DDD & Microservices At last, some boundaries! Eric Evans - PowerPoint PPT Presentation

DDD & Microservices At last, some boundaries! Eric Evans @ericevans0 domainlanguage.com

Why do I like microservices? • Autonomous teams with isolated implementation. • Acknowledge the rough and tumble of enterprises. • Cattle not pets. • A philosophical break from the past — gives us a chance to shake assumptions.

Possibly, some other people mean something else: What I’m saying may not apply to that!

Services and Messages S m m m m m m

m m m A B m m m

a a a A B b b b How do they understand the messages?

Bounded Context • context The setting in which a word or statement appears that determines its meaning • bounded context The conditions under which a particular model is defined and applicable.

a a a A B b b b Context Map partners B A

C a a a a a a A B b b b Context Map partners B A

Asymmetrical Relationships relationship Context Name Context Name (point toward power)

C a a a a a a A B b b b Context Map C partners B A

C a a a a a a A B b b b Context Map C conforms partners B A

D C a a a a a a a a a A B b b b Context Map C conforms partners B A

D C a a a a a a a a a A B b b b Context Map D C AC conforms partners B A

D C a a a a a a a a a E A B a a b b b Context Map D C AC conforms partners B A

D C a a a a a a a a a E A B a a b b b Context Map D C AC conforms partner conforms B E A s

D C a a a a a a a a a a a a E A B a a b b b Context Map D C AC conforms partner conforms B E A s

D C F f a a a a b a a a a b a a a a E A B a a b b b Context Map D C AC conforms partner conforms B E A s

D C F f a a a a b a a a a b a a a a E A B a a b b b Context Map D C F conforms AC AC conforms partners conforms B E A

Models need to be clear, 
 not big. • Useful models need crisp definitions. • Definitions require clear context. • Useful models need simple assertions. • Assertions require boundaries.

There are always multiple models.

D C F f a a a a b a a a a b a a a a E A B a a b b b Context Map D C F conforms AC AC conforms partners conforms B E A

D C F f a a a a b a a a a b a a a a E A B a a b b b Context Map D C F conforms AC AC conforms partners conforms B E A

D C F f a a a a b a a a a b a a a a E A B a a b b b Context Map D C F conforms BBoM AC AC conforms partners conforms B E A

D C F f a a a a b a a a a b a a a a E A B a a b b b Context Map D C F conforms BBoM BBoM AC AC conforms partners conforms B E A

D C F f ae a a ae b a a ae a b a a a a E A B a e a e b b b Context Map D Fiction! C F conforms BBoM BBoM Map what is . AC AC conforms partners conforms B E A

Not all of a large system will be well designed.

D C F f ae a a ae b a a ae a b a a a a E A B a e a e b b b Context Map D Patterns describe C F conforms good places to be. BBoM BBoM AC AC ??? conforms partners ??? B E A BBoM

D C F f ae a a ae b a a ae a b a a a a E A B a e a e b b b Context Map D C F conforms AC BBoM BBoM AC AC conforms partners B E A AC BBoM

Interchange context

D C F f a a a a b a a a a b a a a a E A B a a b b b Context Map D C F conforms AC AC conforms partners conforms B E A

D C F f i i i i b i i i b i i a a a E A B i i b b b Context Map D C F conforms AC AC conforms partners conforms B E A

D C F f i i i i b i i i b i i a a a E A B i i b b b Context Map D C F conforms AC AC conforms conforms E I partners B A AC

Why not logical boundaries? • Smart people point out that most of what I want is the logical partitioning of the system. • We’ve had decades to get that to work. • Some techniques are too subtle to survive the rough and tumble.

Wrap up • Subtle design (such as DDD) requires concrete boundaries. • Lots of services recreate some of the old problems. • Context Maps help visualize and communicate about those problems. • Modest use of interchange contexts is one way of organizing coherent services.

Not all of a large system will be well designed.

DDD & Microservices At last, some boundaries! Eric Evans @ericevans0 domainlanguage.com