I have done some google search on this topic and couldn't find the answer to my question.
What I want to achieve is the following:
the client make an asynchronous call to a function in the server
the server runs that function in the background (because that function is time consuming), and the client is not hanging in the meantime
the client constantly make a call to the server requesting the status of the background job
Can you please give me some advices on resolving my issue?
Thank you very much! ^-^
You are not specifying what language the asynchronous call is in, but I'm assuming PHP on both ends.
I think the most elegant way would be this:
HTML page loads, defines a random key for the operation (e.g. using rand() or an already available session ID [be careful though that the same user could be starting two operations])
HTML page makes Ajax call to PHP script to start_process.php
start_process.php executes exec /path/to/scriptname.php to start the process; see the User Contributed Notes on exec() on suggestions how to start a process in the background. Which one is the right for you, depends mainly on your OS.
long_process.php frequently writes its status into a status file, named after the random key that your Ajax page generated
HTML page makes frequent calls to show_status.php that reads out the status file, and returns the progress.
Have a google for long running php processes (be warned that there's a lot of bad advice out there on the topic - including the note referred to by Pekka - this will work on Microsoft but will fail in unpredicatable ways on anything else).
You could develop a service which responds to requests over a socket (your client would use fsockopen to connect) - some simple ways of acheiving this would be to use Aleksey Zapparov's Socket server (http://www.phpclasses.org/browse/package/5758.html) which handles requests coming in via a socket however since this runs as a single thread it may not be very appropriate for something which requiers a lot of processing. ALternatively, if you are using a non-Microsoft system then yo could hang your script off [x]inetd however, you'll need to do some clever stuff to prevent it terminating when the client disconnects.
To keep the thing running after your client disconnects then the PHP code must be running from the standalone PHP executable (not via the webserver) Spawn a process in a new process group (see posix_setsid() and pcntl_fork()). To enable the client to come back and check on progress, the easiest way to achieve this is to configure the server to write out its status to somewhere the client can read.
C.
Ajax call run method longRunningMethod() and get back an idendifier (e.g an id)
Server runs the method, and sets key in e.g. sharedmem
Client calls checkTask(id)
server lookup the key in sharedmem and check for ready status
[repeat 3 & 4 until 5 is finished]
longRunningMethod is finished and sets state to finished in sharedmem.
All Ajax calls are per definition asynchronous.
You could (although not a strictly necessary step) use AJAX to instantiate the call, and the script could then create a reference to the status of the background job in shared memory (or even a temporary entry in an SQL table, or even a temp file), in the form of a unique job id.
The script could then kick off your background process and immediately return the job ID to the client.
The client could then call the server repeatedly (via another AJAX interface, for example) to query the status of the job, e.g. "in progress", "complete".
If the background process to be executed is itself written in PHP (e.g. a command line PHP script) then you could pass the job id to it and it could provide meaningful progress updates back to the client (by writing to the same shared memory area, or database table).
If the process to executed it's not itself written in PHP then I suggest wrapping it in a command line PHP script so that it can monitor when the process being executed has finished running (and check the output to see if was successful) and update the status entry for that task appropriately.
Note: Using shared memory for this is best practice, but may not be available if you are using shared hosting, for example. Don't forget you want to have a means to clean up old status entries, so I would store "started_on"/"completed_on" timestamps values for each one, and have it delete entries for stale data (e.g. that have a completed_on timestamp of more than X minutes - and, ideally, that also checks for jobs that started some time ago but were never marked as completed and raises an alert about them).
Related
I'm trying to figure out the best way to send a response to an incoming API POST to our system, without returning.
We receive an XML post from another service, and consume it, and then our response to them is in XML also. What we are currently doing is digesting the incoming post, do some stuff on our end, then do a php return with the XML.
I would like to change this so that we can respond to their call with the XML, but then do some processing after the fact, without making some type of exec/background call.
What's the best way to send a response in PHP without returning? If we do an "echo" will that close the connection and allow us to process more afterwards without the other server "waiting?
Calling PHP's echo will not close the connection, in fact you can call echo multiple times in your PHP script and the output will be added to the response. The connection will only close when
The end of the script is reached
exit() or the alias die() are called
A fatal/parse error or an uncaught Exception occurs or your server runs out of memory
The maximum script execution time which you can set in php.ini is exceeded
Usually, the calling client code will also have some kind of timeout, so if your 'digesting' code could take longer and you want to take care about this as well as point 4 in the list, you can store the request data for later processing, for example in a database or serialized in files. Having successfully stored the data, you then have basically 2 options to go:
Option 1: Spawn a background PHP process
To spawn a background PHP process that will survive the livecycle of the calling script, use exec and nohup. The basic usage could look like this:
exec('RESOURCE_ID=123 nohup /path/to/your/php/executable your_script.php > /dev/null');
Within the first segment of the command, RESOURCE_ID=123, you can pass a unique identifier of the previously stored request data, maybe a database entry id or the storage filename, to the background script. Use getenv('RESOURCE_ID') in your background script to retrieve the variable.
[EDIT] > /dev/null for output redirection is crucial for running the process in the background, otherwise the parent script will wait for the output of the background process. I also suggest to write the output as well as error output to an actual file like &> my_script.out, this will have the same effect. You could also get the process id of the background process by appending & echo $! to the command, exec() will then return it.
After starting the background script, you can send your 'OK' response and exit the parent script.
Option 2: Cronjob for processing, as suggested by Jim Panse
The more complex your system grows, you probably need more control over the execution of your 'digesting' code. Perhaps you want to balance server load peaks, or restart failed tasks, or throttle malicious usage of your API. If you need this kind of control, you are better off with this option.
Since i guess you want your system-to-system communication synchronously there are multiple things you can consider.
Even though time consuming requests you usually still want a fast response.
To satisfy this you can't process the request immediately.
So, just save the request and process it later (give the client a 202 response back). Systems like queues are very popular to save time consuming jobs for running them later. Another time controlled script (cronjob) could then do polling and process the stacked messages/data.
If you want to provide the results to the client too, return them a unique resource id on the initial rest call and implement another resource with exactly this parameter as the input. If your system finished processing, the result will appear there.
Spawning a process from within another php script isn't very handy since it's very difficult to debug and error-prone.
I personally would't go for this solution.
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've got the following problem at hand:
I'm having users on two seperate pages, but saving page input to the same text file. While one user is editing, the other can't. I'm keeping track of this with sessions and writing changes and who's turn to edit it is in a file.
Works fine so far, the output in the end is quite similar to a chat. However, right now I'm having users manually actualize their page and reload the file. What I'd like to do is have the page execute a redirect when the file-timestamp changes (to indicate that the last user has saved their edits and its another users turn). I've looked into javascript shortpolling a little, but then found the php filmtime function and it looks much easier to use. Well - here's what I got:
while(true){
$oldtimestamp=filemtime(msks/$session['user']['kampfnr'].txt);
$waittimer=2;
$waittimer++;
sleep($waittimer);
$newtimestamp=filemtime(msks/$session['user']['kampfnr'].txt);
if ($eintragszeit2 > $eintragszeit1){
addnav("","kampf_ms.php?op=akt");
redirect("kampf_ms.php?op=akt");
}}
In theory, while the user sees the output "it's ... turn to edit the file." this should loop in the background, checking if the file has already been updated, and if yes, redirect the user.
Practically this heavily affects server perfomance (I'm on shared hosting) until it breaks with a memory exceeded error message.
Is something wrong with the code? Or is it generally a bad idea to use a while loop in this case?
Thanks in advance!
PHP language should be only used to generate web content (client do a request to the server => server calls the required script, and returns the response to the client).
Once page is loaded and displayed to the client, the connection is closed, so Internet can die, the client isn't informed...
So with an infinite loop, not only the client can wait for response... an infinite time, but also the server may be heavy impacted because of load... Effectively It is a really bad idea :)
PHP can't be used to make a bidirectional communication: it is just called to build web pages that client demands, and so he can't do anything "in the background" (not directly, effectively you can call an external script, but not for notify a client...)
Also, to do a bidirectional communication, php and "regular" http is not good, because of client / server architecture (the server only answers client request, it is passive)
I can suggest to use WebSocket protocol, to do a chat application:
http://socket.io/
https://en.wikipedia.org/wiki/WebSocket
But for that, you need to use an "active" server solution, such as node.js or ruby (depends of your server capabilities...)
The other way if you want to stay in php is that client makes Ajax request every 10 seconds, for example, to call a php script which check the file, and send back a message to the client if file is updated, but it is really deprecated, because of heavy performance loss, so forget it immediately.
Do all comet style applications require a loop somewhere in the application on the serverside to detect updates/changes? If no, please could you explain how the logic behind a loopless comet style application would work?
This kind of application will always require a loop, you need to periodically check for new data etc. Of course you can make the "loop" non-blocking by using an even-loop based approach, but in the end there's still a loop somewhere.
Just think about it for a moment, how would you make it work without a loop? I sure can't imagine a way that doesn't utilize a loop somewhere.
Short answer is, no, not all require a loop on the serverside.
Instead you can use long-polling AJAX calls from the browser to request data,
at which the server simply responds with the data and the browser waits until the response is gotten before sending a new request.
The solution could be stream_set_blocking. Use any possible blocking resource to be suspended by OS and wait for appropriate interruption.
Client side:
Ajax call to endpoint script (timeout for ajax e.g. 30 seconds - after 30 seconds initiate another one - after 30 seconds you will get response from server - script execution time reached)
If you will get response during 30 seconds handle response (async) and open new connection (as in comet done - I saw it in cometD client)
Server setup:
Setup apache timeouts (between request and data sent to 30-31 second), this is so apache will allow you to wait so much
set apache to allow lot of child instances (concurrent users * 1.5), but you need to be sure that you have enough memory for this amount of apache instances (+ memory used by php children)
Script one:
execution_time = 28
set shutdown_function in order to send response (timeout, but formatted and understandable for ajax if You need it)
you need to open file, empty one
enable blocking mode using stream_set_blocking for file stream
try read from file and you will get suspended until other process will write to file or timeout be reached.
As soon as script gets content in file written from other process it will get back and will send response. (this will trigger another ajax call and another slept process)
Worst thing is that you need to think how to get multiple reader scripts reading from same bus (file) without disturbing each other.
Also there could be that timeout will be exactly at that time when message will be written into bus.
(hope that this solution is not as bad as my English)
My iPhone client app uploads a data to the server, which runs on PHP. There is a code to invoke a .exe program on the server side on PHP. The .exe program will take the uploaded data and run on a process on its own. That means the PHP execution will end without waiting for the .exe program to finish. After the .exe program finished processing the uploaded data and have an output, I want this output to be sent back to the iPhone.
Normally, if we call the .exe program to be run inside the php without making it a seperate process, we have to wait for the program to finish and we can send the output back to the iPhone client.
By running the .exe program as a seperate process, it is impossible to send the data back via PHP that invokes the .exe program. The question is, if we have the .exe program running on a seperate process rather than on the PHP script, what are the possible methods to send the output back to the iPhone client?
That's a need problem you've outlined. Let me explain a couple of ideas.
First of all, if you terminate the initial upload request, the only resonable way to check for it being done is to poll every few seconds from the iPhone. Send a request to "get-update.php" every 5 seconds to see if you have data.
By using $_SESSION, you should be able to store a token that will identify the data when it has finished processing.
Regarding the actually process, you may be able to accomplish that in a number of ways. One is to do a fairly standard double-fork, detaching the child process from the parent, so it will continue after the parent exits.
Another (recommended) would be to author a backend server process that would watch your database for requests, fetch them, process them, and update the database. So when the inital upload script actually uploads the data, have PHP put it in the database, store the record ID in $_SESSION, and return to the user.
The back end process will notice that there is a record to process, read the data, call the executable, and update the database with the result.
The get-update.php script will read $_SESSION for the record id, and check the database if the data has been processed (or what the status is).
If you do not have the ability to run a background process, and you are constrained to using PHP, you could do the double-fork magic, and fork of another PHP process to do the database read / exe / database update.
Feel free to comment with questions.
You need (a) a good way to pass the data to the program, and (b) a good way to get the data back.
I would say this is a perfect case for an AJAX snippet frequently polling data from, say, a text file the .exe writes its status in.
The upload script you call could return a unique identifier of some sort to the uploading client. Using that identifier, the client would poll the exe's status (e.g. "does the output file xyz already exist?") until it gets positive feedback.
You're going to have a hard time reconnecting with the iPhone once you've severed the connection. It may be out of coverage, it may have changed IP address, ......
Your best bet is to have the iPhone reconnect back to the server and poll for it's information.
You could do this by using Apple's Push Notification service, but that's probably overkill, unless you think the data processing is going to take a long time, and/or you want to update the app icon when the processing is done, even if the app isn't running.
Do you expect the user to just be patiently waiting for the result, or are they going to fire off the data, and check back later? If it's only going to take a couple of seconds, you could just have the iPhone app poll for the result after waiting a little while (while displaying a progress indicator).