ASP.NET MVC AJAX chat

[nccsbim071]

I built an ajax chat in one of my mvc website. everything is working fine. I am using polling. At certain interval i am using $.post to get the messages from the db. But there is a problem. The message retrieved using $.post keeps on repeating. here is my javascript code and controller method.

var t;        
        function GetMessages() {        
            var LastMsgRec = $("#hdnLastMsgRec").val();
            var RoomId = $("#hdnRoomId").val();
            //Get all the messages associated with this roomId
            $.post("/Chat/GetMessages", { roomId: RoomId, lastRecMsg: LastMsgRec }, function(Data) {
                if (Data.Messages.length != 0) {
                    $("#messagesCont").append(Data.Messages);
                    if (Data.newUser.length != 0)
                        $("#usersUl").append(Data.newUser);
                    $("#messagesCont").attr({ scrollTop: $("#messagesCont").attr("scrollHeight") - $('#messagesCont').height() });
                    $("#userListCont").attr({ scrollTop: $("#userListCont").attr("scrollHeight") - $('#userListCont').height() });
                }
                else {
                }
                $("#hdnLastMsgRec").val(Data.LastMsgRec);
            }, "json");
            
           
            t = setTimeout("GetMessages()", 3000);
        }

and here is my controller method to get the data:

public JsonResult GetMessages(int roomId,DateTime lastRecMsg)
        {
            StringBuilder messagesSb = new StringBuilder();
            StringBuilder newUserSb = new StringBuilder();            
            List<Message> msgs = (dc.Messages).Where(m => m.RoomID == roomId && m.TimeStamp > lastRecMsg).ToList();
            if (msgs.Count == 0)
            {
                return Json(new { Messages = "", LastMsgRec = System.DateTime.Now.ToString() });
            }            
            foreach (Message item in msgs)
            {
                messagesSb.Append(string.Format(messageTemplate,item.User.Username,item.Text));
                if (item.Text == "Just logged in!")
                    newUserSb.Append(string.Format(newUserTemplate,item.User.Username));
            }            

            return Json(new {Messages = messagesSb.ToString(),LastMsgRec = System.DateTime.Now.ToString(),newUser = newUserSb.ToString().Length == 0 ?"":newUserSb.ToString()});
        }

Everything is working absloutely perfect. But i some messages getting repeated. The first time page loads i am retrieving the data and call GetMessages() function. I am loading the value of field hdnLastMsgRec the first time page loads and after the value for this field are set by the javascript.

I think the message keeps on repeating because of asynchronous calls. I don't know, may be you guys can help me solve this.

or you can suggest better way to implement this.

[nccsbim071]

I got my answer here: ASP.NET AJAX CHAT

The names i am referring to below is from above link:

i think the actual problem was with the timestamp thing and asynchronous behaviour of $.post. after calling "GetMessages()" method, even if the previous request to retrive chat message was not complete anathor call to same method used to fire due to setting timeout for "GetMessages()" method outside the $.post method. In my question you can see that timeout for "GetMessages()" method is set outside the $.post method. Now i set the timeout for "GetMessages()" method inside the $.post method. so that next call to "GetMessages()" only occur after 3 seconds of completion of current $.post method. I have posted the code below.

var t;
        function GetMessages() {
            var LastMsgRec = $("#hdnLastMsgRec").val();
            var RoomId = $("#hdnRoomId").val();
            //Get all the messages associated with this roomId
            $.post("/Chat/GetMessages", { roomId: RoomId, lastRecMsg: LastMsgRec }, function(Data) {
                if (Data.LastMsgRec.length != 0)
                    $("#hdnLastMsgRec").val(Data.LastMsgRec);
                if (Data.Messages.length != 0) {
                    $("#messagesCont").append(Data.Messages);
                    if (Data.newUser.length != 0)
                        $("#usersUl").append(Data.newUser);
                    $("#messagesCont").attr({ scrollTop: $("#messagesCont").attr("scrollHeight") - $('#messagesCont').height() });
                    $("#userListCont").attr({ scrollTop: $("#userListCont").attr("scrollHeight") - $('#userListCont').height() });
                }
                else {
                }
                t = setTimeout("GetMessages()", 3000);
            }, "json");

        }

I addition to that i also changed few things. As suggested by ignatandrei i placed $("#hdnLastMsgRec").val(Data.LastMsgRec); immediately after function(Data) {.

and also

as said by MikeSW i changed the data retrieval process. Previously i was extracting data on the basis of timespan(retrieve all the data associated with
this room id that has greater timespan than last data retrieved message timespan) but now i keep track of the messageid. Now i retrieve only those data that
has message id greater than last retrieved message id.

and guess what no repeataion and perfectly working chat application so far on my intranet.

I still got to see it's performance when deployed on internet.

i think it solved my problem.

i will still test the system and let u guys know if there is any problem.