Simple background loops framework

loops is a small and lightweight framework for Ruby on Rails, Merb and other ruby frameworks created to support simple background loops in your application which are usually used to do some background data processing on your servers (queue workers, batch tasks processors, etc).

Warning: If you use some pre-2.0 version of this plugin, read a dedicated paragraph below.

What would you use it for?

Originally loops plugin was created to make our own loops code a bit more organized. We used to have dozens of different modules with methods that were called with script/runner and then used with nohup and other painful backgrounding techniques. When you have such a number of loops/workers to run in background it becomes a nightmare to manage them on a regular basis (restarts, code upgrades, status/health checking, etc).

After a few takes on writing our scripts in a more organized way we were able to generalize most of the code so now our loops started looking like a classes with a single mandatory public method called run. Everything else (spawning many workers, managing them, logging, backgrounding, pid-files management, etc) is handled by the plugin itself.

But there are dozens of libraries like this! Why do we need yet another one?

The major idea behind this small project was to create a deadly simple and yet robust framework to be able to run some tasks in background and do not think about spawning many workers, restarting them when they die, etc. So, if you need to be able to run either one or many copies of your worker and you do not want to think about re-spawning your scripts when they die and you do not want to spend megabytes of RAM on separate copies of Ruby interpreter (when you run each copy of your loop as a separate process controlled by monit/god/etc), then you should try this framework – you’re going to like it.

How to install?

Add the loops gem to your Gemfile:

gem 'loops'

And then run bundle command.

To install loops as a Rails plugin you need to do the following:

./script/plugin install git://github.com/kovyrin/loops.git

This will install the whole package in your vendor/plugins directory. For merb applications, just check out the code and place it to the vendor/plugins directory.

After you are done with the installation, you need to generate binary and configuration files by running:

./script/generate loops

This will create the following list of files:

  • ./script/loops – binary file that will be used to manage your loops

  • ./config/loops.yml – example configuration file

  • ./app/loops/simple.rb – REALLY simple loop example

  • ./app/loops/queue_loop.rb – simple ActiveMQ queue worker

How to use?

Here is a simple loop scaffold for you to start from (put this file to app/loops/hello_world_loop.rb):

class HelloWorldLoop < Loops::Base
  def run
    with_period_of(1) do # period is in seconds
      debug("Hello, debug log!")
      sleep(config['sleep_period']) # Do something "useful" and make it configurable
      debug("Hello, debug log (yes, once again)!")
    end
  end
end

When you have your loop ready to use, add the following lines to your (maybe empty yet) config/loops.yml file loops section:

loops:
  hello_world:
    type: simple
    sleep_period: 10

This is it! To manage your loop, just run one of the following commands:

  • To list all configured loops:

    $ ./script/loops list

  • To run all enabled (actually non-disabled) loops in foreground:

    $ ./script/loops start

  • To run all enabled loops in background:

    $ ./script/loops start -d

  • To run specific loop in background:

    $ ./script/loops start hello_world -d

  • To see all possible options:

    $ ./script/loops help

Notice: If you use loops as a gem, you will get a system binary called loops which you could use instead of your <tt>./script/loops<tt> binary:

$ loops help

How to run more than one worker?

If you want to have more than one copy of your worker running, that is as simple as adding one option to your loop configuration:

loops:
  hello_world:
    type: simple
    sleep_period: 10
    workers_number: 1

This workers_number option would tell loops manager to spawn more than one copy of your loop and run them in parallel. The only thing you’d need to do is to remember about concurrent work of your loops. For example, if you have some kind of database table with elements you need to process, you can create a simple database-based locks system or use any memcache-based locks.

How to run more than one loop using the same class?

You can run the same loop class with different configuration parameters by explicitly identifying the loop class to execute:

loops:
  hello:
    type: simple
    loop_name: some_module/my_worker
    language: English

  salut:
    type: simple
    loop_name: some_module/my_worker
    language: French

Now two independent sets of loops are using the same class SomeModule::MyWorkerLoop customized by the language parameter.

How to initialize the loop before workers run?

You can run initialization code before starting loop workers by implementing the initialize_loop class method. If initialize_loop raises an error, then the loop is not started and the error is logged.

class HelloWorldLoop < Loops::Base
  def self.initialize_loop(config)
    raise "Missing required dependency" unless File.exist?(config['my_dependency'])
  end

  def run
    with_period_of(1) do # period is in seconds
      debug("Hello, debug log!")
      sleep(config['sleep_period']) # Do something "useful" and make it configurable
      debug("Hello, debug log (yes, once again)!")
    end
  end
end

How do I detect if my code is running in a loop?

We provide an api call Loops.running? that returns true if your code is running as a part of a loops process. For example, if you want your Rails.logger to write logs into your Loops logger (when you run it in a loop), you could do something like the following in your environment files:

# Switch logger to loops if we are in a loops process
config.logger = Loops.logger if Loops.running?

I want to keep my loop running on machine reboots. How to do it?

We use monit to keep loop monitors runnings. You could use something like this in your configs:

check process loop-slow_logs with pidfile /your/project/current/tmp/pids/loop-slow_logs.pid
  group loops
  start program "/bin/bash -c 'cd /your/project/current && /usr/bin/bundle exec loops start slow_logs -p /your/project/shared/pids/loop-slow_logs.pid -e loops -d'"
  stop  program "/bin/bash -c 'cd /your/project/current && /usr/bin/bundle exec loops stop -p /your/project/shared/pids/loop-slow_logs.pid'"

If you do not use bundler, you can do:

check process loop-slow_logs with pidfile /your/project/current/tmp/pids/loop-slow_logs.pid
  group loops
  start program "/your/project/current/script/loops start slow_logs -e loops -p tmp/pids/loop-slow_logs.pid -d"
  stop  program "/your/project/current/script/loops stop  slow_logs -e loops -p tmp/pids/loop-slow_logs.pid"

ActiveMQ-based workers? What’s that?

In some of our worker loops we poll ActiveMQ queue and process its items to perform some asynchronous operations. So, to make it simpler for us to create such a workers, we’ve created really simple loops class extension that wraps your code with basic queue polling/acknowledging code and as the result, you can create a loops like this:

class MyQueueLoop < Loops::Queue
  def process_message(message)
    debug "Received a message: #{message.body}"
    debug "sleeping..."
    sleep(0.5 + rand(10) / 10.0) # do something "useful" with the message :-)
    debug "done..."
  end
end

With configs like this:

loops:
  # An example of a STOMP queue-based loop
  my_queue:
    type: queue
    host: 127.0.0.1
    port: 61613
    queue_name: blah

Of course, this solution scales perfectly and to make your queue processing faster you just need to add more workers (by adding workers_number: N option).

Warning: This type of loops requires you to have the stomp gem installed in your system.

There is this workers_engine option in the config file. What do you use it for?

There are two so called “workers engines” in loops: fork and thread. They’re used to control the way process manager would spawn new loops workers: with the fork engine we’ll load all the loops classes and then fork ruby interpreter as many times as many workers we need. With the thread engine we’d do Thread.new instead of forking. Thread engine could be useful if you are sure your loop won’t lock ruby interpreter (it does not do native calls, etc) or if you use some interpreter that does not support forks (like jruby).

The default engine is fork.

What Ruby implementations does it work for?

We’ve tested and used the plugin on MRI 1.8.6/1.8.7/1.9.3 and on JRuby 1.4.0. At this point we do not support demonization in JRuby. Obviously because of JVM limitations you won’t be able to use fork workers engine in JRuby, but threaded workers do pretty well.

We have a continuous integration server configured for the project: <img src=“https://secure.travis-ci.org/kovyrin/loops.png?branch=master” alt=“Build Status” />

Recommended version of ruby ro run loops is Ruby Enterprise Edition. This is because we have a support for their Copy-On-Write friendly garbage collector that makes your loops much smaller in memory (since they share the most of the code). Even with one loop process you’d save some memory because your loop monitor process would share most of the memory with the loop itself. When you run on RubyEE, you could use loops stats command to get detailed loops memory stats:

[root@analyics current]# ./script/loops stats

--------- Loops processes ----------
PID   PPID  VMSize    Private  Name
------------------------------------
9062  1     199.3 MB  32.4 MB  loops monitor: activemq
9234  9062  211.9 MB  37.5 MB  loop worker: activemq
9251  9062  213.3 MB  38.4 MB  loop worker: activemq
9267  9062  211.9 MB  37.1 MB  loop worker: activemq
9268  9062  211.9 MB  38.0 MB  loop worker: activemq
### Processes: 5
### Total private dirty RSS: 183.33 MB

Migrating from pre-2.0 releases

Before version 2.0 has been released, this code was developed as a Rails plugin only and did not have any versions numbering system in place. So we call all those old versions a pre-2.0 releases. If you use one of those relases (if your loops plugin does not have the VERSION.yml file in the root directory), be careful when upgrading because there are a few incompatible changes we have made in the loops command: -h, -a, -s, -L and -l options were deprecated and replaced with a friendlier word commands. Use loops help to get help.

Who are the authors?

This plugin has been created in Scribd.com for our internal use and then the sources were opened for other people to use. All the code in this package has been developed by Oleksiy Kovyrin, Dmytro Shteflyuk and Alexey Verkhovsky for Scribd.com and is released under the MIT license. For more details, see LICENSE file.