CQRS innovations (English version)

CQRS - Innovative solution for problems
of modern enterprise architectures.

Andrey Lomakin (lomakin.andrey@gmail.com)
Artem Orobets (enisher@gmail.com)
January 1, 2013 www.ExigenServices.com

Table of content

1. Why CQRS ?
a) Milestones of enterprise architecture history
b) Pitfalls of CRUD
c) CQRS as solution
2. CQRS internals
3. Axon framework as CQRS implementation

2 www.ExigenServices.com

Why CQRS ?

I. Why CQRS ?


Back to the beginning

Lets talk about evolution of enterprise
architecture
?


Evolution of presentation of documents

Epoch of paper documents

Evolution of presentation of documents

Epoch of electronic storages

Evolution of data processing

Epoch of business processes


However…

CRUD approach is oriented on data
manipulation, but not on business
logic modeling .


Pitfalls of CRUD

II. Pitfalls of CRUD


Problem #1. Usage of JavaBeans ….

JavaBean – “Reusable software components
that can be manipulated visually in a builder
tool”.


… results in …

1. Broken encapsulation of business
objects.
2. Worse code readability.
3. Harder maintainability.
4. All business logic
is placed in service
methods.


Problem #2. Performance
optimization and consequences.

Usage of ORM tools with
denormalization dilutes


Problem #3. Scalability

During business data processing we always chose
C in CAP theorem


Problem #4:In real life concurrent data
modification conflicts do not exist.

 Real life business processes do not have
concurrent data modification conflicts.
 Detection of such conflict is sign of flaw in
implementation of business logic.
 CRUD does not take this into account.


CQRS as solution

III. CQRS as solution

CQRS

CQRS - Command Query
Responsibility Segregation


Problem # 1: Usage of JavaBeans

JavaBeans are still used on query side only
but JavaBean != Domain Entity.


Problem #2. Performance optimization and
consequences.

Data denormailzation is used
on query side only


Data on query side

Each table – reflection of presentation of data on user
screen.

Database are not restricted to be SQL only.

Possible alternatives - Apache Cassandra, HBase,
OrientDB ….


Problem #3. Scalability

 Data consistency is needed on business
logic side only.
 On query side eventual consistency is
totally acceptable.


Problem #4:In real life concurrent data
modification conflicts do not exist.

Two approaches to present object state

1. In the form of values of variables inside object



2. In form of event queue

+11.2 M $ + 5.3 M $ - 8.2 M $

Sum: 8.3 M $



 Each aggregate (DDD) – event container.
 There is no need to use relational
databases.
 It is preferable to have DB with ACID
properties.



Advantages:
 System monitoring support.
 Ability rollback system state till any moment in
the past.
 Framework for data replication and conflict
resolution.


Conflict resolution

Correct
customer
address

Conflict Customer
resolver reallocated

Customer
reallocated


Architecture of CQRS applications

IV. Architecture of CQRS applications


CQRS – is approach only

CQRS is approach only,
how it will be implemented, depends
on you.


Queries

Multilayer architecture CQRS

DB query DB query

Conversion to domain model Conversion to domain model

Conversion to DTO Conversion to DTO

Transmutation of data Transmutation of data
to client to client


Commands

Command – presentation of business action, action
which is meaningful for end user.

Advantages of usage of commands:
1. Orientation on business problems.
2. Convenient facilities for monitoring and scalability.


….advantages…..

1. Good preconditions for DDD usage.
2. Usage of complex event processing
possibilities.
3. Simplicity of distribution of responsibilities
between highly specialized commands.


CQRS frameworks

V. CQRS frameworks


Axon framework

Axon framework - most popular and
feature rich.


Example of CQRS application

Address Book – address
list management


Command creation and submission


Command processing


Business action


Event processing on query side


Query side


Simplicity of configuration


Axon – repository и event store.

Repository

JPA

Event Store

File System JPA Mongo DB

Event Stores – pros and cons

JPA Event Store

Mongo DB, File system

Transactional but slow

There is no ACID support
better performance


Questions


Authors

 Artem Orobets. twitter: @Dr_EniSh ,
enisher@gmail.com, skype: dr_enish
 Andrey Lomakin. twitter: @Andrey_Lomakin ,
lomakin.andrey@gmail.com , skype: lomakin_andrey


Sources

1. First CQRS introduction http://www.infoq.com/presentations/greg-
young-unshackle-qcon08
2. CQRS architecture overview -
http://elegantcode.com/2009/11/11/cqrs-la-greg-young/
3. Greg Young blog - http://codebetter.com/gregyoung/
4. Race conditions does not exist
http://www.udidahan.com/2010/08/31/race-conditions-dont-exist/
5. Domain Driven Design Aggregator -
http://domaindrivendesign.org/
6. Axon framework home page -
http://code.google.com/p/axonframework/
7. Mark Nijhof blog http://cre8ivethought.com/blog


CQRS innovations (English version)

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