Keep getting and posting /socket.io/?EIO=3&transport=polling&t=

Question

My backend in ExpressJS and NodeJS used to work well. I just realized that logs exploded the disk (the backend is still functional), because when the backend is on, it keeps trying:

kpi.js GET /socket.io/?EIO=3&transport=polling&t=NBiaEK6
index.js GET /socket.io/?EIO=3&transport=polling&t=NBiaEK6
index.js router.get *
kpi.js POST /socket.io/?EIO=3&transport=polling&t=NBiaER6
index.js POST /socket.io/?EIO=3&transport=polling&t=NBiaER6
Error: Not Found
    at /opt/funfun/app.js:99:13
    at Layer.handle [as handle_request] (/opt/funfun/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/opt/funfun/node_modules/express/lib/router/index.js:317:13)
    at /opt/funfun/node_modules/express/lib/router/index.js:284:7
    at Function.process_params (/opt/funfun/node_modules/express/lib/router/index.js:335:12)
    at next (/opt/funfun/node_modules/express/lib/router/index.js:275:10)
    at /opt/funfun/node_modules/express/lib/router/index.js:635:15
    at next (/opt/funfun/node_modules/express/lib/router/index.js:260:14)
    at /opt/funfun/routes/index.js:18:2
    at Layer.handle [as handle_request] (/opt/funfun/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/opt/funfun/node_modules/express/lib/router/index.js:317:13)
    at /opt/funfun/node_modules/express/lib/router/index.js:284:7
    at Function.process_params (/opt/funfun/node_modules/express/lib/router/index.js:335:12)
    at next (/opt/funfun/node_modules/express/lib/router/index.js:275:10)
    at Function.handle (/opt/funfun/node_modules/express/lib/router/index.js:174:3)
    at router (/opt/funfun/node_modules/express/lib/router/index.js:47:12)
    at Layer.handle [as handle_request] (/opt/funfun/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/opt/funfun/node_modules/express/lib/router/index.js:317:13)
    at /opt/funfun/node_modules/express/lib/router/index.js:284:7
    at Function.process_params (/opt/funfun/node_modules/express/lib/router/index.js:335:12)
    at next (/opt/funfun/node_modules/express/lib/router/index.js:275:10)
    at /opt/funfun/node_modules/express/lib/router/index.js:635:15
kpi.js GET /socket.io/?EIO=3&transport=polling&t=NBiaF8A
index.js GET /socket.io/?EIO=3&transport=polling&t=NBiaF8A
index.js router.get *
kpi.js POST /socket.io/?EIO=3&transport=polling&t=NBiaFFz
index.js POST /socket.io/?EIO=3&transport=polling&t=NBiaFFz
Error: Not Found
    at /opt/funfun/app.js:99:13
    at Layer.handle [as handle_request] (/opt/funfun/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/opt/funfun/node_modules/express/lib/router/index.js:317:13)
    at /opt/funfun/node_modules/express/lib/router/index.js:284:7
    at Function.process_params (/opt/funfun/node_modules/express/lib/router/index.js:335:12)
    at next (/opt/funfun/node_modules/express/lib/router/index.js:275:10)
    at /opt/funfun/node_modules/express/lib/router/index.js:635:15
    at next (/opt/funfun/node_modules/express/lib/router/index.js:260:14)
    at /opt/funfun/routes/index.js:18:2
    at Layer.handle [as handle_request] (/opt/funfun/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/opt/funfun/node_modules/express/lib/router/index.js:317:13)
    at /opt/funfun/node_modules/express/lib/router/index.js:284:7
    at Function.process_params (/opt/funfun/node_modules/express/lib/router/index.js:335:12)
    at next (/opt/funfun/node_modules/express/lib/router/index.js:275:10)
    at Function.handle (/opt/funfun/node_modules/express/lib/router/index.js:174:3)
    at router (/opt/funfun/node_modules/express/lib/router/index.js:47:12)
    at Layer.handle [as handle_request] (/opt/funfun/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/opt/funfun/node_modules/express/lib/router/index.js:317:13)
    at /opt/funfun/node_modules/express/lib/router/index.js:284:7
    at Function.process_params (/opt/funfun/node_modules/express/lib/router/index.js:335:12)
    at next (/opt/funfun/node_modules/express/lib/router/index.js:275:10)
    at /opt/funfun/node_modules/express/lib/router/index.js:635:15

kpi.js GET ... and index.js GET ... are what I print. Here is the code of opt/funfun/app.js:

// catch 404 and forward to error handler
app.use(function (req, res, next) {
  var err = new Error('Not Found');  // line 99
  err.status = 404;
  next(err);
});

Does anyone know what may be the reason?

Answer 1

This may happen if you have a socket.io client trying to connect to your backend, but your backend does not have a socket.io server configured or is not properly configured to accept connection and you are logging any errors in the error handler.

Make sure the socket.io client and server are properly configured. Please see the socket.io docs on how to setup the client and the server.

By default the client keeps trying to reconnect infinitely when the connection fails, you can prevent it from polling the server infinitely by setting the reconnectionAttempts options.

For example, this will prevent the client from polling the server after 10 failed attempts

const socket = io(serverURL, { reconnectionAttempts: 10 });

Also to minimize the size of your logs, don't log the entire error object when you are in production, you could update the error handler to log only relevant details

// catch errors and forward to error handler
app.use(function (req, res, next) {
    ...
    next(err);
});

//In the error handler 
app.use(function (err, req, res, next)  {
    //log only relevant details
    logger.info(err.message)
})