I wrote a web spider to spider pages concurrently. For each link that the spider finds, I want to fork off a new child that starts the process all over again.
I don't want to overload the target server so I created a static array that all objects can access. Each child can add their PID to the array, and either parent or child should check the array to see if $maxChildren have been met, and if so, patiently wait until any child finishes.
As you see, I have $maxChildren set to 3. I am expecting to see 3 simultaneous processes at any given time. However, that's not the case. The linux top command shows 12 to 30 processes at any given time. In concurrent programming, how can I regulate the number of simultaneous processes? My logic is currently inspired by how Apache handles it's max children, but I'm not exactly sure how that works.
As pointed out in one of the answers, globally accessing the static variable brings up issues with race conditions. To deal with this, the $children array takes the unique $PID of the process as both the key and it's value, thereby creating a unique value. My thinking is that since any object can only deal with one $children[$pid] value, locking is not necessary. Is this not true? Is there a chance that two processes could try to unset or add the same value at some point?
private static $children = array();
private $maxChildren = 3;
public function concurrentSpider($url) {
// STEP 1:
// Download the $url
$pageData = http_get($url, $ref = '');
if (!$this->checkIfSaved($url)) {
$this->save_link_to_db($url, $pageData);
}
// STEP 2:
// extract all hyperlinks from this url's page data
$linksOnThisPage = $this->harvest_links($url, $pageData);
// STEP 3:
// Check the links array from STEP 2 to see if this page has
// already been saved or is excluded because of any other
// logic from the excluded_link() function
$filteredLinks = $this->filterLinks($linksOnThisPage);
shuffle($filteredLinks);
// STEP 4: loop through each of the links and
// repeat the process
foreach ($filteredLinks as $filteredLink) {
$pid = pcntl_fork();
switch ($pid) {
case -1:
print "Could not fork!\n";
exit(1);
case 0:
if ($this->checkIfSaved($filteredLink)) {
exit();
}
//$pid = getmypid();
print "In child with PID: " . getmypid() . " processing $filteredLink \n";
$var[$pid]->concurrentSpider($filteredLink);
sleep(2);
exit(1);
default:
// Add an element to the children array
self::$children[$pid] = $pid;
// If the maximum number of children has been
// achieved, wait until one or more return
// before continuing.
while (count(self::$children) >= $this->maxChildren) {
//print count(self::$children) . " children \n";
$pid = pcntl_waitpid(-1, $status);
unset(self::$children[$pid]);
}
}
}
}
This is written in PHP. I know that the pcntl_waitpid function with argument of -1 waits for any child to complete regardless of the parent (http://php.net/manual/en/function.pcntl-waitpid.php).
What's wrong with my logic and how can I correct it so that only $maxChildren processes are running simultaneously? I'm also open to improving the logic in general if you have suggestions.
First thing to note: if this is truly a global being shared among multiple threads, it's possible that multiple threads are adding to it at once and you're running afoul of a race condition. You need some sort of concurrency control to ensure that only one process is accessing your global array at once.
Also, try the simple debugging trick of having each process write out (to the console or to a file) its PID and the full contents of the global array each time a new spider is forked. It will help you to check your assumptions (which are plainly wrong at some point) and figure out what's going wrong.
EDIT: (In response to the comments)
I'm not a PHP developer, but if I had to guess, based on the fact that you're using an OS tool that counts OS-level processes, I'd guess that your fork is spawning multiple processes, but your static array is global within the current process. Implementing system-wide shared memory is a lot more complicated!
If you just want to count something and ensure that instances of a shared resource don't grow out of control, look into semaphores, and see if you can find a way in PHP to create a named semaphore object that can be shared between multiple instances of your spider.
Use a real programming language ;)
Step 1 is kind of bad why are you downloading if it might be in the db. Put that inside the if and see if you can put a mutex around it. Maybe so something in sql to imitate one.
I hope harvest_links uses a proper html processor with css selector support (i like fizzler for .NET). I guess regular expression would be fine if its just to get links but it is possible to mess up.
I see step 4 and i don't think its bad but personally i'd do it a different way.
I'd have something like step one to insert url,page,flag into a db. Then i'd have another process or the same one ask the db for unprocessed pages and set the flag to some value if it errors and another if its successful. This is so if something fails of the process exits (shutdown, crash, power out, etc) it can pick it up easily and don't need to scan every page to find where it left off. It just ask the database for the next link and redoes what it didnt finish
PHP doesn't support multithreading, therefore it doesn't support mutexes or any other synchronization methods. As others have said in their answers, this will lead to a race condition.
You'll have to write a wrapper in C or bash. That way, the PHP script can submit targets to the wrapper, and the wrapper will handle scheduling.
Another approach is to rewrite your spider in Python or Ruby, both of which support multithreading. That will eliminate the need for interprocess communication.
Edit: On second thought, the best way is to write the wrapper in Python or Ruby and reuse your existing PHP code as a black box. That's a compromise of the solutions above.
If the spider is for practical purposes, you might want to google "curl multithread"
cURL Multi Threading with PHP
Related
im wondering if theres a way to run a code in a loop as a process and interacting with it from a different script. I know sockets listen to incoming requests but im referring to internal usage, without requests.
Standard approach:
Use pcntl-signal() and posix-kill() functions to interact by standard or user-defined signals.
Pros:
PHP built-in, ready for use functions. No need to reinvent wheels.
POSIX compatibility.
Cons:
You can only send defined signals to a script. Not values.
One-way interaction.
Example of listening script:
<?php
pcntl_signal(SIGTERM, 'sig_handler');
pcntl_signal(SIGUSR1, 'sig_handler');
echo 'Run... PID: ' . getmypid() . PHP_EOL;
$finish = false;
while (!$finish) {
pcntl_signal_dispatch();
}
echo 'Shutdown...' . PHP_EOL;
function sig_handler($signal) {
global $finish;
echo 'Received signal: ' . $signal . PHP_EOL;
switch ($signal) {
case SIGTERM:
$finish = true;
break;
case SIGUSR1:
echo 'Processing SIGUSR1 signal...' . PHP_EOL;
break;
}
}
Non-standard approach:
You can implement interaction with script using tools like database, files, sockets, pipes.
Pros:
Functionality depends only on realization.
Cons:
You need to implement protocol for interaction and support it in your script.
I would say first you set up your script to run in the background. You can implement it yourself (using fork) or use existing libraries.
https://www.google.co.jp/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=php%20daemon
Then you define a protocol to communicate. There are many way to implements that, from simple to complex, depending on your needs.
A simple way for example would be to define a folder somewhere in the server that your script reads on the regular basis (loop + sleep). When a file is added, the script reads it, execute the instruction in it, and delete it.
Nope, you cannot directly access another running script, as each script runs on it's own resource sandbox (memory, file descriptors). You'd also need to know it's PID in order to access it, and then after, you'd need some hacking tools to get into that process space.
This may not full fill your requirement. But will works:
Try logging the status to a text file from the first script and read it frequently from the second script to know what is the current status of first script.
I have a situation, which is with similar approach.
I'm starting independent processes by forking and not wait for them - only start. But I've created control structure in a database. There every children process is storing it's state but also observes a control flag and when this flag is up - then children stops immediately. The children processes stores time-stamped records in another table and that let's me to check what process where is with it's task.
In control structure is stored also processes pids and it is possible to send signals to them.
I think this can be useful in your situation too.
when php process starts it take all its requirements for example variables linked pages etc etc.. and show result after it finishes its execution, and after it is completed execution you cant do anything but to re-execute.
its like going to picnic on moon and coming back to home after its over, so no one can disturb you while you are in your picnic. :D
I want to achieve the following:
Initialize an array. Child process adds some elements to the array. Parent process adds some elements to the array. Finally before exiting, print all elements.
Following is the code that I wrote:
<?php
$values=array();
$pid = pcntl_fork();
if (!$pid) {
sleep(2);
$values[]="Put by child";
exit(0);
}
$values[]="Put by parent";
pcntl_waitpid($pid, $status);
print_r($values);
?>
However, it only prints one value - Put by parent. Can someone please explain the behavior and suggest the right code?
Regards,
JP
(sorry for crossposting)
I suggest a look at socket_create_pair().
In the PHP manual is a very short & easy example of interprocess communication (IPC) between a fork()-parent and the child.
And using serialize() und unserialize() You could even transfer complex data types like arrays...
Forked children will gain their own dedicated copy of their memory space as soon as they write anywhere to it - this is "copy-on-write". While shmop does provide access to a common memory location, the actual PHP variables and whatnot defined in the script are NOT shared between the children.
Doing $x = 7; in one child will not make the $x in the other children also become 7. Each child will have its own dedicated $x that is completely independent of everyone else's copy.
a local domain socket is easiest. have the parent open one with fsockopen for each child immediately before the fork. that way you can have one comm channel per child: http://php.net/manual/en/transports.unix.php and http://php.net/manual/en/transports.unix.php.
You could also shared memory, or open a bi-directional communications channel between the two processes and build a little api to send data back and forth.
As long as father and children know the key/keys of the shared memory segment is ok to do a shmop_open before pcnlt_fork. But remember that pcnlt_fork returns 0 in the child's process and -1 on failure to create the child (check your code near the comment /confusion/). The father will have in $pid the PID of the child process just created.
Check it here:
http://php.net/manual/es/function.pcntl-fork.php
The child's code is missing the print_r() statement.
The parent won't print what the child added to values, as the addition was done after the child process had been fork()ed off, and with this it had gotten its own copy of the prcoess' memory.
From the fork-tag's excerpt (emphasis by me):
The fork() function is the Unix/Linux/POSIX way of creating a new process by duplicating the calling process.
This behaviour of forking is different from threading where all threads share the same address space.
Apologies if this has been covered before - I did my searching but possibly may not know the correct terms to have used.
This process is handled with PHP.
Here's the situation:
I have a large array of file names. The script I have opens these files and enters their content into a database. Processing these files one at a time takes over 24 hours, and these files are updated on a daily basis.
Breaking the single large array into four smaller arrays and running concurrent processes finishes the job before the 24 hour window elapses, but sometimes one or two processes will finish hours before the others because file sizes vary on a daily basis.
Much like people who stock retail shelves (who else has worked that nightmare before?) pitch in to help out with what's left after finishing their own tasks, I'd like to have a script in place where these "agents" do the same.
Here's some basics of what I have figured out - it could be wrong, and I'm not too proud to protest if I am :-)
$files = array('file1','file2','file3','file4','file5');
//etc... on to over 4k elements
while($file = array_pop($files)){
//Something in here... I have no idea what.
}
Ideas? Something like four function calls or four loops within that overarching 'while' has crossed my mind, but I'm pretty sure it's going to wait on executing subsequent calls until the previous one(s) finish.
Any help is appreciated. I'm seriously stuck on this one!
Thanks!
A database-backed message queue seems the obvious solution but I think that's overkill in this case. I would simply put the files to be processed into a single dedicated queue directory, then use the DirectoryIterator class to scan it. Something like this:
while (true) {
look in the queue directory for a file
if you don't fine one, exit the script, all processing is done
if you find one, rename it or move it to a work directory
if the rename/move command succeeded, process the file
if the rename/move command failed, one of the other threads got it first
}
Edit:
Regarding launching the workers, you could use a simple shell script to spawn the PHP processes in the background:
NUM_WORKERS=5
for WORKER in $(seq 1 ${NUM_WORKERS})
do
echo "starting worker ${WORKER}"
php -f /path/to/my/process.php &
done
Then, create a cron entry to run this launcher, for example, at midnight:
0 0 * * * /path/to/launcher.sh
You want what's called a "message queue". Something like beanstalkd
You'll basically create a list of messages that include your individual filenames. You'll then create a set of processors to process them. Each processor will handle one file then go back to the queue to see if there are more messages/files waiting to be processed.
EDIT:
Here's an analogy to help explain message queues. Your first idea is like a human manager taking a stack of files, dividing them into four piles and then handing each of his four employees a pile to process. A message queue is more like this: the manager puts all the files on a table and tells each employee to take a single file from the table and process it. He tells them when they're done with the first file to keep taking files until there are no more files on the table. When all the files are done, the employees can go home.
One employee might end up with really large files and only handle a few, while another employee might get smaller files and handle many. It doesn't matter how many each employee handles, they'll all keep working until the table is empty.
I would have a socket server master script that hands out file paths to x number of slave scripts, until there are no files left to process. This way, all the slave scripts will keep running, and you can hand out file paths dynamically as they are requested.
Something like this:
master.php
<?php
// load the array of files to process (however you do this)
$fileList = file('filelist.txt');
// Create a listening socket on localhost
$serverSocket = stream_socket_server('tcp://127.0.0.1:7878');
$sockets = array($serverSocket);
$clients = array();
// Loop while there are still files to process
while (count($fileList)) {
// Run a select() call on the existing sockets' read buffers
// Skip to next iteration if no sockets are waiting for handling
if (stream_select($read = $sockets, $write = NULL, $except = NULL, 1) < 1) {
continue;
}
// Loop sockets with data to read
foreach ($read as $socket) {
if ($socket == $serverSocket) {
// Accept new clients
$sockets[] = $clients[] = stream_socket_accept($serverSocket);
} else if (trim(fgets($socket)) == 'next') {
// Hand out a new file path to the client
fwrite($socket, array_shift($fileList)."\n");
if (!count($fileList)) {
break 2;
}
}
}
}
// When we're done, disconnect the clients
foreach ($clients as $socket) {
#fclose($socket);
}
// ...and close the listen socket
#fclose($serverSocket);
slave.php
<?php
$socket = fsockopen('127.0.0.1', 7878);
while (!feof($socket)) {
// Get a new file path from the master
fwrite($socket,"next\n");
$path = trim(fgets($socket));
if (is_file($path)) {
// Process the file at $path here
}
}
You then just need to start master.php, then when it is running, you can start however many instances of slave.php as you want, and they will all keep running until there are no more files to process.
Obviously, this has no error handling, but it should provide a basic framework to get you started. This relies on blocking function calls (stream_select() and fgets()) to avoid a race condition - this may or may not be sufficient for your purposes.
Can PHP call a function and don't wait for it to return? So something like this:
function callback($pause, $arg) {
sleep($pause);
echo $arg, "\n";
}
header('Content-Type: text/plain');
fast_call_user_func_array('callback', array(3, 'three'));
fast_call_user_func_array('callback', array(2, 'two'));
fast_call_user_func_array('callback', array(1, 'one'));
would output
one (after 1 second)
two (after 2 seconds)
three (after 3 seconds)
rather than
three (after 3 seconds)
two (after 3 + 2 = 5 seconds)
one (after 3 + 2 + 1 = 6 seconds)
Main script is intended to be run as a permanent process (TCP server). callback() function would receive data from client, execute external PHP script and then do something based on other arguments that are passed to callback(). The problem is that main script must not wait for external PHP script to finish. Result of external script is important, so exec('php -f file.php &') is not an option.
Edit:
Many have recommended to take a look at PCNTL, so it seems that such functionality can be achieved. PCNTL is not available in Windows, and I don't have an access to a Linux machine right now, so I can't test it, but if so many people have advised it, then it should do the trick :)
Thanks, everyone!
On Unix platforms you can enable the PCNTL functions, and use pcntl_fork to fork the process and run your jobs in child processes.
Something like:
function fast_call_user_func_array($func, $args) {
if (pcntl_fork() == 0) {
call_user_func_array($func, $args);
}
}
Once you call pcntl_fork, two processes will execute your code from the same position. The parent process will get a PID returned from pcntl_fork, while the child process will get 0. (If there's an error the parent process will return -1, which is worth checking for in production code).
You can check out PHP Process Control:
http://us.php.net/manual/en/intro.pcntl.php
Note: This is not threading, but the handling of separate processes. There is more overhead attached.
Wouldn't it solve your problem to fork, keeping the parent process free for other connections & actions? See http://www.php.net/pcntl_fork. If you need an answer back you could possibly listen to a socket in the parent, and write with the child. A simple while(true) loop with a read could possibly do, and probably you already have that basic functionality if you run a permanent TCP server. Another option would be to keep track of your childprocess-ids, keep a accessable store somewhere (file/database/memcached etc), with a pcnt_wait in the main process with a WNOHANG to check which process has exited, and retrieve the data from the store.
You can do some threading in PHP if you use the method pcntl_fork.
http://ca.php.net/manual/en/function.pcntl-fork.php
I have never use this myself, but the are some good example of how to use it on php.net.
PHP doesn't have this functionality as far as I know
You can emulate the function using a different technique, like this one:
Parallel functions in PHP
PHP does not support multi-threading, so there's no other option than taking advantage of the OS or the web server multi processing capabilities. Note that actually you can fetch both the result and output of exec:
string exec ( string $command [,
array &$output [, int &$return_var
]] )
You can, at least, prevent the parent process from hanging until the child process is done by ignoring the child signals using pcntl_signal(SIGCHLD, SIG_IGN).
So, let's say you want to fork a process and execute another PHP function that takes a while without making the parent wait for it to finish (since you want the main process to finish in a timely manner):
pcntl_signal(SIGCHLD, SIG_IGN);
$pid = pcntl_fork();
if ($pid < 0) {
exit(0);
} elseif (!$pid) {
my_slow_function();
exit(0);
}
// Parent keeps executing and finishes before the child does
If you want to execute a slow external script as the child process, pcntl_exec is handy:
$script = array('/path/to/my/script'); // E.g. /home/my_user/my_script.php
pcntl_exec('/path/to/program/executable',$script); // E.g. /usr/bin/php
Which design pattern exist to realize the execution of some PHP processes and the collection of the results in one PHP process?
Background:
I do have many large trees (> 10000 entries) in PHP and have to run recursive checks on it. I want to reduce the elapsed execution time.
If your goal is minimal time - the solution is simple to describe, but not that simple to implement.
You need to find a pattern to divide the work (You don't provide much information in the question in this regard).
Then use one master process that forks children to do the work. As a rule the total number of processes you use should be between n and 2n, where n is the number of cores the machine has.
Assuming this data will be stored in files you might consider using non-blocking IO to maximize the throughput. Not doing so will make most of your process spend time waiting for the disk. PHP has stream_select() that might help you. Note that using it is not trivial.
If you decide not to use select - increasing the number of processes might help.
In regards to pcntl functions: I've written a deamon with them (a proper one with forking, changing session id, the running user, etc...) and it's one of the most reliable piece of software I've written. Because it spawns workers for every task, even if there is a bug in one of the tasks, it does not affect the others.
From your php script, you could launch another script (using exec) to do the processing. Save status updates in a text file, which could then be read periodically by the parent thread.
Note: to avoid php waiting for the exec'd script to complete, pipe the output to a file:
exec('/path/to/file.php | output.log');
Alternatively, you can fork a script using the PCNTL functions. This uses one php script, which when forked can detect whether it is the parent or the child and operate accordingly. There are functions to send/receive signals for the purpose of communicating between parent/child, or you have the child log to a file and the parent read from that file.
From the pcntl_fork manual page:
$pid = pcntl_fork();
if ($pid == -1) {
die('could not fork');
} else if ($pid) {
// we are the parent
pcntl_wait($status); //Protect against Zombie children
} else {
// we are the child
}
This might be a good time to consider using a message queue, even if you run it all on one machine.
The question seems to be a bit confused.
I want to reduce the absolute execution time.
Do you mean elapsed time? Certainly use of the right data-structure will improve throughput, but for a given data-structure, the minmimum order of the algorithm is absolute, and nothing to do with how you implement the algorithm.
Which design pattern exist to realize....?
Design Patterns are something which code is, not a template for writing programs, and a useful tools for curriculum design. To start with a pattern and make your code fit it is in itself an anti-pattern.
Nobody can answer this question withuot knowing a lot more about your data and how its structured, however the key driver for efficiency will be the data-structure you use to implement your tree. If elapsed time is important then certainly look at parallel execution, however it may also be worth considering performing the operation in a different tool - databases are highly optimized for dealing with large sets of data, however note that the obvious method for describing a tree in a relational database is very inefficient when it comes to isolating sub-trees and walking the tree.
In response to Adam's suggesting of forking you replied:
I "heard" that pcntl isnt a good solution. Any experiences?
Where did you hear that? Certainly forking from a CGI or mod_php invoked script is a bad idea, but nothing wrong with doing it from the command line. Do have a google for long running PHP processes (be warned there is a lot of bad information out there). What code you write will vary depending on the underlying OS - which you've not stated.
I suspect that you could solve a large part of your performance issues by identifying which parts of the tree need to be checked and only checking those parts AND triggering the checks when the tree is updated, or at least marking the nodes as 'dirty'.
You might find these helpful:
http://mikehillyer.com/articles/managing-hierarchical-data-in-mysql/
http://en.wikipedia.org/wiki/Threaded_binary_tree
C.
You could use a more efficient data structure, such as a btree. I used once in Java but not in PHP. You can try this script: http://www.phpclasses.org/browse/file/708.html, it is an implementation of btree.
If it is not enough, you can use Hadoop to implement a Map/Reduce pattern, as Michael said. I would not fork PHP process, it does not seem to help for performace.
Personally, I would use PHP as client and put everything in Hadoop. This tutorial might help: http://www.lunchpauze.com/2007/10/writing-hadoop-mapreduce-program-in-php.html.
Another solution can be to use a Java implementation of Btree: http://jdbm.sourceforge.net/. JDBM is an object database using a Btree+ data astructures. Then you can search with PHP by exposing data with a web service or by accessing it directly with Quercus
Using web or CLI?
If you use web, you could intergrate that part in Quercus Then you could use the advantages of JAVA multithreading.
I don't actually know how reliable Quercus is though. I'd also suggest using a kind of message queue and refactoring the code, so it doesn't need the scope.
Maybe you could rebuild the code to a Map/Reduce pattern. You then can run the PHP code in Hadoop Then you can cluster the processing through a couple of machines.
I don't know if it's useful, but I came across another project, called Gearman. It's also used to cluster PHP processes. I guess you can combine that with a reduce script as well, if Hadoop is not the way you want to go.
pthreads
There is a rather new (since 2012) PHP extension available: pthreads. It can be installed via PECL.
Simple Implementation in PHP Code: extend from Thread Class. Add a run() method and execute the start() method.
<?php
// Example from http://www.phpgangsta.de/richtige-threads-in-php-einfach-erstellen-mit-pthreads
class AsyncOperation extends Thread
{
public function __construct($threadId)
{
$this->threadId = $threadId;
}
public function run()
{
printf("T %s: Sleeping 3sec\n", $this->threadId);
sleep(3);
printf("T %s: Hello World\n", $this->threadId);
}
}
$start = microtime(true);
for ($i = 1; $i <= 5; $i++) {
$t[$i] = new AsyncOperation($i);
$t[$i]->start();
}
echo microtime(true) - $start . "\n";
echo "end\n";
Outputs
>php pthreads.php
0.041301012039185
end
T 1: Sleeping 3sec
T 2: Sleeping 3sec
T 3: Sleeping 3sec
T 4: Sleeping 3sec
T 5: Sleeping 3sec
T 1: Hello World
T 2: Hello World
T 3: Hello World
T 4: Hello World
T 5: Hello World
Try this: PHPThreads
Code Example:
function threadproc($thread, $param) {
echo "\tI'm a PHPThread. In this example, I was given only one parameter: \"". print_r($param, true) ."\" to work with, but I can accept as many as you'd like!\n";
for ($i = 0; $i < 10; $i++) {
usleep(1000000);
echo "\tPHPThread working, very busy...\n";
}
return "I'm a return value!";
}
$thread_id = phpthread_create($thread, array(), "threadproc", null, array("123456"));
echo "I'm the main thread doing very important work!\n";
for ($n = 0; $n < 5; $n++) {
usleep(1000000);
echo "Main thread...working!\n";
}
echo "\nMain thread done working. Waiting on our PHPThread...\n";
phpthread_join($thread_id, $retval);
echo "\n\nOur PHPThread returned: " . print_r($retval, true) . "!\n";
Requires PHP extensions:
posix
pcntl
sockets