I'm creating a PHP script that will allow a user to log into a website and execute database queries and do other actions that could take some time to complete. If the PHP script runs these actions and they take too long, the browser page times out on the user end and the action never completes on the server end. If I redirect the user to another page and then attempt to run the action in the PHP script, will the server run it even though the user is not on the page? Could the action still time out?
In the event of long-running server-side actions in a web application like this, a good approach is to separate the queueing of the actions (which should be handled by the web application) from the running of the actions (which should be handled by a different server-side application).
In this case it could be as simple as the web application inserting a record into a database table which indicates that User X has requested Action Y to be processed at Time Z. A back-end process (always-running daemon, scheduled script, whatever you prefer) would be constantly polling that database table to look for new entries. ("New" might be denoted by something like an "IsComplete" column in that table.) It could poll every minute, every few minutes, every hour... whatever is a comfortable balance between server performance and the responsiveness of an action beginning when it's requested.
Once the action is complete, the server-side application that ran the action would mark it as complete in the database and would store the results wherever you need them to be stored. (Another database table or set of tables? A file? etc.) The web application can check for these results whenever you need it to (such as on each page load, maybe there could be some sort of "current status" of queued actions on each page so the user can see when it's ready).
The reason for all of this is simply to keep the user-facing web application responsive. Even if you do things like increase timeouts, users' browsers may still give up. Or the users themselves may give up after staring at a blank page and a spinning cursor for too long. The user interface should always respond back to the user quickly.
You could look at using something like ignore_user_abort but that is still not ideal in my opinion. I would look at deferring these actions and running them through a message queue. PHP comes with Gearman - that is one option. Using a message queue scales well and does a better job ensuring the request actions actually get completed.
Lots on SO on the subject... Asynchronous processing or message queues in PHP (CakePHP) ...but don't use Cake :)
set_time_limit() is your friend.
If it were me, I would put a loading icon animation in the user interface telling them to wait. Then I would execute the "long process" using an asynchronous AJAX call that would then return an answer, positive or negative, that you would pass to the user through JavaScript.
Just like when you upload pictures to Facebook, you can tell the user what is going on. Very clean!
Related
I currently submit a process to a class.php page doing some calculations, which get written to a MySql database for the users to use.
Depending on the clients internet speed the calculation do take long and the user is waiting for it to finish.
Is there a way to submit the process and allow the users to continue browsing while it finish in the background.
The calculation only need to be submitted once a week
I have
rank.php
inside it $ranklist=$admin->do_ranking()
then inside admin.class
function do-ranking(){
the calculations
}
return $ranking
}
There are two wide spread strategies for this:
send response early and detach
You can send the response (whatever that is, might also be empty) right away and forward the task to be computed to a background task. Several options for that, either based on a cron task periodically checking for waiting jobs to be computed. So we are talking about a simple job scheduler, you will find easy examples for that. Or you can spawn a system process and detach from that. how exactly depends on the operating system you use, on Linux for example you can push the process into background, thus return immediately to the spawning php script.
use a ajax request to transmit the data
This certainly is the much more elegant approach: instead of doing a full page reload at all (by sending a form form example) you use some simple client side logic (read: javascript) to send the data (or an empty request) via an ajax request. That means the page loaded into the browser is not unloaded at all and the user can do whatever he likes.
I have made a simple chat application in PHP/Ajax/MySQL.
I am regularly calling these two methods-
setInterval("GetChatParticipants()",5000);
setInterval("GetChatMessages()",1000);
Now as per client design, there's no Logout or Sign Out button, so I want to detect when user closes the browser and at that time I have to delete the participant record from the database. I dont want to use onbeforeunload as Chrome refuses to accept Ajax requests on window unload event.
So I am thinking of implementing in Ajax by making repeated reqests. But the question is how exactly shall I do that. Shall I update the Participant table with last_active_time and if the difference between current time and that last_active_time is , say, more than 10 min, then I should delete the user? Is it a practical solution or could there be a better one?
You have the best solution IMO. In Javascript create a "ping" function that pings every minute / two minutes etc with a session ID unique to that browser/session. Have a sessions table on your server and update when you get a ping. Have another script that looks for entries that have not been pinged for long periods and close the sessions.
I've had this structure running for thousands of concurrent users on one site, and I had to drop the ping down from 1 minute to every 2 minutes when load got heavyish (that was on 2 load balanced servers while running the rest of the site too). Obviously you make your ping approx 45% of the time-out time (so if one fails, a second should hit). It's a simple process that can handle the load.
Edit: don't use "setInterval" for the ping, but user "setTimeout" when each ping either returns or fails. Otherwise, with setInterval, if your server gets too loaded then the pings queue, never respond and queue some more. You get a meltdown of the server as all server sockets are used, then you can't connect by ssh to fix it... not a moment I was proud of.
Yes it is practical solution and then only one you can rely on, so, no matter what else you use in addition to that, this one final check should be in your logic at all times. Said that, combining several methods will give you more possibilities to detect user leaving early. Add onbeforeunload for browsers that support it. Add "log out" button for clients coming from insecure location that want to log out right now when leaving PC and finally, check for inactivity.
Yes, it is practical solultion. The only difference is where to store information about user ping. You can do it in database or in key-value storage like memcached. I prefer the second one as I think it takes lesser resources.
I have a PHP script something like:
$i=0;
for(;$i<500;++i) {
//Do some operation with files numbered 0 to 500;
}
The thing is, the script works and displays the end results, but the operation takes a while and watching a blank screen can be frustrating. I was thinking if there is some way I can continuously update the page at the client's end, detailing which file is currently being worked upon. That is, can I display and continuously update what is the current value of $i?
The Solution
Thanks everyone! The output buffering is working as suggested. However, David has offered valuable insight and am considering that approach as well.
You can buffer and control the output from the PHP script.
However, you may want to consider the scalability of this design. In general, heavy processes shouldn't be done online. Your particular case may be an edge in that the wait is acceptable, but consider something like this as an alternative for an improved user experience:
The user kicks off a process. This can be as simple as setting a flag on a record in the database or inserting some "to be processed" records into the data.
The user is immediately directed to a page indicating that the process has been queued.
An offline process (either kicked off by the PHP script on the server or scheduled to run regularly) checks the data and does the heavy processing.
In the meantime, the user can refresh the page (manually, by navigating elsewhere and coming back to check, or even use an AJAX polling mechanism to update the page) to check the status of the processing. In this case, it sounds like you'd have several hundred records in a database table queued for processing. As each one finishes, it can be flagged as done. The page can just check how many are left, which one is current, etc. from the data.
When the processing is completed, the page shows the result.
In general this is a better user experience because it doesn't force the user to wait. The user can navigate around the site and check back on progress as desired. Additionally, this approach scales better. If your heavy processing is done directly on the page, what happens when you have many users or the data processing load increases? Will the page start to time out? Will users have to wait longer? By making the process happen outside of the scope of the website you can offload it to better hardware if needed, ensure that records are processed in serial/parallel as business rules demand (avoid race conditions), save processing for off-peak hours, etc.
Check out PHP's Output Buffering.
Try to use:
flush();
http://php.net/manual/ru/function.flush.php
Try the flush() function. Calling this function forces PHP to send whatever output it has so far to the client, instead of waiting for the script to end.
However, some web servers will only send the output once the entire page is done being built, so calling flush() would have no effect in this case.
Also, browsers themselves buffer input, so you may run into problems there. For example, certain versions of IE won't start displaying the page until 256 bytes has been received.
Ok, I didn't really now how to formulate this question, and especially not the title. But i'll give it a try and hope i'm being specific enough while trying to keep it relevant to others.
I you want to run a php script in the background (via ajax) every X seconds that returns data from a database, how do you do this the best way without using to much of the server resources?
My solution looks like this:
A user visits a webpage, ever x seconds that page runs a javascript. The javascript calls a PHP script/file that calls the database, retrieves the data and returns the data to the javascript. The javascript then prints the data to the page. My fear is that this way of solving it will put a lot of pressure on the server if there is a lot (10 000) simultaneous visitors on the page. Is there another way to do this?
That sounds like the best way, given the spec/requirement you set out.
Another way is to have an intermediary step. If you are going to have a huge amount of traffic (otherwise this does not introduce any benefit, but to the contrary may overcomplicat/slow the process), add another table that records the last time a dataset was pulled, and a hard file (say, XML) which if the 'last time' was deemed too long ago, is created from a new query, this XML then feeds the result returned to the user.
So:
1.Javascript calls PHP script (AJAX)
2.PHP pings DB table which contains last time data was fully output
3.If time is too great, 'main' query is rerun and XML file is regenerated from output
ELSE skip to 4
4.Fetch the XML file and output as appropriate for returned AJAX
You can do it the other way, contacting the client just when you need it and wasting less resources.
Comet it's the way to go for this option:
Comet is a programming technique that
enables web servers to send data to
the client without having any need for
the client to request it. This
technique will produce more responsive
applications than classic AJAX. In
classic AJAX applications, web browser
(client) cannot be notified in real
time that the server data model has
changed. The user must create a
request (for example by clicking on a
link) or a periodic AJAX request must
happen in order to get new data fro
the server.
I've a particularly long operation that is going to get run when a
user presses a button on an interface and I'm wondering what would be the best
way to indicate this back to the client.
The operation is populating a fact table for a number of years worth of data
which will take roughly 20 minutes so I'm not intending the interface to be
synchronous. Even though it is generating large quantities of data server side,
I'd still like everything to remain responsive since the data for the month the
user is currently viewing will be updated fairly quickly which isn't a problem.
I thought about setting a session variable after the operation has completed
and polling for that session variable. Is this a feasible way to do such a
thing? However, I'm particularly concerned
about the user navigating away/closing their browser and then all status
about the long running job is lost.
Would it be better to perhaps insert a record somewhere lodging the processing record when it has started and finished. Then create some other sort of interface so the user (or users) can monitor the jobs that are currently executing/finished/failed?
Has anyone any resources I could look at?
How'd you do it?
The server side portion of code should spawn or communicate with a process that lives outside the web server. Using web page code to run tasks that should be handled by a daemon is just sloppy work.
You can't expect them to hang around for 20 minutes. Even the most cooperative users in the world are bound to go off and do something else, forget, and close the window. Allowing such long connection times screws up any chance of a sensible HTTP timeout and leaves you open to trivial DOS too.
As Spencer suggests, use the first request to start a process which is independent of the http request, pass an id back in the AJAX response, store the id in the session or in a DB against that user, or whatever you want. The user can then do whatever they want and it won't interrupt the task. The id can be used to poll for status. If you save it to a DB, the user can log off, clear their cookies, and when they log back in you will still be able to retrieve the status of the task.
Session are not that realible, I would probably design some sort of tasks list. So I can keep records of tasks per user. With this design I will be able to show "done" tasks, to keep user aware.
Also I will move long operation out of the worker process. This is required because web-servers could be restrated.
And, yes, I will request status every dozens of seconds from server with ajax calls.
You can have JS timer that periodically pings your server to see if any jobs are done. If user goes away and comes back you restart the timer. When job is done you indicate that to the user so they can click on the link and open the report (I would not recommend forcefully load something though it can be done)
From my experience the best way to do this is saving on the server side which reports are running for each users, and their statuses. The client would then poll this status periodically.
Basically, instead of checkStatusOf(int session), have the client ask the server of getRunningJobsFor(int userId) returning all running jobs and statuses.