A short talk on the key lessons I've learned on DDD over the last 4 years. Presented at DDDEU 2018.

1. DDD Lessons
Domain modelling lessons that I wish I’d learned earlier

2. Who am I?
Lead Developer and Solutions Architect
Writing PHP web apps professionally for 12 yrs
Recovered Architecture Astronaut
A little DDD and EventSourcing obsessed (I will talk for hours)
Organiser of PHPDublin @barryosull
barry@tercet.io
http://barryosull.com

3. Lesson 1
Recognising Domain Experts

4. Recognising Domain Experts
? ? ?
? ? ? ?
Solution Expert Domain Expert

5. Lesson 2
The UI is not a proxy for a Domain Expert

6. The UI is not a proxy for a domain expert
Developer
How
does “X”
work?

7. Lesson 3
When interviewing Domain Experts,
refine the constraints they define

8. When Interviewing Domain Experts, refine defined constraints
? ? ?
? ? ? ?
Domain ExpertDeveloper
What
happens
if X?
Well,
then it
should . .
...
...

9. Lesson 4
Sub Domains and Bounded Contexts
are VERY different things

10. Domains and SubDomains are composable
Domain: Running a Travel Agency
We want to create and sell trips to customers
SubDomain: TripBuilding
Building trips is expensive
SubDomain: Searching
Searching across all the services
(flight/hotel/car/etc..) is time
consuming and error prone
SubDomain: Booking
Booking trips is time consuming
SubDomain: Finance
We don't know how effective we are
at turning quotes into bookings
SubSubDomain: Tracking
We don't keep track of what we've
quoted
SubSubDomain: Reporting
We have no way of aggregating
this information

11. Bounded Contexts are independent
Trip
Building
IAM
CRM Reporting
Searching

12. Lesson 5
Domains and contexts do not always align

13. Unaligned sub domains and bounded contexts
Payroll Accounting
Payroll
Accounts
Manager
HR Reporting
Accounting
Payroll

14. Other Lessons that I don’t have time
to explore
Ohh lightning talks, there’s never enough time

15. Other lessons
6: Temporal constraints are far more costly to implement than Value constraints
7. Constant refactoring is key to understanding a domain
8: Hold event storming sessions, even if you're not event sourcing
9: Applying DDD works and it makes you a better developer
10: If it's not your core domain, don't build it yourself, use an existing solution

16. Thank you
Any questions?
(Please leave feedback on the meetup page!)