c-626a05ff264650183a73c98a","user-agent":"AmazonAPIGateway_hd5zymklr8","content-type":"application/json","accept":"application/json","host":"NLB-docloud-internal-ea0692d1e2c8186c.elb.ap-northeast-1.amazonaws.com","connection":"Keep-Alive"}

The request is indeed sent to the endpoint, but why is the request body empty?

How is the content lost?

Answer 1

I have done some research on this.

As it has been discussed in this post: AWS API Gateway Websockets -- where is the connectionID? , the accepted answers (the solution I was trying in my question above) are assuming your are using Lambda as the backend integration.

However, I am using Express JS running in EC2 instance and VPC Link.

In my case, the issue actually is solved by one of the upvoted answers (but not the accepted one): https://stackoverflow.com/a/65639742/3703783 .

The official documentation is here: Setting up data mapping for WebSocket APIs .

Update

I also found that in my original question, instead of doing

{
    "myConnectionId": "$context.connectionId",
    "body": "$context"
}

I should simply do:

{
  "integration.request.header.connectionId":"$context.connectionId",
}

and this will fix the issue.