Curiosity

Legacy system

Legacy code: the code most tightly coupled to your revenue stream.

@dobbse

Legacy code is where the lifetime of the code is not coincident with the team lifetime.

@mfeathers at #xprolo

Honor legacy code. It exists because it is bringing value. We work on it because we want it to keep bringing value.

@woodyzuill

Some people call it legacy code. I like to call it revenue code.

@ryanflorence

Unsuccessful code doesn't survive long enough to become legacy! Legacy code is the sign of being successful.

@aklecha

CQRS & ES

Command Query Responsibility Segregation

CQRS is not a top level architecture

just a way to implement part of the system

Event Sourcing

CQRS could be used without ES,
ES without CQRS not

Aggregate

foldl(events)

Execute command

f(aggregate, command) => events

Projection

f(state, event) => state

Process manager

f(state, event) => maybe(command)

First domain event


  RailsEventStore::Client.new.read_all_streams_forward(:head, 1)
  => ProductItemEvents::ProductItemSold
      @event_id="74eb88c0-8b97-4f27-9234-ed390f72287c",
      @metadata={:timestamp=>2014-11-12 22:20:24 UTC},
      @data={:order_id=>23456, :product_item_id=>123456,
        :attributes=>{
          "id"=>123456, "order_id"=>23456, "product_type_id"=>98765,
          "price"=>50, "barcode"=>"1234567890",
          "scanned_at"=>nil, "serialized_type"=>nil,
          "order_line_id"=>3456789, "code_id"=>nil,
          "updated_at"=>2014-11-12 22:20:24 UTC, "created_at"=>2014-11-12 22:20:24 UTC}}

Trials & errors

Anatomy of the Domain Event

Rule #1

the naming is hard, really hard

Rule #2

don’t be CRUDy

Rule #3

your event is not your entity

Rule #4

be explicit

Rule #5

natural is better

Rule #6

time is a modelling factor

Rule #7

when in doubt

First domain event

(done right)


=> TicketSold
     @event_id=“74eb88c0-8b97-4f27-9234-ed390f72287c”,
     @metadata={:timestamp=>2014-11-12 22:20:24 UTC},
     @data={:barcode=>”1234567890”,
            :order=>{SaleIdentification value object},
            :type=>{Ticket type value object},
            :price=>{Price value object}}