I'm sorry, I've not had time to look at this yet. I'll take a look at the weekend.

I've recreated the problem with your code, and I think that I understand the problem.

The error that is reported, Invalid state; requests can only be made in the LoggedIn state, not the SentClientRequest state, indicates that there has been an attempt to execute a second request on a connection while one is already in progress. Concurrent requests (on a single connection) are not supported. This should probably be made clearer in the documentation.

Your code always causes this problem on the second request. Each page request results in a call to this code.

controller.execute = function(session, admin, sqlCommand) {
    var MSSqlConnection = require('./MSSqlConnection');

    MSSqlConnection.on('connected', function(connection){
            _performQuery(connection, sqlCommand.toString());
    });

    ...
}

Your MSSqlConnection variable is a singleton, because require caches modules. So when this code is called for the second page request, a second listener is added for the MSSqlConnection's connected event. When the connected event is emitted from MSSqlConnection, both listeners are called. This results in two calls to the Tedious Connection object's execSql function. The second call occurs before the first request has completed, and the state machine in Connection detects and reports the problem.

I will leave the issue open until I have updated the documentation to make it clear that only one request can be performed on a connection at a time.

Documentation updated.

commit 6eb911a

Here's the problem with this issue: There's no signal in the code (such that I can see) as to when I can successfully call again.

I'm trying to make multiple queries in a row, and even if I wait for request.end before calling the next, I'm still running into this error.

Edit: oops. As it turned out I had accidentally called my callback in the callback of the request without checking if it was an error.

Disregard!

You made me realise that the documentation doesn't make it clear enough that the Request's callback signifies the completion of the request, and that another request may not be made until it is called. As a result I've updated the documentation to make it clearer.

Hi,
I understood the above clearly, and tried to stick to it in my application.
The code below, do the following:

1. Every 1 second, fetsh the data from the server (fetchStock function) and save it in temporary array,
2. Every 1 second, send the data saved in the temporary array to the client using the Server-Sent Events (SSE) API.

It looks the 1 second, is not enough, so I got getting the error mentioned here, many times, so i increased it to 5 second, but still git it from time to time.

Is there a way to use Promise().then to be sure the fetchStock function is not re-called before the first call be completely executed.

var Request = require('tedious').Request;
var Connection = require('tedious').Connection;
var config = {
     userName: 'sa',
     password: 'pswd',
    server: 'xx.xxx.xx.xxx',
    options: {
        database: 'DB',
             rowCollectionOnRequestCompletion: 'true',
             rowCollectionOnDone: 'true'
     },
 };
var sql = new Connection(config);

var addElem = (obj, elem)=> [].push.call(obj, elem);
var result = {}, tmpCol = {}, tmpRow = {};
module.exports = {

  displayStock: function (es) {
      var dloop = setInterval(function() {
      if(result.error !== null)
            if (es) es.send(JSON.stringify(result), {event: 'rmSoH', id: (new Date()).toLocaleTimeString()});
      if(result.error === null)
            if (es) es.send('connection is closed');
      }, 1000);
  },

  fetchStock: function () {
            request = new Request("SELECT ItemCode, WhsCode, OnHand FROM OITW where OnHand > 0 and (WhsCode ='RM' or WhsCode ='FG');", function(err, rowCount, rows) {
            if (err) {
            result = {'error': err};
            console.log((new Date()).toLocaleTimeString()+' err : '+err);
             }
             if(rows)
             rows.forEach(function(row){
                 row.forEach(function(column){
                     var colName = column.metadata.colName;
                     var value = column.value;
                     addElem(tmpCol, {colName: value})
                 });
             addElem(tmpRow,{'item': tmpCol[0].colName, 'Whs': tmpCol[1].colName, 'Qty': tmpCol[2].colName});
             tmpCol = {};
           });
           result = tmpRow;
           tmpRow={}
       });
    sql.execSql(request);
  }
}

I am still getting the same issue, please look at the code below:

var Connection = require('tedious').Connection;
var Request = require('tedious').Request;
var TYPES = require('tedious').TYPES;

var config2 = {
    userName: 'dbuser',
    password: 'dbpass',
    server: 'mysqlserver.com',
    options: {
        database: 'msdb'
    }
};

var connection = new Connection(config2);
    connection.on('connect', function(err) {
        if(err) {
            console.log("Database connection is not established: \n"+err);
            process.exit(0);
        } else {
            console.log("Connected");  // If no error, then good to proceed.
            createTableOperation();
        }
    });  

function createTableOperation() {
    var query = "CREATE TABLE ClientDetailsTest(CLIENT_ID CHAR(5) PRIMARY KEY, CLIENT_NAME VARCHAR(25), CLIENT_CHANNEL VARCHAR(25))";

    request = new Request(query, function(err){
        if(err) {
            console.log("Error while creating table: "+err);
            process.exit(0);
        }
    });
    connection.execSql(request);
}

@swapnilkadu What error are you seeing exactly? Do you see any of the console.log statements?

I am wondering how the above issue of @swapnilkadu fixed? I am having the same issue. Any pointers?

Comment out the
connection.execSql(request);
as it will probably be masking the true error (which in my case was bad credentials).