Howler

An asynchronous message queue that's always chewing on something.

Advantages

• On-the-fly Configuration (via Howler::Config)
• Simple message queueing interface.
• Powerful and Fine-grained message retry logic.
• Dashboard for managing and tracking message processing.
• No need for an external Exception Notification Service.
• Simple Message passing between Actors

Usage

0. Rails 3 + Redis
1. gem 'howler' in your Gemfile.
2. bundle install.
3. From the root of the Rails project run [bundle exec] howler.

Configuration

Howler listens to configuration and updates accordingly.

  # Scale Workers
  Howler::Config[:concurrency] = 75

Queueing Interface

class User [< ActiveRecord::Base]
  async :fetch_content

  def self.fetch_content(user_id)
    ...
  end
end

User.async_fetch_content(user.id)
#=> true

Message Retry Handling

• Retry a message every minute for up to 10 minutes

  def self.fetch_content(user_id)
    user = User.find(user_id)

    unless user.fetchable?
      raise Howler::Message::Retry(:after => 1.minute, :ttl => 10.minutes)
    end

    ... # fetch content
  end

Exception Notification

• Notify when an external API is down

  def self.fetch_content(user_id)
    ...
    begin
      # Try to fetch /home_timeline for the user
    rescue Twitter::Error::ServiceUnavailable => error
      raise Howler::Message::Notify.new(erorr)
    end

    ... # process the timeline
  end

Message Passing

• Pass messages by setting values into the shared configuration (key, value).

  def fetch_content(user_id)

    ... # done fetching content
    Howler::Config[user_id] = {:fetched_at => Time.now, :status => 'success'}.to_json

    # Then to delete a key simply assign nil
    Howler::Config[user_id] = nil
  end

Dashboard (In Development)

• Global settings management.
• Change the default message retry handling.
• Increase or Decrease the number of workers.
• Explicitly retry, delete, or reschedule messages
• Change the log-level (seeing higher error rates, so switch to the debug level)

Get rid of your Exception Notifier

• Simply raise a Howler::Message::Notify exception
• Raise with custom attributes and Howler will take care of the rest.
• The Notifications tab will give you access to errors in real-time.