ddd & microservices - goto...
TRANSCRIPT
DDD & MicroservicesAt 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
Sm mm m mm
Am
m
m m
m mB
Aa
b
a a
b bB
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.
Aa
b
a a
b bB
partnersA B
Context Map
partnersA B
Context Map
Aa
b
a a
b bB
C
a
a
a
Asymmetrical Relationships
Context Name Context Namerelationship
(point toward power)
Context Map
Aa
b
a a
b bB
C
a
a
a
partnersA B
C
Context Map
Aa
b
a a
b bB
C
a
a
a
partnersA B
C
conforms
Context Map
partnersA B
C
conforms
Aa
b
a a
b bB
C
a
a
a
D
aa
a
Context Map
Aa
b
a a
b bB
C
a
a
a
D
aa
a
partnersA B
C
conforms
D
AC
Context Map
partnersA B
C
conforms
D
AC
Aa
b
a a
b bB
C
a
a
a
D
aa
a
E a a
Context Map
Aa
b
a a
b bB
C
a
a
a
D
aa
a
E a a
partnersA B
C
conforms
D
AC
E conforms
Context Map
partnersA B
C
conforms
D
AC
E conforms
Aa
b
a a
b bB
C
a
a
a
D
aa
a
E a aa
a
a
Context Map
partnersA B
C
conforms
D
AC
E conforms
Aa
b
a a
b bB
C
a
a
a
D
aa
a
E a a
F
bb
a
a
a
f
Context Map
Aa
b
a a
b bB
C
a
a
a
D
aa
a
E a a
F
bb
a
a
a
f
partnersA B
C
conforms
D
AC
E conforms
Fconforms
AC
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.
Context Map
Aa
b
a a
b bB
C
a
a
a
D
aa
a
E a a
F
bb
a
a
a
f
partnersA B
C
conforms
D
AC
E conforms
Fconforms
AC
Context Map
partnersA B
C
conforms
D
AC
E conforms
Fconforms
AC
Aa
b
a a
b bB
C
a
a
a
D
aa
a
E a a
F
bb
a
a
a
f
Context Map
Aa
b
a a
b bB
C
a
a
a
D
aa
a
E a a
F
bb
a
a
a
f
partnersA B
C
conforms
D
AC
E conforms
Fconforms
AC
BBoM
Context Map
partnersA B
C
conforms
D
AC
E conforms
Fconforms
AC
BBoMBBoM
Aa
b
a a
b bB
C
a
a
a
D
aa
a
E a a
F
bb
a
a
a
f
Context Map
partnersA B
C
conforms
D
AC
E conforms
Fconforms
AC
BBoMBBoM
Aa
b
a a
b bB
C
a
a
a
D
aa
a
E ae ae
F
bb
ae
ae
ae
f
Fiction! Map what is.
Not all of a large system will be well designed.
Context Map
Aa
b
a a
b bB
C
a
a
a
D
aa
a
E ae ae
F
bb
ae
ae
ae
f
partnersA B
C
conforms
D
AC
E
Fconforms
AC
BBoMBBoM
BBoM
???
???
Patterns describe good places to be.
Context Map
Aa
b
a a
b bB
C
a
a
a
D
aa
a
E ae ae
F
bb
ae
ae
ae
f
partnersA B
C
conforms
D
AC
E
Fconforms
AC
BBoMBBoM
AC
ACBBoM
Interchange context
Context Map
Aa
b
a a
b bB
C
a
a
a
D
aa
a
E a a
F
bb
a
a
a
f
partnersA B
C
conforms
D
AC
E conforms
Fconforms
AC
Context Map
partnersA B
C
conforms
D
AC
E conforms
Fconforms
AC
Aa
b
a a
b bB
C
i
i
i
D
ii
i
E i i
F
bb
i
i
i
f
Context Map
Aa
b
a a
b bB
C
i
i
i
D
ii
i
E i i
F
bb
i
i
i
f
partnersI
B
C
conforms
D
AC
E conforms
Fconforms
AC
AAC
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 & MicroservicesAt last, some boundaries!
Eric Evans @ericevans0
domainlanguage.com