Forgive me if this is a duplicate question, but I've spent the better part of an hour looking and couldn't find anything specific. Or I didn't know how to word the question.
Problem:
I administer a website which runs a PHP application using the CakePHP framework. We have an issue where if you want to open multiple tabs for the same domain, you have to wait for the 1st request's data to return before the other tab's data is returned.
e.g. lets say I click a link which runs a heavy DB query and takes 5 seconds to return and render the page. But after clicking that link, I also click some other links for very light pages. These other tabs will not render until the first tab receives data back from the server.
I also know that the server side processing is done async. because even if I load 2 heavy tabs, as soon as the first returns the 2nd also returns and renders the page.
So... my question is, is this a code related issue or a config issue and how would I go about correcting this?
Stack:
Apache 2.2.14
PHP 5.3.2
CakePHP 2.4
Ubuntu 10.04
Any help or pointers in the right direction would be much appreciated.
P.S. I know its possible because about a year or two ago, I managed to get this to work, but it messed something up with our sessions so I had to revert what I worked on.
Its actually a browser feature, though I can not find a good reference currently to it.
https://productforums.google.com/forum/#!topic/chrome/H0AHdeAXSFo
is a topic but doesnt really show much. Is it possible to load the page and then load the results in though AJAX ? You do not then suffer from the same issues.
EDIT:
Answer above is wrong...
This is caused by session locking.
Example 4 on http://php.net/manual/en/function.session-start.php
<?php
// If we know we don't need to change anything in the
// session, we can just read and close rightaway to avoid
// locking the session file and blocking other pages
session_start([
'cookie_lifetime' => 86400,
'read_and_close' => true,
]);
Related
Case: A php page with SQL needs to be refreshed every 30 seconds, but due to hardware limitations, there is a 0.5-2 second blank screen. Values do need to be retrieved from the database on every refresh, but the idea is to use a kind of seamless transitional effect, but where the transition is from one page to itself. 15 years ago or so, I experimented with what is described here: https://www.hongkiat.com/blog/meta-tag-hidden-features/ and I need something similar, though I am not even entirely sure that it would work. The page is very light and does not contain any images, so the load time is not the issue.
So, in short: How do I make it "impossible" to see that the page is refreshed, though values might have changed (or at least without the page going momentarily blank)?
I have tried numerous approaches with js and ajax functions to take care of the reloading instead of <meta http-equiv="refresh">, but none of them seem to be able to do what I want.
As suggested by #ivar and $usman-afzal, the issue was resolved using Ajax. Having no experience with Ajax/jQuery whatsoever, it took a while to get all the different values right and put them in their correct positions, but I succeeded, so thanks for the tip.
Here is the problem we have been facing for the past few weeks.
1/ Our setup
PHP 5.4 + MySQL
2 dedicated servers, load-balanced
Sessions are replicated between the 2 servers using memcached
3 applications running on these servers :
One custom-developped application, using default php session settings
Another custom-developped application, using different session settings (cookie name, path)
One Wordpress CMS
2/ The problem
The problem occurs on our first application.
Some of our users reported that they sometimes get disconnected after a few minutes (when the session is setup to last 3 hours). It can happen to them several time in the same day, then no disconnection for a few days, but the problem always comes back.
So far the fraction of users impacted is small, but I would like to solve this before it "spreads" to other users.
The problem seems to occur in different places of the application, though we have identified 3 scenarii where most of the errors occur :
Some involve submitting a form ($_SESSION variable is modified)
Other simply involve opening a popup page, with no modification of the session data
We have tried to reproduce the different scenarii described by the users : sometimes we have been able to, but most of the time we don't have any problem, which makes it hard to debug.
Other notes :
The problem is recent, this application had been running for years without any problem.
It doesn't seem to be related to our server load, because the problem still occured during the summer break when our trafic was low
It only affects one session/users at a time: all the other users logged in at the same time don't experience this problem
The problem occured on all the different browsers (IE, Firefox, Chrome)
3/ Technical analysis
When a disconnect occurs, the user is redirected to a page "Your session has expired or you don't have the right to view". When this page is loaded, we get a technical email with a dump of the $_SESSION variable.
When a session expires the normal way, the email we get shows that the $_SESSION variable is empty (normal behavior).
When an unexpected disconnect occurs, what is interesting is that the $_SESSION is not entirely empty : out of the ~20 elements the array contained, only one is left (always the same).
So this would mean the session is not expired, but not enough data is left to "identify" the user, hence the "no rights" page displayed. As a confirmation when this occurs, we can check in memcached that this session still holds some data.
These are the potential problem causes we have identified so far, and what we have done to rule them out :
Memcached indicates between 70 et 80% freespace, so we don't think it is the problem.
We removed Memcached and went back to using a NFS shared directory for session files: the problem actually got worse. This would point to an applicative bug, because NFS being slower to write data, session loss would occur more often.
We have browsed all the different forums (including SO) talking about PHP session data loss, and reviewed our code accordingly. The code base is big, but we have used automated tools and scripts to avoid missing a file.
session_start() is called at the beginning of each page.
exit() is called after each header("Location...")
register_globals is Off
We have tested the possible interractions between our 2 other applications and the problematic one, though they don't share any code, database or session handling. Nothing identified there.
We have analyzed our access logs around the times of the disconnections, to check for behavior patterns : no luck here either.
So we have no idea what causes this problem, as it seems to occur randomly, so my questions are :
The problem could come from our code: did we miss anything to check ? This solutions seems unlikely as the code works most of the time for all our users, but I am still considering it.
The problem could come from another application/process that would "empty" part of the session variable array. We have also reviewed the code from the other applications, but didn't find anything that could cause this.
And if another process is doing this, why would it only empty some sessions and not all of them ?
Thanks for your help.
I don't think you'll get a definitive answer to your question. There are too many probable causes and you haven't shown any code.
Still, my guess is that you have memcached.sess_locking turned Off, or if you have a custom session implementation - that it doesn't implement locking at all.
Eventually, this leads to a race condition between two simultaneous HTTP requests.
My guess is based on the often seen bad advice to turn off locks or free them as soon as possible, in order to achieve higher performance.
If this problem "suddenly" occurred, check what has changed. Did you do any work on the application? If so check committed code (you talked about automated tools so I expect there to be a repository which would allow for accurate finding of code changes).
Did you change anything on the server? Like upgrade software, upgrade/change hardware, make changes to the other two applications ?
One thing that popped to mind, did you check the drives you use for caching? It could be a corrupted part of the file system. Which would explain the random user part.
I couple of things I always to is:
Try to determine the moment of first occurrence as accurate as possible. At my work this occasionally triggers someone saying "oh yeah that might have to do with when I changed/updated/created this or that" so this might help. On the other hand it can sometimes takes days, weeks or more before something gets noticed so start expanding that time-frame if nothing comes up.
You have already a couple of scenario, find the common factor in these. If they don't share any code, stop looking there. If they DO share code search there. Of course sharing (part of) it here might allow us to help you search.
Do an organised search. I usually do the main application check when I am the one working most on the application (or even better when I created it). A colleague will check surrounding applications that might have influence on it. In your case those 2 other applications. Finally our sysadmin will check for newly installed or updated software on the server(s) and he will also check with our network guys if anything changed hardware wise or network related (for other people this could be the hosting provider).
It could be as simple as a WordPress plugin that uses sessions and calls either session_name() or session_id() with a different value, overlapping your custom applications with default session settings.
Since WordPress itself does not use sessions, plugins are often written from the perspective of having free rein with sessions. I just did a search on a WordPress test site and found sessions used in a gallery plugin, a plugin for putting a background image on the page, a shopping cart plugin, and a plugin I was writing that needed to carry an uploaded file from one admin page to another.
I want to make a page that will show all the users who are looking at that page right now. Ideally, a database would store which users are on the page and websockets would be used to update when users enter and leave.
The problem is that I don't know how to tell when users are viewing the page. I was thinking that I would be able to tell when they arrived and when they exited and add/remove accordingly. It's easy to tell when they arrive; however, it's difficult to tell when they leave - especially because users might do something like open multiple tabs, close one but keep the other open.
What is the best way to do this? I am using PHP as my server-side language.
You can use the blur and focus events on the window to toggle a variable. However, IE does some quirks which you will need to work around.
As a fallback to not working focus events, you might add a mousemove handler to the document. This might also throttle an automatic timeout which detects the loss of focus just by the fact that there was no user interaction for a specific period of time. However, you will never be able to detect a distracted user that has the site open but looks at something else...
To detect the window closing, you can hook on the unload event. This might be unreliable, but as you are using Websockets your server can easily detect a closed connection.
Well, one thing you could do, especially if you are using websockets is do a heartbeat/ping to the server every few seconds if you really wanted. If you don't get that heartbeat, you know they are not on the page anymore.... however, getting a response doesn't mean they are looking at the page, it would just mean that it is open, possibly in another tab. There is no way that I know of that will send a response to the server if the person loses focus on the page and opens another tab/window.
As Tim mentioned, Firefox and IE will run javascript in the background tabs, so there's no surefire way by simple polling to tell if the user is actually "looking" at the page or just has it open somewhere. Although I know of no way to tell if the user is actually looking at your page, a potential solution might be to bind polling to actions. So you might have some variable
var timesincelastaction=0;
var threshhold = 20;
Then
setInterval("timesincelastaction++",100);
Then
function keepAlive() {
if(timesincelastaction > threshhold) {
$.ajax(...tell the server you are alive...);
timesincelastaction = 0;
}
}
Then start thinking of actions like
$('a').mouseover(keepAlive);
$('div').mouseover(keepAlive);
$(window).scroll(keepAlive);
$(video).play(keepAlive); // okay this isn't a real one but you get the picture
So then you just brainstorm on everything the user can possibly be doing on the page that requires their attention and use those as your benchmark.
This seems a little intense I know, there's probably some nice ways to optimize it. Just thinking out loud. Curious to see what others come up with.
Every time one of your PHP scripts run, some user or entity has requested to view a page on your site (this usually occurs every time your script runs).
It is more challenging to detect when a user has left your page, which is why most online indicators are simply a timeout, i.e. if you have not been active on the website in the past 5 minutes, you are no longer considered online.
You can get information about the user who requested the page with variables like $_SERVER['REMOTE_ADDR'] or if you already have an authentication system you should be able to pull a users username, store this info in a database with the username/ip as a unique key with a timestamp, and simply not count them as online if their timestamp is older than 5 minutes.
You could probably use jQuery- ajax, unload and load. The ajax request will trigger on load telling you that they are viewing, and trigger on unload telling you they are no longer viewing. The rest is left to you to figure out because it sounds like you already have a grip on it.
Note. same result should be achievable in plain JS. Such as, onunload. Google will find you the rest.
I'm new to PHP (and web development in general) and have come across something that to me seems really, really bizarre!
Background
I am currently designing and developing an online enrolment form for my employer, a training company. The form consists of 3 pages - pages 1 and 2 are for data input whilst page 3 is a summary page with only one input, a box to check that the customer agrees to our T&Cs. Page 3 also includes a box where calculations are made as to the price of the selected training course based on choices made earlier in the form.
The form displayed on page 2, and some of the text on page 3, changes depending on a choice made on page one - that is, whether they are paying for the course themselves or their employer is funding it. If they choose employer, they get one form, if they choose self funding they get a different one. However, both of these forms are contained within a single .php file, using session variables to decide which one should be displayed.
I am using sessions to transfer the data between the pages. Each page has been made sticky using session variables. Real-time validation is carried out using javascript on each individual page, then a final PHP validation check is run on the whole thing when the customer tries to submit the final page. Javascript is also used on page one to calculate the price of the course in real time so the customer can see how much he/she will be paying before proceeding further.
The Problem
I find this really strange:
In Firefox, everything works perfectly
In Internet Explorer, when JavaScript is turned off, everything is fine. But when JS is switched on, the PHP validation on the final page seems to go haywire, thus making it impossible to submit the form.
In Chrome, again when JS is switched off, everything works, but when switched on, I can't even get past the first page. Instead of loading the correct form for page 2, I just get a blank screen.
Can anyone help? This is my first major project and I've been working on it for weeks, coming up against all sorts of problems and tearing my hair out but managing to solve them. Now I'm completely stumped, when I'm almost within touching distance of completion!!
I can't post the code here as it's 3 separate documents and very complicated, but if you want to have a look at the form itself go to:
http://testing.xenongroupadmin.com/testing/enrolment.php
I hope someone can help! Thanks very much!
You are most likely getting errors on the page2.php,
See error reporting to display and debug those:
http://ca.php.net/manual/en/function.error-reporting.php
When I have validation errors in non-Firefox browsers, it's usually because I have some error-reporting code in my JavaScript. Just something like
console.log(testValue);
can bring IE to its knees. If you have any code like that, remove it before testing in other browsers.
Also make sure that all variable declarations are done with "var", and that you're using semicolons at the end of each line (the spec should really enforce this, but I guess it doesn't, and browsers handle it differently).
I've only just seen all these answers, so apologies for not responding sooner.
I managed to fix the problem - turns out it was all to do with JS, no problems with my PHP at all. There were two issues. Firstly, a few syntax errors (which it took forever to find). Secondly, a lot of my HTML elements had id's and classes with identical names (e.g. id = "example" class = "example") which seemed to be throwing the javascript a bit.
Anyway, thanks for all the responses, and also apologies for not being very clear on some aspects - 'going haywire' was a pretty poor way of explaining the problem!!
Cheers
I'm working on a simple PHP application, using CouchDB and PHP-on-Couch to access some views, and it's working great. My next step is to introduce Ajax to update the frontend with data from the database.
I understand you can use the _changes notifications to detect any changes made on the database easily enough. So, its a matter of index.html monitoring for changes (through long polling), which calls loadView.php to update the page content.
Firstly, I hope the above is the correct method of going about it...
Secondly, when browsing to index.html, the page seems to never fully load (page load bar never completes). When a change is made, Firebug shows a the results as expected, but not any subsequent changes. At this time, the page seems to have stopped the infinite loading.
So far, i'm using jQuery to make the Ajax call...
$.getJSON('http://localhost:5984/db?callback=?', function(db) {
console.log(db.update_seq);
$.getJSON('http://localhost:5984/db/_changes?since='+db.update_seq+'&feed=continuous&callback=?', function(changes) {
console.log(changes);
});
});
Any ideas what could be happening here?
I believe the answer is simple enough.
A longpoll query is AJAX, guaranteed to respond only once, like fetching HTML or an image. It may take a little while to respond while it waits for a change; or it may reply immediately if changes have already happened.
A continuous query is COMET. It will never "finish" the HTTP reply, it will keep the connection open forever (except for errors, crashes, etc). Every time a change happens, zoom, Couch sends it to you.
So in other words, try changing feed=longpoll to feed=continuous and see if that solves it.
For background, I suggest the CouchDB Definitive Guide on change notifications and of course the excellent Couchbase Single Server changes API documentation.