DDD for beginners

Domain Driven Design focusses on modelling the domain logic. This talk looks at the components of the model layer of your web application and the options you have. How are you supposed to organise your models in an MVC application? What goes where? What is a service class, a mapper or an entity? We'll discuss the terminology and take a look at what Domain Driven Design is and see how you use service layer to provide the business logic for your application and hide your persistence code from your controllers. By the end of this session you will be equipped to create excellent, maintainable models in your projects.

This talk was presented at CodeMash, Ohio on 12 January 2017

Transcript

1. DDD for Beginners Rob Allen @akrabat ~ January 2017 ~ http://akrabat.com (slides at http://akrabat.com/talks)
2. The business logic is the hard part Rob Allen ~ @akrabat
3. MVC Rob Allen ~ @akrabat
4. MVC Rob Allen ~ @akrabat
5. DDD is an approach to help you manage the model Rob Allen ~ @akrabat
6. Domain Driven Design A set of patterns for the model: • Language • Strategy • Tactical Rob Allen ~ @akrabat
7. Domain The domain is the "real-world" subject of the project The model is an abstraction of the domain We communicate the model using: • diagrams • use-cases • specifications • etc. Rob Allen ~ @akrabat
8. Is DDD the right choice? DDD requires time, effort and collaboration with the business experts. Benefits: • Useful model of the problem domain • Collaboration between the business & the software teams • Better user experiences with the software • Better architecture through better understanding Rob Allen ~ @akrabat
9. "We have really everything in common with America nowadays except, of course, language" Oscar Wilde Rob Allen ~ @akrabat
10. Ubiquitous Language • A common language between the developers and the business • Limited to how the domain problem • Describes how business itself thinks and operates • Ensures that the team are all on the same page The UL is the agreed concepts, meanings and terms for the project. Rob Allen ~ @akrabat
11. Creating the Ubiquitous Language Conversations in the team exploring how the business operates. • Identify the business processes • Find the inputs and outputs • Document it all! pictures, use-cases, glossary, workflow diagrams, etc. Rob Allen ~ @akrabat
12. Bounded Contexts & Context Maps The context where our UL is valid is called the Bounded Context Insurance example: • Quotation • Policies • Underwriting • Billing • Customer management Rob Allen ~ @akrabat
13. Putting it into practice Rob Allen ~ @akrabat
14. What's in the model? Rob Allen ~ @akrabat
15. Organising the model Rob Allen ~ @akrabat
16. A problem A customer wants to plan a journey between two stations. Rob Allen ~ @akrabat
17. How do we model this? Rob Allen ~ @akrabat
18. Identify the key objects • Customer • Journey • Station • Line Rob Allen ~ @akrabat
19. Identify the relationships Rob Allen ~ @akrabat
20. Entities represent things in the problem-space Rob Allen ~ @akrabat
21. Entity • Means something to the customer • An object defined primarily by its identity • Mutable • Persisted • Has a life cycle Rob Allen ~ @akrabat
22. Identity • The identity of an object is what it means to the customer • Unique within the scope of the domain For a tube station, this is as much its name, as its database ID. My customer is never going to talk to me about station 43, they are going to say "Euston Square". Rob Allen ~ @akrabat
23. Value objects • Defined primarily by its attributes • Immutable • Simple! • Do not exist without an entity Rob Allen ~ @akrabat
24. Entity vs Value Object “ When people exchange dollar bills, they generally do not distinguish between each unique bill; they only are concerned about the face value of the dollar bill. In this context, dollar bills are value objects. However, the Federal Reserve may be concerned about each unique bill; in this context each bill would be an entity.” Wikipedia Rob Allen ~ @akrabat
25. Aggregates A group of Entities and Value objects that need to be consistent when persisted. Example: • Order • Line item Rob Allen ~ @akrabat
26. A station has a location Rob Allen ~ @akrabat
27. A station has a location Rob Allen ~ @akrabat
28. Domain services If a SERVICE were devised to make appropriate debits and credits for a funds transfer, that capability would belong in the domain layer. Eric Evans Rob Allen ~ @akrabat
29. Domain services • We map the business processes to services • Represents behaviour in the domain • A service does not have internal state • Usually a point of connection for many objects Rob Allen ~ @akrabat
30. Domain services Conditions for using a domain service: • Calculations requiring input from multiple domain objects • Performing a significant business process • Transforming a domain object from one composition to another Rob Allen ~ @akrabat
31. Let's look at some code Rob Allen ~ @akrabat
32. An entity 1 class Journey { 2 function getStart() {} 3 function setStart(Station $start) {} 4 5 function getStop() {} 6 function setStop(Station $stop) {} 7 8 function getRoute() {} 9 function setRoute(Route $route) {} 10 } Rob Allen ~ @akrabat
33. A service 1 class RoutingService { 2 pubic function calculateRoute(Station $from, Station $to) : Route {} 3 } Rob Allen ~ @akrabat
34. Anaemic domain model When you look at the behavior, and you realize that there is hardly any behavior on these objects, making them little more than bags of getters and setters. Instead there are a set of service objects which capture all the domain logic. Martin Fowler Rob Allen ~ @akrabat
35. Entity with behaviour 1 class Journey { 2 public function __construct (Station $from, Station $to) {} 3 4 public function calculateRoute() : Route {} 5 } Rob Allen ~ @akrabat
36. What happens if calculateRoute() is complex? Rob Allen ~ @akrabat
37. Implement as a helper The entity calls the helper, passing a reference to itself. 1 // Helper class 2 class JourneyRouter { 3 public function calculateRoute(Journey $journey) : Route {} 4 } 5 6 // Journey class 7 class Journey { 8 function calculateRoute() : Route { 9 $route = $this->journeyRouter->calculateRoute($this); 10 } 11 } Rob Allen ~ @akrabat
38. Persistence Rob Allen ~ @akrabat
39. Persistence options A simple domain model can use TDG or Data Mapper; a complex one will require Data Mapper or an ORM. Aggregates need an ORM Pick the right one! Rob Allen ~ @akrabat
40. But don't use ActiveRecord pattern! Rob Allen ~ @akrabat
41. But don't use ActiveRecord pattern! It integrates the database code into your domain model Rob Allen ~ @akrabat
42. Table Data Gateway • Operates on a single database table • Contains all the SQL for accessing the table • Doesn't know anything about the entity • Simple to implement Rob Allen ~ @akrabat
43. Table Data Gateway 1 class JourneyGateway { 2 function __construct($dbAdapter) {} 3 4 function find($id) {} 5 function findForStartingStation($stationId) {} 6 7 function insert(array $data) {} 8 function update($id, array $data) {} 9 } Rob Allen ~ @akrabat
44. Data Mapper • Class to transfer data from objects to the database and back • Entity aware • Isolates the domain model from the database • Not limited to a single table Rob Allen ~ @akrabat
45. Data Mapper 1 class JourneyMapper { 2 function __construct($dsn, $username, $password) {} 3 4 function find($id) : Journey {} 5 function findForStartingStation($stationId) : [Journey] {} 6 7 public function save(Journey $journey) {} 8 } Rob Allen ~ @akrabat
46. Increased scope: ORM Data mapping that works with the object graphs is known as an Object Relational Mapping Must-have for aggregates, but use a pre-written ORM library! Rob Allen ~ @akrabat
47. ORM Persistence layer is more complicated: • Storage of entire object graphs to the database • Identity map to hold loaded objects • Unit of Work to track changed objects for saving Rob Allen ~ @akrabat
48. Repositories • 1-1 relationship between repository and aggregate • Manage the loading and storing of aggregates • Often oriented around collections of aggregates Rob Allen ~ @akrabat
49. Web services • The persistence storage could be a web service • Data mappers work really well Rob Allen ~ @akrabat
50. Integrating our model into the application Rob Allen ~ @akrabat
51. The application layer* It does not contain business rules or knowledge, but only coordinates tasks and delegates work to collaborations of domain objects in the next layer down. Eric Evans * (Known as service layer in PoEAA) Rob Allen ~ @akrabat
52. Application layer We can sub-divide: • Application services • Infrastructural services Rob Allen ~ @akrabat
53. Application services If the banking application can convert and export our transactions into a spreadsheet file for us to analyze, this is an application SERVICE. Eric Evans Rob Allen ~ @akrabat
54. Application service 1 class JourneyCreator { 2 public function createJourney(Customer $customer, Station $from, Station $t 3 { 4 $journey = $customer->createJourney($from, $to); 5 $journey->calculateRoute(); 6 7 $this->notifier->send($journey); 8 $this->auditor->log("Journey created", $journey); 9 } 10 } Rob Allen ~ @akrabat
55. Beware the Fat service Decouple using Observer pattern Rob Allen ~ @akrabat
56. Infrastructural services A bank might have an application that sends out an email to a customer when an account balance falls below a specific threshold. The interface that encapsulates the email system, and perhaps alternate means of notification, is a SERVICE in the infrastructure layer. Eric Evans Rob Allen ~ @akrabat
57. Infrastructural service • Standard: there's no business or application logic • Reusable across applications • Do not write your own - this is what the ecosystem is for Rob Allen ~ @akrabat
58. To sum up Rob Allen ~ @akrabat
59. To sum up Ubiquitous Language: Agreed vocabulary with the business Bounded Context: Area of the problem space where the Ubiquitous Language is valid Rob Allen ~ @akrabat
60. To sum up Entity: Object with identity that do stuff Value object: Immutable with no identity Domain service: Behaviours that don't belong to an entity Rob Allen ~ @akrabat
61. To sum up Mappers / Repository: Transfer your model to and from persistence Application services: Isolate your domain model from your controllers Infrastructure services: Support services for your application Rob Allen ~ @akrabat
62. Final point Rob Allen ~ @akrabat
63. The success of a design is not necessarily marked by its longevity. It is the nature of software to change. Eric Evans Rob Allen ~ @akrabat
64. Thank you! Please rate this talk on EventsXD Rob Allen ~ @akrabat ~ akrabat.com