I have a personal web site that crawls and collects MP3s from my favorite music blogs for later listening...
The way it works is a CRON job runs a .php scrip once every minute that crawls the next blog in the DB. The results are put into the DB and then a second .php script crawls the collected links.
The scripts only crawl two levels down into the page so.. main page www.url.com and links on that page www.url.com/post1 www.url.com/post2
My problem is that as I start to get a larger collection of blogs. They are only scanned once ever 20 to 30 minutes and when I add a new blog to to script there is a backup in scanning the links as only one is processed every minute.
Due to how PHP works it seems I cannot just allow the scripts to process more than one or a limited amount of links due to script execution times. Memory limits. Timeouts etc.
Also I cannot run multiple instances of the same script as they will overwrite each other in the DB.
What is the best way I could speed this process up.
Is there a way I can have multiple scripts affecting the DB but write them so they do not overwrite each other but queue the results?
Is there some way to create threading in PHP so that a script can process links at its own pace?
Any ideas?
Thanks.
USE CURL MULTI!
Curl-mutli will let you process the pages in parallel.
http://us3.php.net/curl
Most of the time you are waiting on the websites, doing the db insertions and html parsing is orders of magnitude faster.
You create a list of the blogs you want to scrape,Send them out to curl multi. Wait and then serially process the results of all the calls. You can then do a second pass on the next level down
http://www.developertutorials.com/blog/php/parallel-web-scraping-in-php-curl-multi-functions-375/
pseudo code for running parallel scanners:
start_a_scan(){
//Start mysql transaction (needs InnoDB afaik)
BEGIN
//Get first entry that has timed out and is not being scanned by someone
//(And acquire an exclusive lock on affected rows)
$row = SELECT * FROM scan_targets WHERE being_scanned = false AND \
(scanned_at + 60) < (NOW()+0) ORDER BY scanned_at ASC \
LIMIT 1 FOR UPDATE
//let everyone know we're scanning this one, so they'll keep out
UPDATE scan_targets SET being_scanned = true WHERE id = $row['id']
//Commit transaction
COMMIT
//scan
scan_target($row['url'])
//update entry state to allow it to be scanned in the future again
UPDATE scan_targets SET being_scanned = false, \
scanned_at = NOW() WHERE id = $row['id']
}
You'd probably need a 'cleaner' that checks periodically if there's any aborted scans hanging around too, and reset their state so they can be scanned again.
And then you can have several scan processes running in parallel! Yey!
cheers!
EDIT: I forgot that you need to make the first SELECT with FOR UPDATE. Read more here
This surely isn't the answer to your question but if you're willing to learn python I recommend you look at Scrapy, an open source web crawler/scraper framework which should fill your needs. Again, it's not PHP but Python. It is how ever very distributable etc... I use it myself.
Due to how PHP works it seems I cannot just allow the scripts to process more than one or a limited amount of links due to script execution times. Memory limits. Timeouts etc.
Memory limit is only a problem, if your code leaks memory. You should fix that, rather than raising the memory limit. Script execution time is a security measure, which you can simply disable for your cli-scripts.
Also I cannot run multiple instances of the same script as they will overwrite each other in the DB.
You can construct your application in such a way that instances don't override each other. A typical way to do it would be to partition per site; Eg. start a separate script for each site you want to crawl.
CLI scripts are not limited by max execution times. Memory limits are not normally a problem unless you have large sets of data in memory at any one time. Timeouts should be handle gracefully by your application.
It should be possible to change your code so that you can run several instances at once - you would have to post the script for anyone to advise further though. As Peter says, you probably need to look at the design. Providing the code in a pastebin will help us to help you :)
Related
I have a daily cron job which takes about 5 minutes to run (it does some data gathering and then various database updates). It works fine, but the problem is that, during those 5 minutes, the site is completely unresponsive to any requests, HTTP or otherwise.
It would appear that the cron job script takes up all the resources while it runs. I couldn't find anything in the PHP docs to help me out here - how can I make the script know to only use up, say, 50% of available resources? I'd much rather have it run for 10 minutes and have the site available to users during that time, than have it run for 5 minutes and have user complaints about downtime every single day.
I'm sure I could come up with a way to configure the server itself to make this happen, but I would much prefer if there was a built-in approach in PHP to resolving this issue. Is there?
Alternatively, as plan B, we could redirect all user requests to a static downtime page while the script is running (as opposed to what's happening now, which is the page loading indefinitely or eventually timing out).
A normal script can't hog up 100% of resources, resources get split over the processes. It could slow everything down intensly, but not lock all resources in (without doing some funky stuff). You could get a hint by doing top -s in your commandline, see which process takes up a lot.
That leads to conclude that something locks all further processes. As Arkascha comments, there is a fair chance that your database gets locked. This answer explains which table type you should use; If you do not have it set to InnoDB, you probally want that, at least for the locking tables.
It could also be disk I/O if you write huge files, try to split it into smaller read/writes or try to place some of the info (e.g. if it are files with lists) to your database (assuming that has room to spare).
It could also be CPU. To fix that, you need to make your code more efficient. Recheck your code, see if you do heavy operations and try to make those smaller. Normally you want this as fast as possible, now you want them as lightweight as possible, this changes the way you write code.
If it still locks up, it's time to debug. Turn off a large part of your code and check if the locking still happens. Continue turning on code untill you notice locking. Then fix that. Try to figure out what is costing you so much. Only a few scripts require intense resources, it is now time to optimize. One option might be splitting it into two (or more) steps. Run a cron that prepares/sanites the data, and one that processed the data. These dont have to run at syncronical, there might be a few minutes between them.
If that is not an option, benchmark your code and improve as much as you can. If you have a heavy query, it might improve by selecting only ID's in the heavy query and use a second query just to fetch the data. If you can, use your database to filter, sort and manage data, don't do that in PHP.
What I have also implemented once is a sleep every N actions.
If your script really is that extreme, another solution could be moving it to a time when little/no visitors are on your site. Even if you remove the bottleneck, nobody likes a slow website.
And there is always the option of increasing your hardware.
You don't mention which resources are your bottleneck; CPU, memory or disk I/O.
However if it is CPU or memory you can do something this in you script:
http://php.net/manual/en/function.sys-getloadavg.php
http://php.net/manual/en/function.memory-get-usage.php
$yourlimit = 100000000;
$load = sys_getloadavg();
if ($load[0] > 0.80 || memory_get_usage() > $yourlimit) {
sleep(5);
}
Another thing to try would be to set your process priority in your script.
This requires SU though, which should be fine for a cronjob?
http://php.net/manual/en/function.proc-nice.php
proc_nice(50);
I did a quick test for both and it work like a charm, thanks for asking I have cronjob like that as well and will implement it. It looks like the proc_nice only will do fine.
My test code:
proc_nice(50);
$yourlimit = 100000000;
while (1) {
$x = $x+1;
$load = sys_getloadavg();
if ($load[0] > 0.80 || memory_get_usage() > $yourlimit) {
sleep(5);
}
echo $x."\n";
}
It really depend of your environment.
If using a unix base, there is built-in tools to limit cpu/priority of a given process.
You can limit the server or php alone, wich is probably not what you are looking for.
What you can do first is to separate your task in a separate process.
There is popen for that, but i found it much more easier to make the process as a bash script. Let''s name it hugetask for the example.
#!/usr/bin/php
<?php
// Huge task here
Then to call from the command line (or cron):
nice -n 15 ./hugetask
This will limit the scheduling. It mean it will low the priority of the task against others. The system will do the job.
You can as well call it from your php directly:
exec("nice -n 15 ./hugetask &");
Usage: nice [OPTION] [COMMAND [ARG]...] Run COMMAND with an adjusted
niceness, which affects process scheduling. With no COMMAND, print the
current niceness. Niceness values range from
-20 (most favorable to the process) to 19 (least favorable to the process).
To create a cpu limit, see the tool cpulimit which has more options.
This said, usually i am just putting some usleep() in my scripts, to slow it down and avoid to create a funnel of data. This is ok if you are using loops in your script. If you slow down your task to run in say 30 minutes, there won't be much issues.
See also proc_nice http://php.net/manual/en/function.proc-nice.php
proc_nice() changes the priority of the current process by the amount
specified in increment. A positive increment will lower the priority
of the current process, whereas a negative increment will raise the
priority.
And sys_getloadavg can also help. It will return an array of the system load in the last 1,5, and 15 minutes.
It can be used as a test condition before launching the huge task.
Or to log the average to find the best day time to launch huge task. It can be susrprising!
print_r(sys_getloadavg());
http://php.net/manual/en/function.sys-getloadavg.php
You could try to delay execution using sleep. Just cause your script to pause between several updates of your database.
sleep(60); // stop execution for 60 seconds
Although this depends a lot on the kind of process you are doing in your script. Maybe or not helpful in your case. Worth a try, so you could
Split your queries
do the updates in steps with sleep inbetween
References
Using sleep for cron process
I could not describe it better than the quote in the above answer:
Maybe you're walking the database of 9,000,000 book titles and updating about 10% of them. That process has to run in the middle of the day, but there are so many updates to be done that running your batch program drags the database server down to a crawl for other users.
So modify the batch process to submit, say, 1000 updates, then sleep for 5 seconds to give the database server a chance to finish processing any requests from other users that have backed up.
Sleep and server resources
sleep resources depend on OS
adding sleep to allevaite server resources
Probably to minimize you memory usage you should process heavy and lengthy operations in batches. If you query the database using an ORM like doctrine you can easily use existing functions
http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/reference/batch-processing.html
It's hard to tell what exactly the issue may be without having a look at your code (cron script). But to confirm that the issue is caused by the cron job you can run the script manually and check website responsiveness. If you notice the site being down when running the cron job then we would have to have a look at your script in order to come up with a solution.
Many loops in your cron script might consume a lot of CPU resources.
To prevent that and reduce CPU usage simply put some delays in your script, for example:
while($long_time_condition) {
//Do something here
usleep(100000);
}
Basically, you are giving the processor some time to do something else.
Also you can use the proc_nice() function to change the process priority. For example proc_nice(20);//very low priority. Look at this question.
If you want to find the bottlenecks in your code you can try to use Xdebug profiler.
Just set it up in your dev environment, start the cron manually and then profile any page. Also you can profile your cron script as well php -d xdebug.profiler_enable=On script.php, look at this question.
If you suspect that the database is your bottleneck than import pretty large dataset (or entire database) in your local database and repeat the steps, logging and inspecting all the queries.
Alternatively if it possible setup the Xdebug on the staging server where the server is as close as possible to production and profile the page during cron execution.
Here's the situation. I am scrapping a website to get the data from it's articles using a robots page supplied by that website (list of URLs pointing to every article that's posted on the site). So far, I do a database merge to 'upsert' the URLs into my table. I know that each scrapping run will take a good while cause there's over 1400 articles to parse. I need to write an algorithm that will only do a small chunk of the jobs on cron at a time so it doesn't overload my server, etc.
Edit: I think I should mention that I'm using drupal 7. Also, this has to be an ongoing script that happens over time, I'm not so worried about the time it takes for the initial fill of the database. The robots page is dynamic, URLs get added there periodically as articles are added. I'm using hook_cron() currently for this, but I'm open to better methods if there's something better than that for doing it.
You can use the Drupal queue operations API to enqueue each page to scrap as queue item. You can, but are not required, declare your queue as cron-executed. Drupal will then take cares of executing as much queue item at each cron run without reaching the queue declared maximum execution time.
See aggregator_cron for an example of item en-queuing. And aggregator_cron_queue_info for the declaration that will let Drupal process these queued items during its cron.
If queue processing during normal Drupal cron is an issue, you can process your queue independently with the help of modules like Waiting Queue or Beanstalkd integration.
Most likely the http overhead of fetching each article will vastly outweigh the overhead of doing the database operations. Just don't fetch too many articles in parallel and you should be fine. Most webmasters frown on scrapers, especially when they're doing 10, 20, 500+ parallel fetches.
So, you already have the urls in your database. Have a status column in that table - scraped or not. The cron can kick off every so often grabbing the next url that has not been scraped from the table and marking it as scraped.
I've heard about cron job and don't think the actual creation of it will be that hard to make but I've some concerns about how this will work with a large script.
Without going too much off-topic on my project i will stick with the basics about my situation. I need to make a script that every day performs a CURL fetch for data on a remote website and updates an database for each featured member on my website with it. In short, it's approximatively at this time 1000 times the script need to be executed but it will be a larger number as times goes by.
As you can guess, this will take a long time to preform so i'm worried about how the execution will work in a manner of not crashing in the middle of it.
My first thought was to perhaps split the users into groups and make the executions on a small amount of users each time but don't know how this is manageable ( will read on further about the topic when i got some form of confirmation on this).
So, to my question. Do you think there is any way for me to make this happen and do you perhaps have any suggestions on how to make this to work efficiently? All help i can get is appreciated. Thank you for your time.
bigger cron-jobs with php and mysql needs to be fragmented, since there is no way for you to 'nice' them, (reduce their os priority). Even if you nice the script, the mysql-requests will be executed without this concern.
From what you're describing there's two aspects to consider:
Congestion of network bandwith
Congestion of database throughput
I'd recommend a fragmented solution where you call your script from cron more often, and let the script execute only a small amount of the total job. The job should further be canceled (postponed to next run) if i/o-bandwith or cpu-usage is above any limit that may affect response-time to visitors.
regards,
/t
One Way:
I'm usually against putting logic in the database, but in this case a stored procedure might help. It will run your job faster (since it's a large one) and also you want to lock the tables as you do it. That way, if the script that calls the stored procedure gets hit by cron before the original job was over with it wont edit your database while the first one is running.
The actual time can i not give an
straight answer on but based on
previously experiences this will take
longer then the max execution time.
So solve that problem. There's a reason you can have a different php.ini for the command line interface. Then you can simply focus on processing all users in one script.
I solved this program using the files of cron job as differents cron jobs with small pieces. If you are using PHP you can set a cron job to domain/cronjob1.php, domain/cronjob2.php limiting the database lets say 10 with
$sql="SELECT * FROM table LIMIT 10";
to cronjob 1 and the rest in cronjo2
I have a PHP application that currently has 5k users and will keep increasing for the forseeable future. Once a week I run a script that:
fetches all the users from the database
loops through the users, and performs some upkeep for each one (this includes adding new DB records)
The last time this script ran, it only processed 1400 users before dieing due to a 30 second maximum execute time error. One solution I thought of was to have the main script still fetch all the users, but instead of performing the upkeep process itself, it would make an asynchronous cURL call (1 for each user) to a new script that will perform the upkeep for that particular user.
My concern here is that 5k+ cURL calls could bring down the server. Is this something that could be remedied by using a messaging queue instead of cURL calls? I have no experience using one, but from what I've read it seems like this might help. If so, which message queuing system would you recommend?
Some background info:
this is a Symfony project, using Doctrine as my ORM and MySQL as my DB
the server is a Windows machine, and I'm using Windows' task scheduler and wget to run this script automatically once per week.
Any advice and help is greatly appreciated.
If it's possible, I would make a scheduled task (cron job) that would run more often and use LIMIT 100 (or some other number) to process a limited number of users at a time.
A few ideas:
Increase the Script Execution time-limit - set_time_limit()
Don't go overboard, but more than 30 seconds would be a start.
Track Upkeep against Users
Maybe add a field for each user, last_check and have that field set to the date/time of the last successful "Upkeep" action performed against that user.
Process Smaller Batches
Better to run smaller batches more often. Think of it as being the PHP equivalent of "all of your eggs in more than one basket". With the last_check field above, it would be easy to identify those with the longest period since the last update, and also set a threshold for how often to process them.
Run More Often
Set a cronjob and process, say 100 records every 2 minutes or something like that.
Log and Review your Performance
Have logfiles and record stats. How many records were processed, how long was it since they were last processed, how long did the script take. These metrics will allow you to tweak the batch sizes, cronjob settings, time-limits, etc. to ensure that the maximum checks are performed in a stable fashion.
Setting all this may sound like alot of work compared to a single process, but it will allow you to handle increased user volumes, and would form a strong foundation for any further maintenance tasks you might be looking at down the track.
Why don't you still use the cURL idea, but instead of processing only one user for each, send a bunch of users to one by splitting them into groups of 1000 or something.
Have you considered changing your logic to commit changes as you process each user? It sounds like you may be running a single transaction to process all users, which may not be necessary.
How about just increasing the execution time limit of PHP?
Also, looking into if you can improve your upkeep-procedure to make it faster can help too. Depending on what exactly you are doing, you could also look into spreading it out a bit. Do a couple once in a while rather than everyone at once. But depends on what exactly you're doing of course.
I have a cron that for the time being runs once every 20 minutes, but ultimately will run once a minute. This cron will process potentially hundreds of functions that grab an XML file remotely, and process it and perform its tasks. Problem is, due to speed of the remote sites, this script can sometimes take a while to run.
Is there a safe way to do this without [a] the script timing out, [b] overloading the server [c] overlapping and not completing its task for that minute before it runs again (would that error out?)
Unfortunately caching isnt an option as the data changes near real-time, and is from a variety of sources.
I think a slight design change would benefit this process quite a bit. Given that a remote server could time out, or a connection could be slow, you'll definitely run into concurrency issues if one slow job is still writing files when another one starts up.
I would break it into two separate scripts. Have one script that is only used for fetching the latest XML data, and another for processing it. The fetch script can take it's sweet time if it needs to, while the process script continually looks for the newest file available in order to process it.
This way they can operate independently, and the processing script can always work with the latest data, irregardless of how long either script takes to perform.
have a stack that you keep all the jobs on, have a handful of threads who's job it is to:
Pop a job off the stack
Check if you need to refresh the xml file (check for etags, expire headers, etc.)
grab the XML (this is the bit that could take the time hence spreading the load over threads) if need be, this should time out if it takes too long and raise the fact it did to someone as you might have a site down, dodgy rss generator or whatever.
then process it
This way you'll be able to grab lots of data each time.
It could be that you don't need to grab the file at all (would help if you could store the last etag for a file etc.)
One tip, don't expect any of them to be in a valid format. Suggest you have a look at Mark Pilgrims RSS RegExp reader which does a damn fine job of reading most RSS's
Addition: I would say hitting the same sites every minute is not really playing nice to the servers and creates alot of work for your server, do you really need to hit it that often?
You should make sure to read the <ttl> tag of the feeds you are grabbing to ensure you are not unnecessarily grabbing feeds before they change. <ttl> holds the update period. So if a feed has <ttl>60</ttl> then it should be only updated every 60 minutes.