Command Bus

Command Pattern - decoupling what is done from who does it.

Usage

Registering commands and their handlers

require 'arkency/command_bus'

command_bus = Arkency::CommandBus.new
register    = command_bus.method(:register)

{ FooCommand => FooService.new(event_store: event_store).method(:foo),
  BarCommand => BarService.new,
}.map(&register)

Invoking command bus with a command

command_bus.(FooCommand.new)

Will call FooService#foo method with the command you just passed.

New instance of a service for every invoked command

If you need a new instance of a service, every time it is called with a command, or you want to lazily load the responsible services, use Proc when registering commands.

command_bus = Arkency::CommandBus.new
command_bus.register(FooCommand, -> (foo_cmd) { FooService.new(dependency: dep).foo(foo_cmd) })
command_bus.register(BarCommand, -> (bar_cmd) { BarService.new.call(bar_cmd) })

Working with Rails development mode

In Rails development mode when you change a registered class, it is reloaded, and a new class with same name is constructed.

a = User
a.object_id
# => 40737760

reload!
# Reloading...

b = User
b.object_id
# => 48425300

h = {a => 1, b => 2}
h[User]
# => 2

a == b
# => false

so your Hash with mapping from command class to service may not find the new version of reloaded class.

To workaround this problem you can use to_prepare callback which is executed before every code reload in development, and once in production.

config.to_prepare do
  config.command_bus = Arkency::CommandBus.new
  register = command_bus.method(:register)

  { FooCommand => FooService.new(event_store: event_store).method(:foo),
    BarCommand => BarService.new,
  }.map(&register)
end

and call it with

Rails.configuration.command_bus.call(FooCommand.new)

Convenience alias

require 'arkency/command_bus/alias'

From now on you can use top-level CommandBus instead of Arkency::CommandBus.