Im looking to build a distributed video encoding cluster of a few dozen machines. Ive never worked with a messaging queue before, but the 2 that I started playing around with were Gearman and Beanstalkd.
Beanstalk seems to be a lot simpler and easier to use than Gearman, but its not as feature rich as.
One thing I don't understand is... how do you spawn new workers on all the servers? I plan to use php. Is it as simple as running worker.php in CLI with "&" and just have it sit there waiting for work?
I noticed gearman doesn't actually kill the process after a job is done, but Beanstalk does, so I have to restart the script after every job, on every server.
Currently Im more inclined to use Beanstalk, the general flow of things I planned was:
Run a minutely cron on each server that checks if there are pre-defined amount of workers running. If its less than supposed to be, spawn new worker processes. Each process will take roughly 2-30 minutes.
Maybe I have a flaw in my logic here? Let me know what would be a "better" or "proper" way of doing this?
Terminology I will use just to try and be clear...
There is the concept of a producer and a consumer. The producer generates jobs that are put on a queue (i.e. the beanstalk service) that is then read by a consumer.
There are multiple ways to write a consumer. You can either every x time frame via a cron job run the task or just have a consumer running in a while 1 loop via php (or what have you).
Where to install the service is really dependent on what you are going after. For me I normally install the service either on a consumer(s) or on its separate box (with sometimes the latter being overkill depending on your needs).
If you want durability on the queue side then you should use Beanstalk's binlog parameter (-b ). If something happens to your beanstalk service this will allow you to restart with minimal loss of data in the queues (if not no information). Durability on the producer side can come from having multiple queues to try against.
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Would appreciate some help understanding typical best practices in carrying out a series of tasks using Gearman in conjunction with PHP (among other things).
Here is the basic scenario:
A user uploads a set of image files through a web-based interface. The php code responding to the POST request generates an entry in a database for each file, mostly with null entries in the columns, queues a job for each to do analysis using Gearman, generates a status page and exits.
The Gearman worker gets a job for a file and starts a relatively long-running analysis. The result of that analysis is a set of parameters that need to be inserted back into the database record for that file.
My question is, what is the generally accepted method of doing this? Should I use a callback that will ultimately kick off a different php script that is going to do the modification, or should the worker function itself do the database modification?
Everything is currently running on the same machine; I'm planning on using Gearman for background scheduling, rather than for scaling by farming out to different machines, but in any case any of the functions could connect to the database wherever it is.
Any thoughts appreciated; just looking for some insights on how this typically gets structured and what might be considered best practice.
Are you sure you want to use Gearman? I only ask because it was the defacto PHP job server about 15 years ago but hasn't been a reliable solution for quite some time. I am not sure if things have drastically improved in the last 12 months, but last time I evaluated Gearman, it wasn't production capable.
Now, on to the questions.
what is the generally accepted method of doing this? Should I use a callback that will ultimately kick off a different php script that is going to do the modification, or should the worker function itself do the database modification?
You are going to follow this general pattern with any job queue:
Collect a unit of work. In your case, it will be 1 of the images and any information about who that image belongs to, user id, etc.
Submit the work to the job queue with this information.
Job Queue's worker process picks up the work, and starts processing it. This is where I would create records in the database as you can opt to not create them on job failure.
The job queue is going to track which jobs have completed and usually the status of completion. If you are using gearman, this is the gearmand process. You also need something pickup work and process that work, I will refer to this as the job worker. The job worker is where the concurrency happens which is what i think you were referring to when you said "kick off a different php script." You can just kick off a PHP script at an interval (with supervisord or a cronjob) for a kind of poll & fork approach. It's not the most efficient approach, but it doesn't sound like it will really matter for your applications use case. You could also use pcntl_fork or pthreads in PHP to get more control over your concurrent processes and implement a worker pool pattern, but it is much more complicated than just firing off a script. If you are interested in trying to implement some concurrency in PHP, I have a proof-of-concept job worker for beanstalkd available on GitHub that implements a worker pool with both fork and pthreads. I have also include a couple of other resources on the subject of concurrency.
Job Worker (pthreads)
Job Worker (fork)
PHP Daemon Example
PHP IPC Example
I know Laravel's queue drivers such as redis and beanstalkd and I read that you can increase the number of workers for beanstalkd etc. However I'm just not sure if these solutions are right for my scenario. Here's what I need;
I listen to an XML feed over a socket connection, and the data just keeps coming rapidly. forever. I get tens of XML documents in a second.
I read data from this socket line by line, and once I get to the XML closing tag, I send the buffer to another process to be parsed. I used to just encode the xml in base64, and run a separate php process for each xml. shell_exec('php parse.php' . $base64XML);
This allowed me to parse this never ending xml data quite rapidly. Sort of a manual threading. Now I'd like to utilize the same functionality with Laravel, but I wonder if there is a better way to do it. I believe Artisan::call('command') doesn't push it to the background. I could of course do a shell_exec within Laravel too, but I'd like to know if I can benefit from Beanstalkd or a similar solution.
So the real question is this: How can I set the number of queue workers for beanstalkd or redis drivers? Like I want 20 threads running at the same time. More if possible.
A slightly less important question is: How many threads is too many? If I had a very high-end dedicated server that can process the load just fine, would creating 500 threads/workers with these tools cause any problems on the code level?
Well laravel queues are just made for that.
Basicaly, you have to create a Job Class. All the heavy work you want to do on your xml document need to be here.
Then, you fetch your xml out of the socket, and as soon as you have received one document, you push it on your Queue.
Later, a queue worker will pick it up from the queue, and do the heavy work.
The advantage of that is that if you queue up documents faster than you work on them, the queue will take care of that high load moment and queue up tasks for later.
I also don't recommend you to do it without a queue (with a fork like you did). In fact, if too much documents come in, you'll create too many childs threads and overload your server. Bookkeeping these threads correctly is risky and not worth it when a simple queue with a fixed number of workers solve all these problems out of the box).
After a little more research, I found how to set the number of worker processes. I had missed that part in the documentation. Silly me. I still wonder if this supervisor tool can handle hundreds of workers for situations like mine. Hopefully someone can share their experience, but if not I'll be updating this answer once I do a performance test this week.
I tell you from experience that shell_exec() is not the ideal way to run async tasks in PHP.
Seems ok while developing, but if you have a small vps (1-2 GB ram) you could overload your server and apache/nginx/sql/something could brake while you're not around and your website could be down for hours / days.
I recommend Laravel Queues + Scheduler for these kind of things.
right now, we have a single server with a cronjob tab that sends out daily emails. We would like to scale that server. The application is standard zend framework application deployed on centos server in amazon cloud.
We already took care of the load balancing, content management and managing deployment. However, the cronjob is still an issue for us, as we need to grantee that some jobs are performed only once.
For example, the daily emails cronjob must only be executed once by a single server. I'm looking for the best method to grantee only one server will execute it only once.
I'm thinking about 2 solutions, but i was wondering if someone else had the same issue.
Make one of the servers "master", who only sends out the daily emails. That will be an issue, if the server malfunction, and generally we don't want to have a "special" server. It would also means we will need to keep track which server is master.
Have a queue of schedule tasks to be performed. Each server open that queue and sees which tasks needed to be performed. The first server who "grab" the task, will preform the task and mark it as done. I was looking at amazon simple queuing service as a solution for the queue.
Both these solutions have advantages and disadvantages, and i was wondering if someone thought about someone else that might help us here.
When you need to scale out cron jobs, you are better off using a job manager like Gearman
Beanstalkd could also be an option for you.
I had the same problem. What I did was dead simple.
I spun up the cheapest EC2 instance on AWS.
I created the cronjob(s) only on this server.
The cron job just run jobs that only makes a simple request to my endpoint / api (i.e. api.mydomain.com).
On my api, i just have a route watching for these special request that will run the job I want. So basically, all I'm doing instead of running the task using a cronjob, im running the task via a http request.
I hope that makes sense! Now it doesn't matter how many servers you have, it will just scale! Also, your cronjob server's only function is to run dead simple jobs to send a request, nothing more.
My app takes a loooong list of urls, and split it in X (where X = $threads) so then I can start a thread.php and calculate the urls for it. Then it does GET and POST request to retrieve data
I am using this:
for($x=1;$x<=$threads;$x++){
$pid[] = exec("/path/bin/php thread.php <options> > /dev/null & echo \$!");
}
For "threading" (I know its not really threading, is it forking or what?), I save the pids into a file for later checking if N thread is running and to stop them.
Now I want to move out from php, I was thinking about using python because I'd like to learn more about it.
How can I achieve this kind of "threading" with python? (or ruby)
Or is there a better way to launch multiple background threads in python or ruby that runs in parallel (at the same time)?
The threads doesn't need to communicate between each other or with a main thread, they are independent, they do http request and interact with a mysql db, they may need to access/modify the same table entries (I haven't tought about this or how I will solve it yet).
The app works with "projects", each project has a "max threads" variable and I use a web interface to control it (so I could still use php for the interface [starting/stopping threads] in the new app).
I wanted to use
from threading import Thread
in python, but I've been told those threads wont run in parallel but once at a time.
The app is intended to run on linux web servers.
Any suggestion will be appreciated.
For Python 2.6+, consider the multiprocessing module:
multiprocessing is a package that supports spawning processes using an API similar to the threading module. The multiprocessing package offers both local and remote concurrency, effectively side-stepping the Global Interpreter Lock by using subprocesses instead of threads. Due to this, the multiprocessing module allows the programmer to fully leverage multiple processors on a given machine. It runs on both Unix and Windows
For Python 2.5, the same functionality is available via pyprocessing.
In addition to the example at the links above, here are some additional links to get you started:
multiprocessing Basics
Communication between processes with multiprocessing
You don't want threading. You want a work queue like Gearman that you can send jobs to asynchronously.
It's worth noting that this is a cross-platform, cross-language solution. There are bindings for many languages (including Python and PHP) provided officially, and many more unofficially with a bit of work with Google.
The original intent is effectively load balancing, but it works just as well with only one machine. Basically, you can create one or more Workers that listen for Jobs. You can control the number of Workers and the types of Jobs they can listen for.
If you insert five Jobs into the queue at the same time, and there happen to be five Workers waiting, each Worker will be handed one of the Jobs. If there are more Jobs than Workers, the Jobs get handled sequentially. Your Client (the thing that submits Jobs) can either wait for all of the Jobs it's created to complete, or it can simply place them in the queue and continue on.
I've a problem which is giving me some hard time trying to figure it out the ideal solution and, to better explain it, I'm going to expose my scenario here.
I've a server that will receive orders
from several clients. Each client will
submit a set of recurring tasks that
should be executed at some specified
intervals, eg.: client A submits task
AA that should be executed every
minute between 2009-12-31 and
2010-12-31; so if my math is right
that's about 525 600 operations in a
year, given more clients and tasks
it would be infeasible to let the server process all these tasks so I
came up with the idea of worker
machines. The server will be developed
on PHP.
Worker machines are just regular cheap
Windows-based computers that I'll
host on my home or at my workplace,
each worker will have a dedicated
Internet connection (with dynamic IPs)
and a UPS to avoid power outages. Each
worker will also query the server every
30 seconds or so via web service calls,
fetch the next pending job and process it.
Once the job is completed the worker will
submit the output to the server and request
a new job and so on ad infinitum. If
there is a need to scale the system I
should just set up a new worker and the
whole thing should run seamlessly.
The worker client will be developed
in PHP or Python.
At any given time my clients should be
able to log on to the server and check
the status of the tasks they ordered.
Now here is where the tricky part kicks in:
I must be able to reconstruct the
already processed tasks if for some
reason the server goes down.
The workers are not client-specific,
one worker should process jobs for
any given number of clients.
I've some doubts regarding the general database design and which technologies to use.
Originally I thought of using several SQLite databases and joining them all on the server but I can't figure out how I would group by clients to generate the job reports.
I've never actually worked with any of the following technologies: memcached, CouchDB, Hadoop and all the like, but I would like to know if any of these is suitable for my problem, and if yes which do you recommend for a newbie is "distributed computing" (or is this parallel?) like me. Please keep in mind that the workers have dynamic IPs.
Like I said before I'm also having trouble with the general database design, partly because I still haven't chosen any particular R(D)DBMS but one issue that I've and I think it's agnostic to the DBMS I choose is related to the queuing system... Should I precalculate all the absolute timestamps to a specific job and have a large set of timestamps, execute and flag them as complete in ascending order or should I have a more clever system like "when timestamp modulus 60 == 0 -> execute". The problem with this "clever" system is that some jobs will not be executed in order they should be because some workers could be waiting doing nothing while others are overloaded. What do you suggest?
PS: I'm not sure if the title and tags of this question properly reflect my problem and what I'm trying to do; if not please edit accordingly.
Thanks for your input!
#timdev:
The input will be a very small JSON encoded string, the output will also be a JSON enconded string but a bit larger (in the order of 1-5 KB).
The output will be computed using several available resources from the Web so the main bottleneck will probably be the bandwidth. Database writes may also be one - depending on the R(D)DBMS.
It looks like you're on the verge of recreating Gearman. Here's the introduction for Gearman:
Gearman provides a generic application
framework to farm out work to other
machines or processes that are better
suited to do the work. It allows you
to do work in parallel, to load
balance processing, and to call
functions between languages. It can be
used in a variety of applications,
from high-availability web sites to
the transport of database replication
events. In other words, it is the
nervous system for how distributed
processing communicates.
You can write both your client and the back-end worker code in PHP.
Re your question about a Gearman Server compiled for Windows: I don't think it's available in a neat package pre-built for Windows. Gearman is still a fairly young project and they may not have matured to the point of producing ready-to-run distributions for Windows.
Sun/MySQL employees Eric Day and Brian Aker gave a tutorial for Gearman at OSCON in July 2009, but their slides mention only Linux packages.
Here's a link to the Perl CPAN Testers project, that indicates that Gearman-Server can be built on Win32 using the Microsoft C compiler (cl.exe), and it passes tests: http://www.nntp.perl.org/group/perl.cpan.testers/2009/10/msg5521569.html But I'd guess you have to download source code and build it yourself.
Gearman seems like the perfect candidate for this scenario, you might even want to virtualize you windows machines to multiple worker nodes per machine depending on how much computing power you need.
Also the persistent queue system in gearman prevents jobs getting lost when a worker or the gearman server crashes. After a service restart the queue just continues where it has left off before crash/reboot, you don't have to take care of all this in your application and that is a big advantage and saves alot of time/code
Working out a custom solution might work but the advantages of gearman especially the persistent queue seem to me that this might very well be the best solution for you at the moment. I don't know about a windows binary for gearman though but i think it should be possible.
A simpler solution would be to have a single database with multiple php-nodes connected. If you use a proper RDBMS (MSql + InnoDB will do), you can have one table act as a queue. Each worker will then pull tasks from that to work on and write it back into the database upon completion, using transactions and locking to synchronise. This depends a bit on the size of input/output data. If it's large, this may not be the best scheme.
I would avoid sqlite for this sort of task, although it is a very wonderful database for small apps, it does not handle concurrency very well, it has only one locking strategey which is to lock the entire database and keep it locked until a sinlge transaction is complete.
Consider Postgres which has industrial strength concurrency and lock management and can handle multiple simultanious transactions very nicely.
Also this sounds like a job for queuing! If you were in hte Java world I would recommend a JMS based archictecture for your solution. There is a 'dropr' project to do something similar in php but its all fairly new so it might not be suitable for your project.
Whichever technoligy you use you should go for a "free market" solution where the worker threads consume available "jobs" as fast as they can, rather than a "command economy" where a central process allocates tasks to choosen workers.
The setup of a master server and several workers looks right in your case.
On the master server I would install MySQL (Percona InnoDB version is stable and fast) in master-master replication so you won't have a single point of failure.
The master server will host an API which the workers will pull at every N seconds. The master will check if there is a job available, if so it has to flag that the job has been assigned to the worker X and return the appropriate input to the worker (all of this via HTTP).
Also, here you can store all the script files of the workers.
On the workers, I would strongly suggest you to install a Linux distro. On Linux it's easier to set up scheduled tasks and in general I think it's more appropriate for the job.
With Linux you can even create a live cd or iso image with a perfectly configured worker and install it fast and easy on all the machines you want.
Then set up a cron job that will RSync with the master server to update/modify the scripts. In this way you will change the files in just one place (the master server) and all the workers will get the updates.
In this configuration you don't care of the IPs or the number of workers because the workers are connecting to the master, not vice-versa.
The worker job is pretty easy: ask the API for a job, do it, send back the result via API. Rinse and repeat :-)
Rather than re-inventing the queuing wheel via SQL, you could use a messaging system like RabbitMQ or ActiveMQ as the core of your system. Each of these systems provides the AMQP protocol and has hard-disk backed queues. On the server you have one application that pushes new jobs into a "worker" queue according to your schedule and another that writes results from a "result" queue into the database (or acts on it some other way).
All the workers connect to RabbitMQ or ActiveMQ. They pop the work off the work queue, do the job and put the response into another queue. After they have done that, they ACK the original job request to say "its done". If a worker drops its connection, the job will be restored to the queue so another worker can do it.
Everything other than the queues (job descriptions, client details, completed work) can be stored in the database. But anything realtime should be put somewhere else. In my own work I'm streaming live power usage data and having many people hitting the database to poll it is a bad idea. I've written about live data in my system.
I think you're going in the right direction with a master job distributor and workers. I would have them communicate via HTTP.
I would choose C, C++, or Java to be clients, as they have capabilities to run scripts (execvp in C, System.Desktop.something in Java). Jobs could just be the name of a script and arguments to that script. You can have the clients return a status on the jobs. If the jobs failed, you could retry them. You can have the clients poll for jobs every minute (or every x seconds and make the server sort out the jobs)
PHP would work for the server.
MySQL would work fine for the database. I would just make two timestamps: start and end. On the server, I would look for WHEN SECONDS==0