Problem: I have a program that will do a lot of post-processing on a file and then sending it ANOTHER web-service for more processing. Does my design smell and is this the right way to ‘tackle a problem’
Rails Accepts file and kicks off a resque_job to do work
The resque job will sends work via REST to another web-service cluster(very slow.. does MORE work) && places the task to be monitored in a monitor_queue within Rabbit MQ for completion . The Resque job will not wait another-webservice to complete task. It exits
There are some smell issues in my design, perhaps gut reactions that could be misguided?
Is it good design to have TWO message queues. The rational is that Rabbit_MQ has a built in method for creating worker_queues (they even give sample code). Resque_Job uses redis, and seems to be the accepted method of having ‘delayed jobs’ with Rails.
What I like about RabbitMQ: It has round-robin tasking abilities (so all threats get tasked) , and guarantees work will not be removed from a QUEUE without a message acknowledgement.
Resque seems to primary suggested solution for launching delayed_jobs within Rails.
Followup: When performing the polling, I was thinking a simple worker_queue of just iterating through the entire queue with seperate 'workers' makes the most sense? Do you agree.
I don't think this is a bad design. It's a type of Service Oriented Architecture, and even though you have separate queuing systems, they're completely separate applications and would only communicate through a specific interface, which has some pros and cons. I didn't quite understand the reasoning for using RabbitMQ, though. Also, a lot of new apps seem to be using Sidekiq; IMHO it is superior in every way to Resque.
Related
So I have a system that receives messages from devices and then it goes through 3 different servers and countless of services are run on each job. From an architecture perspective, whats certain considerations in using sidekiq to make my program async? Are there downsides to making sub processes run using sidekiq. Any advice?
architecture(system design) should be based on the problems you are trying to solve. if your services are design to unique business domains and if they are async compatible then you can spawn sub jobs for each service. but if not or your need flexible transactions among services then job per request is the right choice. so you may have both of these implementations in your system based on the requirements.
The upside to making your program async with sidekiq is that it is easy and produces good reporting in case of an error. The downsides of using sidekiq for this task is that there is a lot of overhead creating and executing the jobs. This could become such a problem that it represents the majority of the resources used.
I want to build a service that notifies me when a url returns status 200. I'm currently using a sidekiq worker, if the status == 200, it updates my database (row.available = true), if not, it raises an exception and retries the worker in n seconds, n amount of times.
Though this works, it doesn't feel efficient or scalable (1000's checks would result in 1000's of exceptions, and on certain platforms that's bad news -- JRuby), and I'm sure there is a way I can build an internal service to manage this url monitoring that doesn't rely on sidekiq (perhaps in Go, or another, more suited Ruby gem). However, I have no idea where to begin, and so I'd appreciate some general direction.
Writing and running a simple link checker is easy. Doing that for 1000s of links quickly, without redundancy, and handling dead and slow-responding links without bogging down your entire system gets harder.
I'd use three threads, plus two queues:
A dispatcher thread that only reads from the database. It is responsible for finding and queuing URLs to be checked in to a "to be checked" queue.
A worker thread that consumes from the first queue and pushes results into the "updated URL results" queue.
An updater/consumer thread that takes the result of a thread in #2 and updates the database.
Ruby has some built-in classes to help:
Thread
Queue
I'd highly recommend Typhoeus and Hydra for use in the middle thread. The documentation for these two classes cover a lot of what you need to do as far as handling multiple threads running in parallel.
I wouldn't write this code as part of a Rails application. There is no value added by Rails to this, nor is it necessary. I would either require Active Record and piggy-back on the existing database.yaml settings and models, or use Rails' "runner" to run the code as an adjunct to the Rails code.
Or, I'd write a small, application-specific, piece of code to run on a different server to avoid bogging down the Rails server. Using something like MySQL or PostgreSQL drivers would let you talk to the same database that Rails uses. In this case I'd use the Sequel gem to act as the ORM, but that's because I prefer it over Active Record.
There are a lot of things to consider as you write this code, including retries of failed URLs, sensing redirections and updating the source URLs to reflect them to avoid wasting time, and not beating up the hosting servers causing you to be banned.
I've written several apps for this purpose over the years and doing it right takes a lot of forethought, so think out your design up front otherwise you could end up with some major rewrites later on.
I'm coming from a PHP environment (at least in terms of web dev) and into the beautiful world of Ruby, so I may have some dumb questions. I imagine there are some fundamentally different options available when not using PHP.
In PHP, we use memcache to store alerts we want to display in a bar along the top of the page. When something happens that generates an alert (such as a new blog post being made), a cron script that runs once every 5 minutes or so puts that information into memcache.
Now when a user visits the site, we look in memcache to find any alerts that they haven't already dismissed and we display them.
What I'm guessing I can do differently in Rails, is to by-pass the need for a cron script, and also the need to look in memcache on every request, by using a Singleton and a polling process running in a separate thread to copy from memcache to this singleton. This would, in theory, be more optimized than checking memcache once-per-request and also encapsulate the polling logic into one place, rather than being split between a cron task and the lookup logic.
My question is: are there any caveats to having some sort of runloop in the background while a Rails app is running? I understand the implications of multithreading, from Objective-C/Java, but I'm asking specifically about the Rails (3) environment.
Basically something like:
class SiteAlertsMap < Hash
include Singleton
def initialize
super
begin_polling
end
# ... SNIP, any specific methods etc ...
private
def begin_polling
# Create some other Thread here, which polls at set intervals
end
end
This leads me into a similar question. We push (encrypted) tasks onto an SQS queue, for things related to e-commerce and for long-running background tasks. We don't use cron for this, but rather we have a worker daemon written in PHP, which runs in the background. Right now when we deploy, we have to shut down this worker and start it again from the new code-base. In Rails, could I somehow have this process start and stop with the rails server (unicorn) itself? I don't think that's something I'd running on the main process in a separate thread, since we often want to control it as a process by itself, but it would be nice if it just conveniently ran when the web application was running.
Threading for background processes in ruby would be a terrible mistake, especially since you're using a multi-process server. Using unicorn with say 4 worker processes would mean that you'd be polling from each of them, which is not what you want. Ruby doesn't really have real threads, it has green threads in 1.8 and a global interpreter lock in 1.9 IIRC. Many gems and libraries are also obnoxiously unthreadsafe.
Using memcache is still your best option and, if you have it set up correctly, you should only see it adding a millisecond or two to the request time. Another option which would give you the benefit of persisting these alerts while incurring minimal additional overhead would be to store these alerts in redis. This would better protect you against things like memcache crashing or server reboots.
For the background jobs you should use a similar approach to what you have now, but there are several off the shelf handlers for this like resque, delayed_job, and a few others. If you absolutely have to use SQS as the backend queue, you might be able to find some code to help you, but otherwise you could write it yourself. This still requires the other daemon to be rebooted whenever there is a code change. In practice this isn't a huge concern as best practices dictate using a deployment system like capistrano where a rule can easily be added to bounce the daemon on deploy. I use monit to watch the daemon process, so restarting it is as easy as telling monit to restart it.
In general, Ruby is not like Java/Objective-C when it comes to threads. It follows the more Unix-like model of process based isolation, but the community has come up with best practices and ways to make this less painful than in other languages. Ruby does require a bit more attention to setting up its stack as it is not as simple as enabling mod_php and copying some files around, but once the choices and architecture is understood, it is easier to reason about how your application works. The process model, in my opinion, is much better for web apps as it isolates code and state from the effects of other running operations. The isolation also makes the app easier to work with in a distributed system.
I'm working on a Rails application that periodically needs to perform large numbers of IO-bound operations. These operations can be performed asynchronously. For example, once per day, for each user, the system needs to query Salesforce.com to fetch the user's current list of accounts (companies) that he's tracking. This results in huge numbers (potentially > 100k) of small queries.
Our current approach is to use ActiveMQ with ActiveMessaging. Each of our users is pushed onto a queue as a different message. Then, the consumer pulls the user off the queue, queries Salesforce.com, and processes the results. But this approach gives us horrible performance. Within a single poller process, we can only process a single user at a time. So, the Salesforce.com queries become serialized. Unless we run literally hundreds of poller processes, we can't come anywhere close to saturating the server running poller.
We're looking at EventMachine as an alternative. It has the advantage of allowing us to kickoff large numbers of Salesforce.com queries concurrently within a single EventMachine process. So, we get great parallelism and utilization of our server.
But there are two problems with EventMachine. 1) We lose the reliable message delivery we had with ActiveMQ/ActiveMessaging. 2) We can't easily restart our EventMachine's periodically to lessen the impact of memory growth. For example, with ActiveMessaging, we have a cron job that restarts the poller once per day, and this can be done without worrying about losing any messages. But with EventMachine, if we restart the process, we could literally lose hundreds of messages that were in progress. The only way I can see around this is to build a persistance/reliable delivery layer on top of EventMachine.
Does anyone have a better approach? What's the best way to reliably execute large numbers of asynchronous IO-bound operations?
I maintain ActiveMessaging, and have been thinking about the issues of a multi-threaded poller also, though not perhaps at the same scale you guys are. I'll give you my thoughts here, but am also happy to discuss further o the active messaging list, or via email if you like.
One trick is that the poller is not the only serialized part of this. STOMP subscriptions, if you do client -> ack in order to prevent losing messages on interrupt, will only get sent a new message on a given connection when the prior message has been ack'd. Basically, you can only have one message being worked on at a time per connection.
So to keep using a broker, the trick will be to have many broker connections/subscriptions open at once. The current poller is pretty heavy for this, as it loads up a whole rails env per poller, and one poller is one connection. But there is nothing magical about the current poller, I could imagine writing a poller as an event machine client that is implemented to create new connections to the broker and get many messages at once.
In my own experiments lately, I have been thinking about using Ruby Enterprise Edition and having a master thread that forks many poller worker threads so as to get the benefit of the reduced memory footprint (much like passenger does), but I think the EM trick could work as well.
I am also an admirer of the Resque project, though I do not know that it would be any better at scaling to many workers - I think the workers might be lighter weight.
http://github.com/defunkt/resque
I've used AMQP with RabbitMQ in a way that would work for you. Since ActiveMQ implements AMQP, I imagine you can use it in a similar way. I have not used ActiveMessaging, which although it seems like an awesome package, I suspect may not be appropriate for this use case.
Here's how you could do it, using AMQP:
Have Rails process send a message saying "get info for user i".
The consumer pulls this off the message queue, making sure to specify that the message requires an 'ack' to be permanently removed from the queue. This means that if the message is not acknowledged as processed, it is returned to the queue for another worker eventually.
The worker then spins off the message into the thousands of small requests to SalesForce.
When all of these requests have successfully returned, another callback should be fired to ack the original message and return a "summary message" that has all the info germane to the original request. The key is using a message queue that lets you acknowledge successful processing of a given message, and making sure to do so only when relevant processing is complete.
Another worker pulls that message off the queue and performs whatever synchronous work is appropriate. Since all the latency-inducing bits have already performed, I imagine this should be fine.
If you're using (C)Ruby, try to never combine synchronous and asynchronous stuff in a single process. A process should either do everything via Eventmachine, with no code blocking, or only talk to an Eventmachine process via a message queue.
Also, writing asynchronous code is incredibly useful, but also difficult to write, difficult to test, and bug-prone. Be careful. Investigate using another language or tool if appropriate.
also checkout "cramp" and "beanstalk"
Someone sent me the following link: http://github.com/mperham/evented/tree/master/qanat/. This is a system that's somewhat similar to ActiveMessaging except that it is built on top of EventMachine. It's almost exactly what we need. The only problem is that it seems to only work with Amazon's queue, not ActiveMQ.
I've been sending emails on my application (ruby 1.8.7, rails 2.3.2) like this
Thread.new{UserMailer.deliver_signup_notification(user)}
Since ruby use green threads, there's any performance advantage doing this, or I can just use
UserMailer.deliver_signup_notification(user)
?
Thanks
Global VM lock will still almost certainly apply while sending that email, meaning no difference.
You should not start threads in a request/response cycle. You should not start threads at all unless you can watch them from create to join, and even then, it is rarely worth the trouble it creates.
Rails is not thread-safe, and is not meant to be from within your controller actions. Only since Rails 2.3 has just dispatching been thread-safe, and only if you turn it on in environment.rb with config.threadsafe!.
This article explains in more detail. If you want to send your message asynchronously use BackgroundRb or its analog.
In general, using green threads to run background tasks asynchronously will mean that your application can respond to the user before the mail is sent. You're not concerned about exploiting multiple CPUs; you're only concerned on off-loading the work onto a background process and returning a web page as soon as possible.
And from examining the Rails documentation, it looks like deliver_signup_notification will block long enough to get the mail queued (although I may be wrong). So using a thread here might make your application seem more responsive, depending on how your mailer is configured.
Unfortunately, it's not clear to me that deliver_signup_notification is necessarily thread-safe. I'd want to read the documentation carefully before relying on that.
Note also that you're making assumptions about the lifetime of a Rails process once a request has been served. Many Rails applications using DRb (or a similar tool) to offload these background tasks onto an entirely separate worker process. The easiest way to do this changes fairly often--see Google for a number of popular libraries.
I have used your exact strategy and our applications are currently running in production (but rails 2.2.2). I've kept a close eye on it and our load has been relatively low (Less than 20 emails sent per day average, with peaks of around 150/day).
So far we have noticed no problems, and this appears to have resolved several performance issues we were having when using Google's mailserver.
If you need something in a hurry then give it a shot, it has been working for us.
They'll be the same as far as I know.