UPDATE: A better way to set the connection pool size.

Watsi has used Puma since the beginning. Even though we run on CRuby 1.9.3 on Heroku (so we don’t get any actual parallelism) I just love how simple & effective Puma is. One of the few options I set with Puma is the thread min & max.

$> bundle exec puma -p $PORT -c 4:8

Since ActiveRecord’s default connection pool size is 5, one might (and we did) see errors that the pool is not big enough when running Puma, or Unicorn, with more than 5 threads/workers. How might we bump that up?

change the database.yml

If you have control over your database.yml file, you can just change the pool size.

  pool: 10

… or establish a new AR connection

I thought I could establish a new connection and pass in an option, but unfortunately that doesn’t work. I wish I could pass options all the way to the ConnectionResolver class but I can’t. Plus, connection_url_to_hash just looks at the query string for options like pool anyway. Kind of a bust.

So, we’re on Heroku, and our only option is to mangle the DATABASE_URL. We could get the DATABASE_URL

$> heroku config | ack DATABASE_URL
DATABASE_URL:      postgres://foo:[email protected]/my_db
$> heroku config:add DATABASE_URL=postgres://foo:[email protected]/my_db?pool=35

(Don’t do this!)

But how would we ever handle a change to our DATABASE_URL by Heroku themselves? Sounds like trouble. Heroku doesn’t recommended this (just asked during YC office hours) so we need to change the DATABASE_URL in a safe way.

What about Sidekiq?

Yes, we also want to set the connection pool for Sidekiq to the total number of workers, like Mike recommends in the wiki.

Sidekiq.configure_server do |config|
  config.redis = { url: 'redis://redis.example.com:7372/12', namespace: 'mynamespace' }

  database_url = ENV['DATABASE_URL']
    ENV['DATABASE_URL'] = "#{database_url}?pool=25"


How we do it

Let’s take a look at our config/application.rb

require File.expand_path('../boot', __FILE__)
require "rails/all"

if defined?(Bundler)
  Bundler.require(:default, :assets, Rails.env)

module Platform

  def set_db_connection_pool_size!(size=35)
    # bump the AR connection pool
    if ENV['DATABASE_URL'] !~ /pool/
      pool_size = ENV.fetch('DATABASE_POOL_SIZE', size)
      db = URI.parse ENV['DATABASE_URL']
      if db.query
        db.query += "&pool=#{pool_size}"
        db.query = "pool=#{pool_size}"
      ENV['DATABASE_URL'] = db.to_s

  module_function :set_db_connection_pool_size!

  class Application < Rails::Application
    if Puma.respond_to?(:cli_config)
      max = Puma.cli_config.options.fetch(:max_threads)
      ::Platform.set_db_connection_pool_size! max

    # ...


With a commit added in puma 2.0.0.b6 we can now get at the max_threads setting. Puma only responds to cli_config when it’s started from the command line, so we’re cool.

For Sidekiq, we do the same thing in config/initializers/sidekiq.rb

Sidekiq.configure_server do |config|

Don’t mangle Heroku’s DATABASE_URL!

That being said, inside your app it’s not so bad to tack on a query param. I did it the safest way I knew how. If you know of a better way, please share!

Hope that helps

This has been deployed for a few weeks now and we don’t see any connection pool issues anymore. If you have a way you do it, please tweet @jc00ke.

- Jesse