Slides from the workshop at O'Reilly Software Architecture NY conference in Feb 2019

1. Splitting the monolith using
Domain Driven Design
Maria Gomez @mariascandella
Tech Principal

2. 3 hours
+
30 minutes break
AGENDA
Intro to Microservices and Domain Driven Design
A couple of group exercises
Coding exercises

3. WHY?

5. Microservices
Architectural style
Collection of loosely coupled services

6. Principles of Microservices
Modeled around business concepts
Culture of automation
Hide internal implementation details
Decentralize all the things
Deploy independently
Isolate Failure
Highly Observable

7. PROS
Technology flexibility
Scalability
Lower the risk of deployment
Small, independent teams
Reusability
Replaceability
CONS
Complex infrastructure
Complex deployment process
Performance
Resilience and monitoring

8. Are Microservices right for me?
Business requirements
Technical maturity
Organizational maturity

9. Continuous Delivery is the ability to get changes
of all types—including new features,
configuration changes, bug fixes and
experiments—into production, or into the
hands of users, safely and quickly in a
sustainable way.
continuousdelivery.com

10. Continuous
Delivery
Technical Architecture Data Management
Continuous Integration
Quality Assurance
Organizational
Alignment
Environments & Deploy
Configuration
Management
Release
Management

11. Testing

12. Evolutionary Architecture
Don’t predict the future, adapt to it!
Build architecture that embrace change

13. ORGANIZATIONAL
ALIGNMENT
“Any organization that designs a system
(defined broadly) will produce a design
whose structure is a copy of the
organization's communication structure.”
CONWAY’S LAW
©ThoughtWorks 2018 Commercial in Confidence
https://martinfowler.com/articles/microservices.html

14. ORGANIZATIONAL
ALIGNMENT
Build teams that look like the architecture
you want (and it will follow)
INVERSE CONWAY MANEUVER
©ThoughtWorks 2018 Commercial in Confidence
https://martinfowler.com/articles/microservices.html

15. HOW?

16. What is Domain Driven Design?
It is an approach to building software that has complex and
ever-changing business requirements

18. Key Concepts
A sphere of knowledge, influence, or activity. What an organisation does, and the world it
does it in.
Domain
A system of abstractions describing selected aspects of a Domain. It is used to solve
problems related to that Domain
Domain Model

19. Key Concepts
A Language structured around the Domain Model and used by all team members.
Ubiquitous Language
An explicit boundary within which a Domain Model exists. Inside, all terms have a specific
meaning and are part of the Ubiquitous Language
Bounded Context

20. https://martinfowler.com/bliki/BoundedContext.html

22. Exercise 1
Teams of 5-6
Duration: 15 minutes
Identify Domain Models & Bounded
Contexts

23. Tales Of A Fourth Grade
An elementary school system comprehensive student management system
Functionality: track absences, tardies and excuses (entered by parents, faculty or staff);
generate reports on student activities; track student grades and assignments (completed
and due); parent-teacher forums
Users: faculty, staff and student parents
(https://archkatas.herokuapp.com/kata.html?kata=TalesOfAFourthGrade.json)

24. As a teacher
I want to take attendance
So that I can identify who is absent
As a teacher
I want to take attendance
So that I can identify who is late
As a teacher
I want to enter the reason for an absence or tardy
So that I can keep a record and observe trends over time
As a teacher
I want to generate reports on student activities
So that I can observe the performance of the students over time
As a teacher
I want to see the list of due assignments
So that I can manage expectations with students and parents
As a teacher
I want to see the list of completed assignments
So that I can observe the performance of the students over time
As a parent
I want to see the grades of my daughter
So that I can see her progress
As a parent
I want to enter the reason for an absence or tardy
So that the school is informed
As a parent
I want to have a conversation on a forum with other parents
So that I can share experiences
As a parent
I want to have a conversation on a forum with the teacher
So that I can ask them questions
As a teacher
I want to have a channel of communication with the parents
So that I can answer their questions

25. Exercise 1
Teams of 5-6
Duration: 15 minutes
Identify Domain Models & Bounded
Contexts

26. What to do when bounded contexts are identified?
1. Isolate them
2. Extract them into services

27. Bounded Context
High Cohesion
Loose Coupling
Represents Business Capability
Individual Tech Stacks & Persistence
Deploy independently
Isolate Failure
Individual Scalability
Microservices+ =

28. Event Storming
EventStorming is a workshop format for
quickly exploring complex business
domains.
http://ziobrando.blogspot.com/2013/11/introducing-event-storming.html
By Henning Schwentner - Own work, CC BY-SA 4.0,
https://commons.wikimedia.org/w/index.php?curid=57766348

29. Key Concepts
Something happened, of interest to a domain expert
Domain Event
An external instruction to do something. It triggers a Domain Event.
Command
The portion of the system that receives commands and decides whether to execute them
or not, thus producing a domain event.
Aggregate

30. Event Storming Legend
A Domain
Event
happened
A Command
or External
Event triggers
the Event
A View or
“read model”
Questions or
puzzles
An Aggregate

31. Ticket
sent by
email
Payment
confirmed
Ticket
bought
Order
created
Ticket
added to
cart
Seat
reserved
Show
selected
Choose
show
Choose
seat
Add
ticket to
cart
Move to
checkout
Insert
checkout
data
Process
paymentBuyer
Accountant
Customer
Service

32. Exercise 2
Teams of 5-6
Duration: 20 minutes
Identify Domain Models & Bounded
Contexts

34. eCommerce Platform
Payment
Gateway
Back office
Stock Service
E
S
B
...
Online
customers
Customer
Service reps

35. Order
created
Order sent
by email
Move to
checkout

36. Exercise 2
Teams of 5-6
Duration: 20 minutes
Identify Domain Models & Bounded
Contexts

37. How to decide where to start?
● Pace of change
● Performance
● Security
● Team structure
● Dependencies (internal or external)
● Technology

38. Instructions for the coding exercises
Git client (for Windows users)
https://git-for-windows.github.io
SDK
http://www.oracle.com/technetwork/java/javase/downloads/jdk8-downloads-2133151.html
Github repo to fork
https://github.com/mariagomez/sacon-microservices

39. SACON NY Library
Browse books
See rating
Borrow books
New initiative: experiment with the
borrow functionality

40. Coding Exercise 1
Duration: 15 minutes
Step by step solution on Exercise1.md
Increase confidence in the code by adding
tests

46. Coding Exercise 2
Duration: 30 minutes
Step by step solution on
Exercise2-part1.md & Exercise2-part2.md
Re-organize monolith and create clear
separation of packages

47. Refactoring Techniques
Composing Methods

48. Refactoring Techniques
Moving features between objects

49. Refactoring Techniques
Organizing data

50. Coding Exercise 2
Duration: 30 minutes
Step by step solution on
Exercise2-part1.md & Exercise2-part2.md
Re-organize monolith and create clear
separation of packages

51. Some useful resources

52. Coding Exercise 3
Duration: 20 minutes
Step by step solution on Exercise3.md
Extract the first microservice

53. Common Antipatterns

54. Chatty Domains
Customer Account
place legal hold (CC)
place legal hold (CL)
Notify customer
Comms
Notify communications

55. Solution: Consolidate
Collections (Bounded Context)
Customer
List<Account>
accounts
Collections
Communication
Preferences
Communications
Notify communications

56. God Domains
Customer Account Comms

57. Anemic Domains
https://github.com/Pragmatists/DDD-ADM/tree/anemic-model/src/main/java/pl/pragmatists/ddd/adm
https://github.com/Pragmatists/DDD-ADM/tree/master/src/main/java/pl/pragmatists/ddd/adm

58. Thanks!
Maria Gomez
@mariascandella