TheLoneDyno

Isolate code to only run on a certain number of Heroku dynos. Then use LISTEN/NOTIFY to trigger custom events on those dynos.

Why would you want to run code on only one dyno? Maybe you want to add some performance monitoring code to a critical path in production, you don't want to slow down all your dynos, so you can retain throughput by isolating to only run on one dyno. Maybe you want to test out a refactoring or change, instead of rolling it out to all of your service, you could run it on a select number. Whatever the reason, if you want to run some code on only a few dynos, this is the gem for you!

Why is this needed? All Heroku dynos operate independently of one another. Once one is running you can't change it. You can $ heroku run bash but this gives you a new dyno that doesn't receieve any web traffic. If you want to run code on only a certain number of dynos it's been difficult to do so until now.

Be warned, only changing behavior on 1 dyno in your app, could cause difficult to reproduce problems. "I'm getting an error, but only once fore every 20 requests". Using this library is an advanced technique and should be implemented with care. Make sure to tell the rest of your team what you're doing, and remove the code from your codebase as soon as you're done.

A big change happened between version 0.1 and 1.0. We are no longer using Postgres advisory locks. Instead we're depending on the DYNO=web.1 environment variables. This makes behavior more predictable and easier to reason about.

Add this line to your application's Gemfile:

gem 'the_lone_dyno'

And then execute:

$ bundle

Or install it yourself as:

$ gem install the_lone_dyno

In an initializer like config/initializers/the_lone_dyno.rb configure your code:

TheLoneDyno.exclusive do
  while true do
    # Code that only runs in 1 dyno in the background
    puts "bump-ba-da-bump-ba-da-bump"
    sleep 1
  end
end

puts "Does not block future code execution"

This code will only run on one dyno (by default in a "web" process type). The code passed into the block will run in the background and the lock will be held for as long as your process is alive. For example if we run the above code we should get:

Does not block future code execution
bump-ba-da-bump-ba-da-bump
bump-ba-da-bump-ba-da-bump
bump-ba-da-bump-ba-da-bump
bump-ba-da-bump-ba-da-bump
bump-ba-da-bump-ba-da-bump
# ...

So your app will continue to load and run as usual.

Note: The Lone Dyno restricts code at the process/machine level. While it will prevent this code from being executed on multiple machines it is not restricted from running multiple times in the same process. If you are calling this code in multiple threads it will execute in every thread. If that's not what you want, you'll need to add your own mutex.

Sometimes you'll need to interact with your Lone Dyno externally. You can trigger events with listen/notify.

TheLoneDyno.exclusive do |signal|
  # Code you only want to run on 1 dyno

  signal.watch do |payload|
    puts "I only get called when I receive the signal: #{payload}"
  end
end

The code you are running inside of the watch block will run in a background thread waiting for a notification. How do you send a signal? You can do so with $ heroku run bash

$ heroku run bash
$ rails console
>
> TheLoneDyno.signal("hi ho silver!")

When you do this 1 and only one dyno will emit

"I only get called when I receive the signal: hi ho silver"

Whatever configuration you pass into TheLoneDyno#exclusive need to be passed into TheLoneDyno#signal such as key_base and dynos. See configuration options below. This is needed since each dyno maintains a unique lock, we re-use the same name to signal the same dyno using postgres' Listen/Notify.

By default the background thread wakes up every 60 seconds and waits 0.1 seconds to see if there is a message. You can customize this behavior using sleep and ttl. So to have it check every 10 seconds, and not wait at all you could run

TheLoneDyno.exclusive do |signal|
  # Code you only want to run on 1 dyno

  signal.watch(sleep: 10, ttl: 0) do |payload|
    puts "I only get called when I receive the signal: #{payload}"
  end
end

By default the_lone_dyno is restricted to only run in the web process type dynos. It does this using the DYNO environment variable. You can set this to another process type:

TheLoneDyno.exclusive(process_type: "worker") do
  # Code you only want to run on 5 dyno
end

This will attempt to lock any DYNO environment variable containing "worker" string.

You can control the number of dynos you run code on by passing in an integer:

TheLoneDyno.exclusive(dynos: 5) do
  # Code you only want to run on 5 dyno
end

If you need to customize the default listen key, for instance if you want to have multiple processes you want to isolate to a set of dynos you can use the key_base: key, for example:

TheLoneDyno.exclusive(key_base: "reticulate splines") do
  # Code for reticulating splines
end

TheLoneDyno.exclusive(dynos: 42, key_base: "extrapolate conclusions") do
  # Code for extrapolating conclusions
end

If you don't want to run your exclusive process in the background you can force it to run syncronously using background: false. For example:

TheLoneDyno.exclusive(background: false) do |signal|
  while true do
    puts "bump-ba-da-bump-ba-da-bump"
    sleep 1
  end
end

puts "Does block execution"

produces

bump-ba-da-bump-ba-da-bump
bump-ba-da-bump-ba-da-bump
bump-ba-da-bump-ba-da-bump
bump-ba-da-bump-ba-da-bump
bump-ba-da-bump-ba-da-bump
# ...

Notice the puts "Does block execution" never gets called because our Lone Dyno block never exits.

Keep in mind that your lock is only held for the duration of your block, so if the code in your block exits, another machine may be able to pick up the lock. For this reason it is recommended to not run short tasks syncronously in the foreground.

By default The Lone Dyno assumes you're using Active Record and already have a connection configured. If you want to use a different ORM, you'll need to provide TheLoneDyno with an object that responds to exec that executes arbitrary SQL. Under the hood this library uses hey_you. So that's how you can configure the connection

connection = Module do
  def self.exec(sql, bind)
    DB.fetch(sql, bind)
  end
end

TheLoneDyno.exclusive(connection: connection)  do

end

You can alternatively set the DEFAULT_CONNECTION

HeyYou::DEFAULT_CONNECTION = Module do
  def self.exec(sql, bind)
    DB.fetch(sql, bind)
  end
end

TheLoneDyno.exclusive  do

end

After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Bug reports and pull requests are welcome on GitHub at https://github.com/[USERNAME]/the_lone_dyno. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.

The gem is available as open source under the terms of the MIT License.