CQRS & Event Sourcing are patterns gaining traction and popularity. In this presentation given at Øredev 2013 it talks about real-world experiences using these patterns, the good, the bad and the ugly.

1. Aesthetics & the Beauty of
an Architecture
Adventures in CQRS & Event Sourcing
Tom Scott
@tomwscott

2. What’s in it for you?
• Understanding of CQRS & Event Sourcing in
the wild
• The mistakes we made…
• … the lessons we learnt …
• … and why we’re still happy!
rob.knight

3. 1
art |ɑːt|
noun
1 [ mass noun ] the expression or application of human creative
skill and imagination, typically in a visual form such as painting
or sculpture, producing works to be appreciated primarily for their
beauty or emotional power

4. 1
art |ɑːt|
noun
1 [ mass noun ] the expression or application of human creative
skill and imagination, typically in a visual form such as painting
or sculpture, producing works to be appreciated primarily for their
beauty or emotional power

5. Aesthetics

6. Aesthetics
Scott’s Purely Arbitrary Criteria, Etceteras
For Evaluating Beautiful Architectures
#SPACE_FEBA

7. Fully Operational

8. Simplicity
echerries

9. Commitment
justageek

10. Deferring Commitment
justageek

11. Metaphor
Petar Pavlov - http://bit.ly/9ySEUt

12. Discipline & Consistency

13. Serendipity
kathryn_rotondo

14. #SPACE_FEBA
• Fully Operational
• Simplicity
• Deferred
Commitment
• Metaphor
• Discipline
• Serendipity

15. Price Comparison
££

16. The evolution of a system
Presentation
Application
Database


|

17. The Solution
archer10

18. Architectural Goals
• Structured
• Horizontal
Scalability
• Availability &
Reliability
• Visibility /
Monitorability
• Flexibility &
Replaceability
archer10

19. The Mandate!
• Service Oriented
Architecture
• Domain Driven
Design
• Micro-Services
• Continuous
Delivery
• Oh and one more
thing!
archer10

20. Domain Driven Design
• Entity
• Value Object
• Aggregate
• Service
• Repository
• Factory
• Bounded Context
archer10

21. Domain Driven Design
• Entity
• Value Object
• Aggregate
• Service
• Repository
• Factory
• Bounded Context
archer10

22. Service Oriented Architecture




23. Understand your Domain



Home
Journey
Panel
Quote
Engine
Risk
Enquiry
Provider
Quote

24. T
d?
En
he

25. Just one more thing…
http://m.cdn.blog.hu/ke/kedvessigmund/image/columbo.jpg

26. CQRS & Event Sourcing
• CQRS or “Why do we use the same schema
for reads and writes?”
• Event Sourcing or “Why do we allow ORMs to
dictate our object model?”
archer10

27. CQRS

28. Command Query Responsibility Segregation


}
}
write
read

29. Use SQL?


{
{
“_id”
:
“590b9902”,
UPDATE
Address
“_id”
:
“590b9902”,
“event”
:
“AddressModified”,
SET
umber
=
10,
n
“event”
:
“PolicySpecified”,
“data”
:
{
street
=
‘Downing
Street’,
“data”
:
{
“number”
:
10,
c
“type”
:
“ContentsInsurance”,
ity
=
‘London’,
“street”
:
“Downing
Street”,
p
ostcode
=
‘SW1A
2AA’
“excess”
:
750
“city”
:
“London”,
WHERE
_id
=
‘590b9902’
}
“postcode”
:
“SW1A
2AA”
}
}
}

30. Event Sourcing (C#)
public
class
Customer
:
AggregateRoot
{
private
readonly
Guid
id;
!
private
Address
currentAddress;
//
Business
Logic
public
void
ChangeAddress(Address
newAddress)
{
if
(newAddress.IsValid())
{
Raise(new
AddressModified(id,
newAddress));
}
}
!
//
State
Transition
public
void
Apply(AddressModified
@event)
{
currentAddress
=
@event.Address;
}
!
!
!
!
......
//
Business
Logic
public
void
SpecifyCommunicationPreferences(CommunicationPrefs
preferences)
{
Raise(new
CommunicationPreferencesSpecified(id,
preferences));

31. Eventual Consistency

POST /risk/new HTTP/1.1 302 !
GET /risk/590b9902:v1
Location: /risk/590b9902:v1

32. What we ended up with:








33. The Conclusion
tim_norris

34. ?
Simple
Metaphor
?
Discipline
Commitment
+
Fully Operational
Serendipity

35. The Power!
public
class
Customer
:
AggregateRoot
{
private
readonly
Guid
id;
!
private
Address
currentAddress;
!
public
void
ChangeAddress(Address
newAddress)
{
Raise(new
AddressModified(id,
newAddress));
}
!
public
void
Apply(AddressModified
@event)
{
currentAddress
=
@event.Address;
}
}

36. The Power!
public
class
Customer
:
AggregateRoot
{
private
readonly
Guid
id;
!
private
Address
currentAddress;
!
public
void
ChangeAddress(Address
newAddress)
{
if
(currentAddress.IsSignificantlyDifferentFrom(newAddress))
{
Raise(new
CustomerMoved(id,
newAddress));
}
else
{
Raise(new
AddressModified(id,
newAddress));
}
}
!
public
void
Apply(CustomerMoved
@event)
{
currentAddress
=
@event.Address;
}
!
public
void
Apply(AddressModified
@event)
{
currentAddress
=
@event.Address;
}
}

37. The Flexibility!
var
connection
=
rabbit.createConnection({
url:
‘amqp://localhost:5672'
})
!
connection.on('ready',
function
()
{
console.info('Connected');
exchange
=
connection.exchange('SOURCE:Exchange',
{
'type':
'topic',
durable:
true
},
function
()
{
var
queue
=
connection.queue('PROJECTION:Map',
{
durable:
false,
exclusive:
true
},
function
()
{
console.info("Joined
Queue");
queue.subscribe(function
(message,
headers,
deliveryInfo)
{
var
policyDetails
=
JSON.parse(message.data.toString());
io.sockets.emit('postcode',
policyDetails.Address.Postcode);
});
queue.bind(exchange.name,
'AddressDetailsSpecified');
});
queue.on('queueBindOk',
function
()
{
console.info('Bound
queue
to
exchange');
});
});
!
});

38. Is it a silver bullet?

39. Lessons Learnt
• Prefer simpler communication protocols
• Messaging == Push == Transient,
• HTTP == Pull == Permanent
• Difficult to evolve Domain Events
• Events as the system contract
rob.knight

40. ha
T
u!
Yo
nk
Tom Scott
@tomwscott