Two Ajax calls from different sources causing one to silently fail [closed] - php

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Edit: Just ran into a wrinkle, I just got this working on an iPhone over the weekend and just out of curiosity ran it on two phones at once and it worked. So it ran happily along (as I expected it to the first time) when one phone was an iPhone (iOS 6) and the other was HTC One (Android 4.3). This was however on another person's WIFI. Could my home WIFI be a factor in blocking two devices going to the same location? (The second phone I was using when it didn't work was a Samsung Galaxy Ace 2 (Android 2.3))
So I've got this problem. I am using PhoneGap and coming up with a basic quiz system to muck around with. I've got it working and I'm having a great time. It works essentially through a web console where I can set up a quiz and cycle through questions and with a not so complex series of status updates to an underlying database I use Ajax calls from my mobile to read the status and display a question or wait for the next one.
Basic call is:
$(function checkForQuestion() {
var postData = "stuff needed in called PHP";
$.ajax({
type: "POST",
data: postData,
url: "my called php",
success: function(data){
var result = JSON.parse(data);
if (result == "question") {
--Display Question
} else {
setTimeout(checkForQuestion, 1000);
return false;
}
});
return false;
});
In my limited experience this seems to be a general use of Ajax with a recursive call back if it needs to wait for a new status.
Now this isn't everything by far, but a taster of what I'm doing. It works extremely well with one mobile device and a web console happily working through quizzes.
When I attempt to add a second mobile (haven't even attempted three yet) what happens is the second mobile just silently fails where it is supposed to be calling this ajax. Now the second mobile can quite happily chug along with this on it's own so it shouldn't be a hardware issue. There are no errors thrown out in the PHP, and with a little logging I can't find anything obviously failing.
At this point I thought, perhaps there is a limit on connections on my web host (I'm just using a shared server web host I'm using to test this out) and there are but that is limited to 20 entry processes with Cloud Linux LVE. However This should not preclude two mobiles calling the same PHP (via wifi) at roughly the same time. It should just run the calls concurrently and one will return slight faster than the other (or that is my understanding).
I thought perhaps this is a lock put on the MySQL database I'm using not allowing multiple users/PHP to query it at roughly the same time, but the consecutive nature of PHP should rule that out and some Googling assures me that calls will be queued.
I'm guessing that what is happening is the two calls come in and without returning any error one silently conflicts with the other causing a fail, but I have no idea hwo to find the fail and fix it.
Any suggestions?
Request for the actual code:
$(function () {
$(document).on("pagebeforeshow", "#page6", function () {
document.getElementById("mob_user_name2").innerHTML = window.localStorage.getItem("mob_local_login_name");
$(function checkForQuestion() {
//sort out the data to be posted
var postData = "mob_quizwait_quizcode=" . concat(window.localStorage.getItem("mob_local_quiz_code"), "&mob_quizwait_email=", window.localStorage.getItem("mob_local_login_email"), "&mob_quizwait_password=", window.localStorage.getItem("mob_local_login_password"), "&mob_quizwait_questionid=", window.localStorage.getItem("mob_question_id"));
$.ajax({
type: "POST",
data: postData,
url: "url of php",
success: function(data){
var mob_quizwait_data = JSON.parse(data);
if (mob_quizwait_data.mob_quizwait_success == "mob quizwait go") {
window.localStorage.setItem("mob_question_id", mob_quizwait_data.mob_quizwait_questionid);
window.localStorage.setItem("mob_question_score", mob_quizwait_data.mob_quizwait_score);
window.localStorage.setItem("mob_question_category", mob_quizwait_data.mob_quizwait_category);
window.localStorage.setItem("mob_question_question", mob_quizwait_data.mob_quizwait_question);
window.localStorage.setItem("mob_answer1", mob_quizwait_data.mob_quizwait_correct);
window.localStorage.setItem("mob_answer2", mob_quizwait_data.mob_quizwait_wrong1);
window.localStorage.setItem("mob_answer3", mob_quizwait_data.mob_quizwait_wrong2);
window.localStorage.setItem("mob_answer4", mob_quizwait_data.mob_quizwait_wrong3);
$.mobile.changePage("#page7", {transition:"slide", changeHash:false});
} else if (mob_quizwait_data.mob_quizwait_success == "mob quizwait stay") {
setTimeout(checkForQuestion, 1000);
return false;
} else if (mob_quizwait_data.mob_quizwait_success == "mob quizwait intermission") {
document.getElementById("mob_intermission").innerHTML = "Intermission";
$.mobile.changePage("#page6", {transition:"none", changeHash:false});
} else if (mob_quizwait_data.mob_quizwait_success == "mob quizwait finish") {
$.mobile.changePage("#page8", {transition:"slide", changeHash:false});
} else {
navigator.notification.alert("Status Check Failed. Please Try Again.", function(){}, "Alert", "OK");
}
}
});
return false;
});
});
});
And the PHP:
<?PHP
include '../open.php';
//take in POST variables
$quizcode = $link->real_escape_string($_POST["mob_quizwait_quizcode"]);
$email = $link->real_escape_string($_POST["mob_quizwait_email"]);
$password = $link->real_escape_string($_POST["mob_quizwait_password"]);
$questionid1 = $link->real_escape_string($_POST["mob_quizwait_questionid"]);
$quizid = 0;
$status = "X";
$questionid = 0;
$sql = "SELECT QUIZ_ID FROM B_QUIZ WHERE QUIZ_CODE = '$quizcode'";
$res = $link->query($sql);
while ($row = $res->fetch_array()) {
$quizid = $row['QUIZ_ID'];
}
$sql = "SELECT USER_ID FROM A_USER WHERE USER_EMAIL = '$email' AND USER_PASSWORD = '$password'";
$res = $link->query($sql);
while ($row = $res->fetch_array()) {
$userid = $row['USER_ID'];
}
$sql = "SELECT QUIZ_STATUS, IFNULL(QUESTION_ID, 0) AS QUESTION_ID FROM B_GAME WHERE QUIZ_ID = $quizid";
$res = $link->query($sql);
while ($row = $res->fetch_array()) {
$status = $row['QUIZ_STATUS'];
$questionid = $row['QUESTION_ID'];
}
if ($questionid == $questionid1) {
$questionid = 0;
}
if ($status != "F") {
if ($quizid != 0 && $status != "X") {
//get details to be written to the profile page
if ($questionid != 0) {
$sql = "SELECT SCORE FROM B_PARTICIPANT WHERE USER_ID = $userid AND QUIZ_ID = $quizid";
$res = $link->query($sql);
while ($row = $res->fetch_array()) {
$score = $row['SCORE'];
}
$sql = "SELECT b.QUESTION_ID, c.CATEGORY, b.QUESTION, b.CORRECT_ANSWER, b.WRONG_ANSWER_1, b.WRONG_ANSWER_2, b.WRONG_ANSWER_3
FROM B_GAME a, B_QUESTION b, D_CATEGORY c
WHERE a.QUESTION_ID = b.QUESTION_ID
AND b.CATEGORY_ID = c.CATEGORY_ID
AND a.QUIZ_ID = $quizid";
$res = $link->query($sql);
while ($row = $res->fetch_array()) {
$questionid = $row['QUESTION_ID'];
$category = $row['CATEGORY'];
$question = $row['QUESTION'];
$correct = $row['CORRECT_ANSWER'];
$wrong1 = $row['WRONG_ANSWER_1'];
$wrong2 = $row['WRONG_ANSWER_2'];
$wrong3 = $row['WRONG_ANSWER_3'];
}
if ($status == "R") {
$arr = array("mob_quizwait_success" => "mob quizwait go",
"mob_quizwait_questionid" => $questionid,
"mob_quizwait_score" => $score,
"mob_quizwait_category" => $category,
"mob_quizwait_question" => $question,
"mob_quizwait_correct" => $correct,
"mob_quizwait_wrong1" => $wrong1,
"mob_quizwait_wrong2" => $wrong2,
"mob_quizwait_wrong3" => $wrong3);
echo json_encode($arr);
} else if ($status == "N") {
$arr = array("mob_quizwait_success" => "mob quizwait stay");
echo json_encode($arr);
} else if ($status == "I") {
$arr = array("mob_quizwait_success" => "mob quizwait intermission");
echo json_encode($arr);
}
} else {
$arr = array("mob_quizwait_success" => "mob quizwait stay");
echo json_encode($arr);
}
} else {
$arr = array("mob_quizwait_success" => "mob quizwait failed");
echo json_encode($arr);
}
} else {
$arr = array("mob_quizwait_success" => "mob quizwait finished");
echo json_encode($arr);
}
include '../close.php';
?>
open.php:
<?PHP
//DATABASE DETAILS//
$DB_ADDRESS = "yeah";
$DB_USER = "not";
$DB_PASS = "gonna";
$DB_NAME = "do it";
//Connect to the MySQL database
$link = new mysqli($DB_ADDRESS, $DB_USER, $DB_PASS, $DB_NAME);
if (mysqli_connect_errno()) {
printf("Connect Failed: %s\n", mysqli_connect_error());
exit();
}
?>
close.php:
<?PHP
mysqli_close($link);
?>

Answer 1

The silent fail may be because there is a server error when processing the ajax call.
Add an error handler to the ajax call.
$.ajax({
type: "POST",
data: postData,
url: 'url of php',
success: successFunction(),
error: function(xhr, status, err) { alert("Ajax request failed"); }
});
You may need to step through PHP code or add additional logging to see the flow of the application.

Answer 2

Try:
<script>
var st_flag = true;
$(function () {
$(document).on("pagebeforeshow", "#page6", function () {
document.getElementById("mob_user_name2").innerHTML = window.localStorage.getItem("mob_local_login_name");
$(function checkForQuestion() {
if(!st_flag)
{
setTimeout(checkForQuestion, 1000);
}
//sort out the data to be posted
var postData = "mob_quizwait_quizcode=" . concat(window.localStorage.getItem("mob_local_quiz_code"), "&mob_quizwait_email=", window.localStorage.getItem("mob_local_login_email"), "&mob_quizwait_password=", window.localStorage.getItem("mob_local_login_password"), "&mob_quizwait_questionid=", window.localStorage.getItem("mob_question_id"));
$.ajax({
type: "POST",
data: postData,
url: "url of php",
st_flag = false;
success: function(data){
var mob_quizwait_data = JSON.parse(data);
if (mob_quizwait_data.mob_quizwait_success == "mob quizwait go") {
window.localStorage.setItem("mob_question_id", mob_quizwait_data.mob_quizwait_questionid);
window.localStorage.setItem("mob_question_score", mob_quizwait_data.mob_quizwait_score);
window.localStorage.setItem("mob_question_category", mob_quizwait_data.mob_quizwait_category);
window.localStorage.setItem("mob_question_question", mob_quizwait_data.mob_quizwait_question);
window.localStorage.setItem("mob_answer1", mob_quizwait_data.mob_quizwait_correct);
window.localStorage.setItem("mob_answer2", mob_quizwait_data.mob_quizwait_wrong1);
window.localStorage.setItem("mob_answer3", mob_quizwait_data.mob_quizwait_wrong2);
window.localStorage.setItem("mob_answer4", mob_quizwait_data.mob_quizwait_wrong3);
$.mobile.changePage("#page7", {transition:"slide", changeHash:false});
st_flag = true;
} else if (mob_quizwait_data.mob_quizwait_success == "mob quizwait stay") {
setTimeout(checkForQuestion, 1000);
return false;
} else if (mob_quizwait_data.mob_quizwait_success == "mob quizwait intermission") {
document.getElementById("mob_intermission").innerHTML = "Intermission";
$.mobile.changePage("#page6", {transition:"none", changeHash:false});
} else if (mob_quizwait_data.mob_quizwait_success == "mob quizwait finish") {
$.mobile.changePage("#page8", {transition:"slide", changeHash:false});
} else {
navigator.notification.alert("Status Check Failed. Please Try Again.", function(){}, "Alert", "OK");
}
}
});
return false;
});
});
});
</script>
Wait for first call to be complete.

Answer 3

Even though your problem is complicated by nature and I've got no reason to come to the conclusion I'm going to say but my bets are on session.auto_start.
As I see you've got no use for session in your code and in your case it might be a problem starting it. Set session.auto_start to false so your code won't use sessions at all and then test your code. It's a long shot but it's the best I've got.
To explain myself, php makes use of files for sessions by default. And it does so by opening the file in exclusive mode. It means if one request is running on the server, the second one with the same session_id will be blocked until the first one realizes it. So each browser can have only one request executed on the server at a time. I know your javascript won't be making the second request before the first one is returned but as I said your problem is complicated and it's the only guess I could come up with and I believe it's worth a test.
So my solution is: make sure you are not using sessions at all.

Answer 4

As Kami said, i think there should be some issues in the server side(PHP) only. And lot of time, PHP don't used to show the errors due to the server settings. I would suggest you to check the apache error logs, so that you can get to know if any unshown error is present. To check this, just load the URL which is having the problem and then check the error log usually present in var/log/ folder.

Answer 5

I have a strange suggestion after your edit. Wifi hotspot hides real adresses by NAT so from server's point of view both your devices has the same IP address. Server sends the answer to your Wifi spot and it route the packets to real addresses. Sometimes I had package missing but I can't remember the situation when all packets had been lost.
You should request the page from both devices and log the scrpit's work result on the server side. If there was 2 requests and all DB requests worked properly that means that one of your clients didn't get the response. It is probably because of your router, probably NAT

Answer 6

This turned out to be an unknown issue with the Web Host provider. Despite more than a dozen detailed requests for what on their side was blocking things I was always referred to how my own equipment or own code was at fault.
Set myself up a VPS and presto change-o I have a working system with no modifications to the code or the equipment.
TIL I don't like people who cover up their incompetency by blaming others.