The document discusses CQRS (Command Query Responsibility Segregation) and Event Sourcing application architectures. It begins with an overview of common application architectures like layered architecture and hexagonal architecture. It then explains event-driven architecture and how event sourcing can be used for data storage. CQRS is introduced as separating read and write interfaces. The document contrasts a traditional architecture with one using CQRS and event sourcing together. It provides a code example and concludes with questions and resources for further reading.

1. CQRS and Event Sourcing
Nikolay
Vasilev

2. CQRS and Event Sourcing
• Applica0on
Architecture
• CQRS
and
Event
Sourcing
• Code
/
Example
• Ques0ons
• Resources
Bulgarian Java User Group
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3. Bulgarian Java User Group
Nikolay Vasilev, CC BY-SA 3.0
Application Architecture

4. Source:
[Evans,
DDD]
Layered Architecture
• One
of
the
oldest
paGerns
• Each
layer
could
couple
only
to
itself
or
below
• Lower
levels
could
loosely
couple
to
upper
via
Observer
or
Mediator
[GoF]
• UI
Layer
– Only
renders
informa0on
– Example:
OHS
(Remote
Façade)
• Applica0on
Layer
– Very
lightweight
– Building
Blocks:
Applica0on
Services
– Coordinate
opera0ons
over
Aggregates
(i.e.
express
Use
Cases)
Bulgarian Java User Group
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5. Source:
[Evans,
DDD]
Layered Architecture
• Applica0on
Layer
– Register
Subscribers
for
Dom.
Events
– Transac0ons,
Email
sending
etc.
– Devoid
of
domain
logic
• Domain
Layer
– Expressive
Behavioural-­‐Rich
Domain
Model
• Infrastructure
Layer
– Low
Level
Services
– Focus
on
technology
specific
decisions
– Persistence,
Messaging,
etc.
Bulgarian Java User Group
Nikolay Vasilev, CC BY-SA 3.0
CQRS + ES Page 5

6. Source:
[Vernon,
DDD]
Hexagonal Architecture (HA)
• Also
known
as
“Ports
and
Adapters”
• Ports
– Outer
hexagon
(HTTP,
Messaging
etc.)
– Each
hexagon’s
side
represents
a
port
– Either
for
input
or
output
– Do
not
implement
ports(use
e.g.
Jersey)
• Adapters
– Transform
client’s
input
into
internal
API’s
“terms”
– Transform
internal
API’s
output
to
input
suitable
for
clients
– Each
client
type
has
an
Adapter
Bulgarian Java User Group
Nikolay Vasilev, CC BY-SA 3.0
CQRS + ES Page 6

7. Source:
[Vernon,
DDD]
Hexagonal Architecture (HA)
• Outer
hexagon
reaches
inner
hexagon
via
applica0on’s
API
• Design
applica0on
inside
per
func0onal
requirements(use
cases
API)
• Layered
Architecture
+
DI
encourages
the
use
of
Hexagonal
Arch.
Bulgarian Java User Group
Nikolay Vasilev, CC BY-SA 3.0
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8. Event Driven Architecture (EDA)
• Event
Sourcing
• Promotes,
detect,
consume
and
react
to
events
• Pipes
and
Filters
Bulgarian Java User Group
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CQRS + ES Page 8
EpisodeAdded
b#1,s#2
SeriesRenamed
b#20,s#1
BrandAc0vated
b#3

9. Event Driven Architecture (EDA)
• EDA
could
be
combined
with
Hexagonal
Architecture
(HA)
– EDA
used
for
integra0on
– HA
used
for
Bounded
Contexts
– LA
could
replace
HA
as
well
Bulgarian Java User Group
Nikolay Vasilev, CC BY-SA 3.0
CQRS + ES Page 9
Source:
[Vernon,
DDD]

10. Event Driven Architecture (EDA)
• Events
as
mechanism
for
Storage
Bulgarian Java User Group
Nikolay Vasilev, CC BY-SA 3.0
CQRS + ES Page 10
Source:
[Vernon,
DDD]

11. Event Driven Architecture (EDA)
• Append-­‐only
architecture
– No
“Delete”
ac0on
• Distribute
easier
than
RDBMS
– Due
to
no
locks
– Sharding
(uses
only
Aggregate
keys)
• Loading
Objects
– Loads
accumulated
events
for
an
Aggregate
– Performance
issue
for
over
than
100
events
– Rolling
Snapshots
Bulgarian Java User Group
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EpisodeAdded
b#1,s#2
SeriesRenamed
b#20,s#1
SeriesRemoved
b#10,s#2
BrandAc0vated
b#3
4
3
snap
2
1

12. CQRS
• Command
and
Query
Responsibility
Segrega0on
(CQRS)
• Bertrand
Mayer’s
CQS
Principle
– Command:
If
method
alters
the
state
should
not
return
value
(i.e.
returns
void)
– Query:
If
you
return
value
you
cannot
mutate
state
• What
if
we
apply
it
at
architectural
level?
;)
Bulgarian Java User Group
Nikolay Vasilev, CC BY-SA 3.0
CQRS + ES Page 12
Read Service
Write Service

13. CQRS
• Stereotypical
vs.
CQRS
Architecture
Bulgarian Java User Group
Nikolay Vasilev, CC BY-SA 3.0
CQRS + ES Page 13
Source:
[Young,
CQRS]
Stereotypical Architecture

14. CQRS
• Stereotypical
vs.
CQRS
Architecture
Bulgarian Java User Group
Nikolay Vasilev, CC BY-SA 3.0
CQRS + ES Page 14
Source:
[Young,
CQRS]
CQRS

15. CQRS and Event Sourcing
Bulgarian Java User Group
Nikolay Vasilev, CC BY-SA 3.0

16. Source:
[Young,
CQRS]
CQRS + ES
• CQRS
+
EDA
with
Domain
Events
Bulgarian Java User Group
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17. CQRS + ES
Bulgarian Java User Group
Nikolay Vasilev, CC BY-SA 3.0
CQRS + ES Page 17
Source:
[Young,
CQRS]

18. Bulgarian Java User Group
Nikolay Vasilev, CC BY-SA 3.0
Code / Example

19. Code / Example
• Example
project
– DDD-­‐CQRS
Sample,
(v.02)
– HLA:
h"p://prezi.com/akrfq7jyau8w/ddd-­‐cqrs-­‐leaven-­‐v20/
– Code
base:
h"ps://github.com/Bo"egaIT/ddd-­‐leaven-­‐v2
– User
Group:
h"ps://groups.google.com/forum/#!forum/ddd-­‐cqrs-­‐sample
• More
projects
on
official
CQRS
site
– hGp://cqrs.wordpress.com/examples/
Bulgarian Java User Group
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CQRS + ES Page 19

20. Bulgarian Java User Group
Nikolay Vasilev, CC BY-SA 3.0
Questions?

21. Bulgarian Java User Group
Nikolay Vasilev, CC BY-SA 3.0
CQRS + ES Page 21
Thank
you
for
your
aGen0on!

22. Resources

23. Resources
• [Evans,
DDD]
"Domain-­‐Driven
Design:
Tackling
Complexity
in
the
Heart
of
Sonware",
Eric
Evans,
Addison
Wesley,
20/08/2003,
ISBN:
0-­‐321-­‐12521-­‐5
• [Vernon,
DDD],
"Implemen0ng
Domain-­‐Driven
Design",
Vaughn
Vernon,
2013,
ISBN-­‐10:
0-­‐321-­‐83457-­‐7,
ISBN-­‐13:
978-­‐0-­‐321-­‐83457-­‐7
• [Young,
CQRS],
"CQRS
Documents
by
Greg
Young",
Greg
Young,
11/2010.
hGp://
cqrs.files.wordpress.com/2010/11/cqrs_documents.pdf
• [Cockburn,
Hex
Arch]
"Hexagonal
Architecture",
h"p://alistair.cockburn.us/Hexagonal
+architecture
• [Fowler,
NoSQL
talk]
"NoSQL
maGers
Cologne
2013
-­‐
Key
Note:
NoSQL
Dis0lled
to
an
hour
-­‐
Mar0n
Fowler",
Mar0n
Fowler,
2013,
h"p://vimeo.com/66052102
• Domain
Driven
Design
Website,
h"p://domaindrivendesign.org/
• DDD
User
Group,
h"p://tech.groups.yahoo.com/group/domaindrivendesign/
• DDD
Community,
h"p://dddcommunity.org/
• CQRS
Official
Website,
h"p://cqrs.wordpress.com/
Bulgarian Java User Group
Nikolay Vasilev, CC BY-SA 3.0
CQRS + ES Page 23

24. Bulgarian Java User Group
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CQRS + ES Page 24