Message bus via the Resque workers you're already using.
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resque-bus.gemspec

README.mdown

Resque Bus

This gem provides an adapter for Resque for use in the queue-bus system. It uses Redis and the Resque that you are already using to allow simple asynchronous communication between apps.

Install

To install, include the 'resque-bus' gem and add the following to your Rakefile:

require "resque_bus/tasks"

Example

Application A can publish an event

# pick an adapter
require 'resque-bus' # (or other adapter)

# business logic
QueueBus.publish("user_created", "id" => 42, "first_name" => "John", "last_name" => "Smith")

# or do it later
QueueBus.publish_at(1.hour.from_now, "user_created", "id" => 42, "first_name" => "John", "last_name" => "Smith")

Application B is subscribed to events

# pick an adapter
require 'resque-bus' # (or other adapter)

# initializer
QueueBus.dispatch("app_b") do
  # processes event on app_b_default queue
  # subscribe is short-hand to subscribe to your 'default' queue and this block with process events with the name "user_created"
  subscribe "user_created" do |attributes|
    NameCount.find_or_create_by_name(attributes["last_name"]).increment!
  end

  # processes event on app_b_critical queue
  # critical is short-hand to subscribe to your 'critical' queue and this block with process events with the name "user_paid"
  critical "user_paid" do |attributes|
    CreditCard.charge!(attributes)
  end

  # you can pass any queue name you would like to process from as well IE: `banana "peeled" do |attributes|`

  # and regexes work as well. note that with the above configuration along with this regex,
  # the following as well as the corresponding block above would both be executed
  subscribe /^user_/ do |attributes|
    Metrics.record_user_action(attributes["bus_event_type"], attributes["id"])
  end

  # the above all filter on just the event_type, but you can filter on anything
  # this would be _any_ event that has a user_id and the page value of homepage regardless of bus_event_type
  subscribe "my_key", { "user_id" => :present, "page" => "homepage"} do
    Mixpanel.homepage_action!(attributes["action"])
  end
end

Applications can also subscribe within classes using the provided Subscriber module.

class SimpleSubscriber
  include QueueBus::Subscriber
  subscribe :my_method

  def my_method(attributes)
    # heavy lifting
  end
end

The following is equivalent to the original initializer and shows more options:

class OtherSubscriber
  include QueueBus::Subscriber
  application :app_b

  subscribe :user_created
  subscribe_queue :app_b_critical, :user_paid
  subscribe_queue :app_b_default, :user_action, :bus_event_type => /^user_/
  subscribe :homepage_method, :user_id => :present, :page => "homepage"

  def user_created(attributes)
    NameCount.find_or_create_by_name(attributes["last_name"]).increment!
  end

  def user_paid(attributes)
    CreditCard.charge!(attributes)
  end

  def user_action(attributes)
    Metrics.record_user_action(attributes["bus_event_type"], attributes["id"])
  end

  def homepage_method
    Mixpanel.homepage_action!(attributes["action"])
  end
end

Note: This subscribes when this class is loaded, so it needs to be in your load or otherwise referenced/required during app initialization to work properly.

Commands

Each app needs to tell Redis about its subscriptions:

$ rake queuebus:subscribe

The subscription block is run inside a Resque worker which needs to be started for each app.

$ rake queuebus:setup resque:work

The incoming queue also needs to be processed on a dedicated or all the app servers.

$ rake queuebus:driver resque:work

If you want retry to work for subscribing apps, you should run resque-scheduler

$ rake resque:scheduler

Heartbeat

We've found it useful to have the bus act like cron, triggering timed jobs throughout the system. Resque Bus calls this a heartbeat. It uses resque-scheduler to trigger the events. You can enable it in your Rakefile.

# resque.rake
namespace :resque do
  task :setup => [:environment] do
    QueueBus.heartbeat!
  end
end

Or add it to your schedule.yml directly

resquebus_heartbeat:
  cron: "* * * * *"
  class: "::QueueBus::Heartbeat"
  queue: bus_incoming
  description: "I publish a heartbeat_minutes event every minute"

It is the equivalent of doing this every minute

seconds = minutes * (60)
hours   = minutes / (60)
days    = minutes / (60*24)

now     = Time.at(seconds)

attributes = {}

now = Time.now
seconds = now.to_i
QueueBus.publish("hearbeat_minutes", {
  "epoch_seconds" => seconds,
  "epoch_minutes" => seconds / 1.minute,
  "epoch_hours"   => seconds / 1.hour,
  "epoch_days"    => seconds / 1.day,
  "minute"        => now.min
  "hour"          => now.hour
  "day"           => now.day
  "month"         => now.month
  "year"          => now.year
  "yday"          => now.yday
  "wday"          => now.wday
})

This allows you do something like this:

QueueBus.dispatch("app_c") do
  # runs at 10:20, 11:20, etc
  subscribe "once_an_hour", 'bus_event_type' => 'heartbeat_minutes', 'minute' => 20 do |attributes|
    Sitemap.generate!
  end

  # runs every five minutes
  subscribe "every_five_minutes", 'bus_event_type' => 'heartbeat_minutes' do |attributes|
    next unless attributes["epoch_minutes"] % 5 == 0
    HealthCheck.run!
  end

  # runs at 8am on the first of every month
  subscribe "new_month_morning", 'bus_event_type' => 'heartbeat_minutes', 'day' => 1, hour' => 8, 'minute' => 0,  do |attributes|
    next unless attributes["epoch_minutes"] % 5 == 0
    Token.old.expire!
  end
end

Local Mode

For development, a local mode is provided and is specified in the configuration.

# config
QueueBus.local_mode = :standalone
or
QueueBus.local_mode = :inline

Standalone mode does not require a separate queuebus:driver task to be running to process the incoming queue. Simply publishing to the bus will distribute the incoming events to the appropriate application specific queue. A separate queuebus:work task does still need to be run to process these events

Inline mode skips queue processing entirely and directly dispatches the event to the appropriate code block.

You can also say QueueBus.local_mode = :suppress to turn off publishing altogether. This can be helpful inside some sort of migration, for example.

TODO

  • Replace local modes with adapters
  • Make this not freak out in development without Redis or when Redis is down
  • We might not actually need to publish in tests
  • Add some rspec helpers for the apps to use: should_ post an event_publish or something along those lines
  • Allow calling queuebus:setup and queuebus:driver together (append to ENV['QUEUES'], don't replace it)