I have read many similar questions concerning cancelling a POST request with jQuery, but none seem to be close to mine.
I have your everyday form that has a PHP-page as an action:
<form action="results.php">
<input name="my-input" type="text">
<input type="submit" value="submit">
</form>
Processing results.php on the server-side, based on the post information given in the form, takes a long time (30 seconds or even more and we expect an increase because our search space will increase as well in the coming weeks). We are accessing a Basex server (version 7.9, not upgradable) that contains all the data. A user-generated XPath code is submitted in a form, and the action url then sends the XPath code to the Basex server which returns the results. From a usability perspective, I already show a "loading" screen so users at least know that the results are being generated:
$("form").submit(function() {
$("#overlay").show();
});
<div id="overlay"><p>Results are being generated</p></div>
However, I would also want to give users the option to press a button to cancel the request and cancel the request when a user closes the page. Note that in the former case (on button click) this also means that the user should stay on the same page, can edit their input, and immediately re-submit their request. It is paramount that when they cancel the request, they can also immediately resend it: the server should really abort, and not finish the query before being able to process a new query.
I figured something like this:
$("form").submit(function() {
$("#overlay").show();
});
$("#overlay button").click(abortRequest);
$(window).unload(abortRequest);
function abortRequest() {
// abort correct request
}
<div id="overlay">
<p>Results are being generated</p>
<button>Cancel</button>
</div>
But as you can see, I am not entirely sure how to fill in abortRequest to make sure the post request is aborted, and terminated, so that a new query can be sent. Please fill in the blanks! Or would I need to .preventDefault() the form submission and instead do an ajax() call from jQuery?
As I said I also want to stop the process server-side, and from what I read I need exit() for this. But how can I exit another PHP function? For example, let's say that in results.php I have a processing script and I need to exit that script, would I do something like this?
<?php
if (isset($_POST['my-input'])) {
$input = $_POST['my-input'];
function processData() {
// A lot of processing
}
processData()
}
if (isset($_POST['terminate'])) {
function terminateProcess() {
// exit processData()
}
}
and then do a new ajax request when I need to terminate the process?
$("#overlay button").click(abortRequest);
$(window).unload(abortRequest);
function abortRequest() {
$.ajax({
url: 'results.php',
data: {terminate: true},
type: 'post',
success: function() {alert("terminated");});
});
}
I did some more research and I found this answer. It mentions connection_aborted() and also session_write_close() and I'm not entirely sure which is useful for me. I do use SESSION variables, but I don't need to write away values when the process is cancelled (though I would like to keep the SESSION variables active).
Would this be the way? And if so, how do I make one PHP function terminate the other?
I have also read into Websockets and it seems something that could work, but I don't like the hassle of setting up a Websocket server as this would require me to contact our IT guy who requires extensive testing on new packages. I'd rather keep it to PHP and JS, without third party libraries other than jQuery.
Considering most comments and answers suggest that what I want is not possible, I am also interested to hear alternatives. The first thing that comes to mind is paged Ajax calls (similar to many web pages that serve search results, images, what-have-you in an infinite scroll). A user is served a page with the X first results (e.g. 20), and when they click a button "show next 20 results" those are shown are appended. This process can continue until all results are shown. Because it is useful for users to get all results, I will also provide a "download all results" option. This will then take very long as well, but at least users should be able to go through the first results on the page itself. (The download button should thus not disrupt the Ajax paged loads.) It's just an idea, but I hope it gives some of you some inspiration.
On my understanding the key points are:
You cannot cancel a specific request if a form is submitted. Reasons are on client side you don't have anything so that you can identify the states of a form request (if it is posted, if it is processing, etc.). So only way to cancel it is to reset the $_POST variables and/or refresh the page. So connection will be broken and the previous request will not be completed.
On your alternative solution when you are sending another Ajax call with {terminate: true} the result.php can stop processing with a simple die(). But as it will be an async call -- you cannot map it with the previous form submit. So this will not practically work.
Probable solution: submit the form with Ajax. With jQuery ajax you will have an xhr object which you can abort() upon window unload.
UPDATE (upon the comment):
A synchronous request is when your page will block (all user actions) until the result is ready. Pressing a submit button in the form - do a synchronous call to server by submitting the form - by definition [https://www.w3.org/TR/html-markup/button.submit.html].
Now when user has pressed submit button the connection from browser to server is synchronous - so it will not be hampered until the result is there. So when other calls to server is made - during the submit process is going on - no reference of this operation is available for others - as it is not finished. It is the reason why sending termination call with Ajax will not work.
Thirdly: for your case you can consider the following code example:
HTML:
<form action="results.php">
<input name="my-input" type="text">
<input id="resultMaker" type="button" value="submit">
</form>
<div id="overlay">
<p>Results are being generated</p>
<button>Cancel</button>
</div>
JQUERY:
<script type="text/javascript">
var jqXhr = '';
$('#resultMaker').on('click', function(){
$("#overlay").show();
jqXhr = $.ajax({
url: 'results.php',
data: $('form').serialize(),
type: 'post',
success: function() {
$("#overlay").hide();
});
});
});
var abortRequest = function(){
if (jqXhr != '') {
jqXhr.abort();
}
};
$("#overlay button").on('click', abortRequest);
window.addEventListener('unload', abortRequest);
</script>
This is example code - i just have used your code examples and changed something here and there.
Himel Nag Rana demonstrated how to cancel a pending Ajax request.
Several factors may interfere and delay subsequent requests, as I have discussed earlier in another post.
TL;DR: 1. it is very inconvenient to try to detect the request was cancelled from within the long-running task itself and 2. as a workaround you should close the session (session_write_close()) as early as possible in your long-running task so as to not block subsequent requests.
connection_aborted() cannot be used. This function is supposed to be called periodically during a long task (typically, inside a loop). Unfortunately there is just one single significant, atomic operation in your case: the query to the data back end.
If you applied the procedures advised by Himel Nag Rana and myself, you should now be able to cancel the Ajax request and immediately allow a new requests to proceed. The only concern that remains is that the previous (cancelled) request may keep running in the background for a while (not blocking the user, just wasting resources on the server).
The problem could be rephrased to "how to abort a specific process from the outside".
As Christian Bonato rightfully advised, here is a possible implementation. For the sake of the demonstration I will rely on Symphony's Process component, but you can devise a simpler custom solution if you prefer.
The basic approach is:
Spawn a new process to run the query, save the PID in session. Wait for it to complete, then return the result to the client
If the client aborts, it signals the server to just kill the process.
<?php // query.php
use Symfony\Component\Process\PhpProcess;
session_start();
if(isset($_SESSION['queryPID'])) {
// A query is already running for this session
// As this should never happen, you may want to raise an error instead
// of just silently killing the previous query.
posix_kill($_SESSION['queryPID'], SIGKILL);
unset($_SESSION['queryPID']);
}
$queryString = parseRequest($_POST);
$process = new PhpProcess(sprintf(
'<?php $result = runQuery(%s); echo fetchResult($result);',
$queryString
));
$process->start();
$_SESSION['queryPID'] = $process->getPid();
session_write_close();
$process->wait();
$result = $process->getOutput();
echo formatResponse($result);
?>
<?php // abort.php
session_start();
if(isset($_SESSION['queryPID'])) {
$pid = $_SESSION['queryPID'];
posix_kill($pid, SIGKILL);
unset($pid);
echo "Query $pid has been aborted";
} else {
// there is nothing to abort, send a HTTP error code
header($_SERVER['SERVER_PROTOCOL'] . ' 599 No pending query', true, 599);
}
?>
// javascript
function abortRequest(pendingXHRRequest) {
pendingXHRRequest.abort();
$.ajax({
url: 'abort.php',
success: function() { alert("terminated"); });
});
}
Spawning a process and keeping track of it is genuinely tricky, this is why I advised using existing modules. Integrating just one Symfony component should be relatively easy via Composer: first install Composer, then the Process component (composer require symfony/process).
A manual implementation could look like this (beware, this is untested, incomplete and possibly unstable, but I trust you will get the idea):
<?php // query.php
session_start();
$queryString = parseRequest($_POST); // $queryString should be escaped via escapeshellarg()
$processHandler = popen("/path/to/php-cli/php asyncQuery.php $queryString", 'r');
// fetch the first line of output, PID expected
$pid = fgets($processHandler);
$_SESSION['queryPID'] = $pid;
session_write_close();
// fetch the rest of the output
while($line = fgets($processHandler)) {
echo $line; // or save this line for further processing, e.g. through json_encode()
}
fclose($processHandler);
?>
<?php // asyncQuery.php
// echo the current PID
echo getmypid() . PHP_EOL;
// then execute the query and echo the result
$result = runQuery($argv[1]);
echo fetchResult($result);
?>
With BaseX 8.4, a new RESTXQ annotation %rest:single was introduced, which allows you to cancel a running server-side request: http://docs.basex.org/wiki/RESTXQ#Query_Execution. It should solve at least some of the challenges you described.
The current way to only return chunks of the result is to pass on the index to the first and last result in your result, and to do the filtering in XQuery:
$results[position() = $start to $end]
By returning one more result than requested, the client will know that there will be more results. This may be helpful, because computing the total result size is often much more expensive than returning only the first results.
I hope I understood this correctly.
Instead of letting the browser "natively" submit the FORM, don't: write JS code that does this instead. In other words (I didn't test this; so interpret as pseudo-code):
<form action="results.php" onsubmit="return false;">
<input name="my-input" type="text">
<input type="submit" value="submit">
</form>
So, now, when the that "submit" button is clicked, nothing will happen.
Obviously, you want your form POSTed, so write JS to attach a click handler on that submit button, collect values from all input fields in the form (actually, it is NOT nearly as scary as it sounds; check out the link below), and send it to the server, while saving the reference to the request (check the 2nd link below), so that you can abort it (and maybe signal the server to quit also) when the cancel-button is clicked (alternatively, you can simply abandon it, by not caring about the results).
Submit a form using jQuery
Abort Ajax requests using jQuery
Alternatively, to make that HTML markup "clearer" relative to its functionality, consider not using FORM tag at all: otherwise, what I suggested makes its usage confusing (why it is there if it's not used; know I mean?). But, don't get distracted with this suggestion until you make it work the way you want; it's optional and a topic for another day (it might even relate to your changing architecture of the whole site).
HOWEVER, a thing to think about: what to do if the form-post already reached the server and server already started processing it and some "world" changes have already been made? Maybe your get-results routine doesn't change data, so then that's fine. But, this approach probably cannot be used with change-data POSTs with the expectation that "world" won't change if cancel-button is clicked.
I hope that helps :)
The user doesn't have to experience this synchronously.
Client posts a request
The server receives the client request and assigns an ID to it
The server "kicks off" the search and responds with a zero-data page and search ID
The client receives the "placeholder" page and starts checking if the results are ready based on the ID (with something like polling or websockets)
Once the search has completed, the server responds with the results next time it's polled (or notifies the client directly when using websockets)
This is fine when performance isn't quite the bottleneck and the nature of processing makes longer wait times acceptable. Think flight search aggregators that routinely run for 30-90 seconds, or report generators that have to be scheduled and run for even longer!
You can make the experience less frustrating if you don't block user interactions, keep them updated of search progress and start showing results as they come in if possible.
You must solve this conceptually first before writing any code. Here are some things that come to mind offhand:
What does it mean to free up resources on the server?
What constitutes to a graceful abort that will free up resources?
Is it enough to kill the PHP process waiting for the query result(s)? If so, the route suggested by RandomSeed could be interesting. Just keep in mind that it will only work on a single server. If you have multiple load balanced servers you won't have a way to kill a process on another server (not as easily at least).
Or do you need to cancel the database request from the database itself? In that case the answer suggested by Christian Grün is of more interest.
Or is it that there is no graceful shutdown and you have to force everything to die? If so, this seems awfully hacky.
Not all clients are going to explicitly abort
Some clients are going to close the browser, but their last request won't come through; some clients will lose internet connection and leave the service hanging, etc. You are not guaranteed to get an "abort" request when a client disconnects or has gone away.
You have to decide whether to live with potentially unwanted behavior, or implement an additional active state tracking, e.g. client pinging server for keepalive.
Side notes
30 secs or greater query time is potentially long, is there a better tool for the job; so you won't have to solve this with a hack like this?
you are looking for features of a concurrent system, but you're not using a concurrent system; if you want concurrency use a better tool/environment for it, e.g. Erlang.
I dont really know how to ask this which is why I am asking it here. So if I was using some code like this:
$.post("/data/something.php", {stuff: 'hi'}, function(data){
$('#box').html(data);
});
Normally if you have php like this you only get 1 result:
<?php echo $_REQUEST['stuff'] ?>
I was wondering if there is any way for the php to send a bit of data, then a little bit more later without it just sending all of it at once like so:
<?php
echo 'Foo';
//Do stuff that takes time
echo 'Bah';
?>
There are 2 ways to accomplish this.
The first uses a standard workflow with the flush command (http://php.net/manual/en/function.flush.php). This means that you can do:
echo "Starting...\n"
flush();
// do long task
echo "Done!\n"
HOWEVER: This often won't work. For example, if your server uses deflate, the Starting likely won't get sent until the request is finished. Many other factors can cause this too (proxies, browser behaviour).
The better option is to use a polling mechanism. Your main script would write its progress to a file (with some session ID related filename), then delete that file when done. You would then add a second script to report the progress in this file (or completion if the file has been deleted) and your JavaScript would send an AJAX request to this checker script (maybe every second or two).
In PHP
<?php
echo 'Foo';
echo '||||';
echo 'Bah';
?>
In Javascript
var responses = data.split('||||');
//you will get
//Foo in responses[0]
//Bar in responses[1]
I expect that php has no problem doing that (as detailed by #Dave). The complicated part, is for javascript to retrieve the first part of the data, before the transmission completes...
I think what you are asking is answered here: Is it possible for an AJAX request to be read before the response is complete?
The way to accomplish this is by listening on the readyState in the the xhr object. When readyState == 3 it means new content has arrived and you can access it. The technique is referred to as Comet.
and...
So finally, yes it is possible, no it is not easy.
To import a js-file is simple... just use >script src='file.js' type='text/javascript'>>/script>.
But then the source code will show a direct url to the contents of the file.
The content should be executable, but not directly viewable by using source url.
What is the best way to load the content of file.js to memory using AJAX.
I've come to the following initial way of working (just and idea, totally flawed?):
function get_contents() {
-> ajax execute PHP {copy 'file.js' to 'token.js' in tmp-directory}
-> ajax get contents of 'tmp/token.js' and load to memory
-> ajax execute PHP {delete 'tmp/token.js' in tmp-directory}
return(true); // content (ie. functions) should now be usable
}
But I'm not sure if the second ajax excute is enough to now be able to succesfully call the functions.
PHP returns content, but javascript 'ajax success' may see it (and stores it) as an variable... doh!
Is this ajax success idea going to work ??
Can someone suggest a better idea ?
Edit:
According to initial responses this way of working is virtually and humanly impossible.
Will solve it by loading common functions the 'normal' unprotected way, and using Jerry's suggestion (see comment) for calculations that happen less often.
Edit #2:
Below mentioned (time consuming) problem can be solved by following next template.
Still making use of the suggested 'hidden PHP code' method.
I am making use of a buffer (or sumthing), like a Youtube video... except 'video data' is 'results from AJAX-PHP functions'.
AJAX request "30 cycle", "60 cycle", "300 cycle", "600 cycle"
store result to buffer
initiate "start cycle"
function cycle() // run every second !!
{
//do stuff... no AJAX needed
//do some more stuff... like animations and small calculations
//per 30 cycle (30 seconds)
if ($cycle==30)
{
perform last "30 cycle" AJAX result [PHP-function set "A"]
... when finished: AJAX request "30 cycle"
store result to buffer in 'background'
}
//per 60 cycle (1 minute)
if ($cycle==60)
{
perform last "60 cycle" AJAX result [PHP-function set "B"]
... when finished: AJAX request "60 cycle"
store result to buffer in 'background'
}
//and so on....
}
Initial question 99% solved (-1 because of developer tools).
Thanks for commenting and suggestions.
Even if you split up your JavaScript files into lots of individual functions it would take very little work to put it all back together again.
With modern browsers even a file is "loaded into memory" you can see exactly what was loaded.
Try using the developer tools that come with browsers, you can use them to see when an Ajax call is made, exactly what was loaded in text format.
If you are mixing PHP and JavaScript then you should put anything that is sensitive in your PHP code whilst using JavaScript for your presentation of the results PHP provides.
EDIT: as per your update, instead of doing "cycles" could you not do this?
function pollAjax()
{
$.ajax({
-- ajax settings --
}).success(function(data) {
// do something with our results
doSomething(data);
// Fire again to get the next set of results.
setTimeout(function() {pollAjax()}, 10);
});
}
This means you're less likely to hang the browser with 1000's of pending ajax requests It will ask for the results, when it gets them, it will ask for the next set of results.
When a user views a certain profile page I need to send the end user an email but would like to do this asynchronously if at all possible. Can this be done with PHP? I really don't want to wait for the PHP function to complete before rendering the rest of the page. It should be seamless to the user.
Or is this a better solution from the client side:
$.post("msg.php?user=xxx", function(data) {
....
}, "json");
Is there a preferred method?
If you want interactivity, go for the ajax() method; if you just want to hit a listener script (and thereby initialize a set piece of PHP functionality), use post() which is a short-hand version of the same method:
$(function() {
// If you just want to activate a listener script (with no interaction)
$.post('/path/to/script.php');
// If you want to receive data back from your script for use in the DOM
$.ajax({
type: "POST",
url: "/path/to/script.php",
data: { name: "John", location: "Boston" }
}).done(function( response ) {
alert( response );
});
});
The best way to have your page perform asynchronous actions is to use AJAX, as you've suggested.
You can execute a script with proc_open or shell_exec if you're stuck with a server side solution. There may be some restrictions on this depending on your environment, but this is generally a quick and dirty way to multi-thread PHP and thus allow for things to happen asynchronously.
That is, somewhere in the script, if a flag is set (such as a notify flag) it will dispatch a process like '../private/notify.php' ... see how to pass arguments to a command line call of php. There is also the option of calling sendmail directly.
Make sure you test it out a bit before trying it; you need to make sure that:
You can safely and successfully spawn the new thread
You do not need to wait for that thread to complete to finish the current script execution
That thread will safely finish and exit on its own, whether with errors or successful
There is a way to handle errors (such as mail failing) so that you can debug problems. A logfile might be the best answer here.
Recently, I am going to make a instant-notification system for my website. I heard COMET is an essential in such cases.
I have been searching about PHP & Comet for a while already, however, the guides & articles I have found seems like just ajax requests in a loop. For example, there is a basic javascript code which gets the value from PHP file every 2 seconds and outputs to HTML. As far as I know, it should be COMET pushing new values to HTML, hence, the loop should be on server side, not client. Half of the articles in my native language was using setInterval() and contact PHP file every X seconds.
So, I have some questions to ask you.
Is there any guides or examples, which doesn't use any external framework like XAJAX/NOLOH that is easy to understand?
What is the performance difference between using COMET in server side, or requesting value from ajax.php every X seconds?
The timed requests I mentioned above can be called as COMET? (ex. Long Polling using jQuery and PHP)
Do I need any extensions to run COMET serverside? (My webhost is using Apache, I personally use Nginx)
You have to use a client-side script (AJAX), because the server has to be polled. The server cannot simply send messages to someone's browser without an open connection.
I'm not too familiar with HTML5 websockets, but I believe this allows you can have a persistent connection with the server, however HTML5 browsers aren't used widely to use this as a solution on a 'public' website.
How long polling works is that an asynchronous request is sent from the browser with a long time-out time (e.g. 30 seconds), when the request arrives at the server, it goes and checks for new messages, but when there are now messages to be displayed, instead of directly outputting the result, it goes into an infinite loop, polling the database e.g. every second (using sleep to postpone the queries), until a message has been found. When a message has been found it terminates the loop and outputs the result. If there have been no messages after 30 seconds, the script times out and sends back an empty request.
So the request can be sent back between 0 and 30 seconds. As soon as the request arrives in the browser, it is handled and a new 30 second request is sent.
As for your questions;
You will need a client-side framework for doing the polling
You cannot use Comet only on server-side. Using longpolling over normal polling (e.g. polling every second) is significant because you make much less server requests
To my understanding; yes
You can use any server-side language, as long as it can keep the connection open while querying for messages.
Also take a look at http://nodejs.org/
I don't know what exactly COMMET is mean. But for this purpose you have many solutions.
One, as you mentioned is long-polling by ajax. is simple. and not requeire new browsers only (HtML5).
One more option is "server-sent -event". It's require browser with HTML5 but it keep connection alive without polling:
client:
if (window.EventSource) {
window.onload = function() {
window.scrollTo(0,1);
setTimeout(
function() {
var source = new EventSource("events.php");
source.onmessage = function (event) {
document.body.innerHTML += event.data + "<br>";
};
}, 1000);
};
} else {
document.write("Please visit this page in a browser that supports EventSource to see the test");
}
server:
if ($_SERVER['HTTP_ACCEPT'] === 'text/event-stream') {
header('Content-Type: text/event-stream');
echo "data: This is the first event\n\n";
flush();
$i = 5;
while (--$i) {
sleep(1);
$time = date('r');
echo "data: The server time is: {$time}\n\n";
flush();
}
} else {
echo 'This demo is for use with an EventSource compatible browser.';
}
goodluck.