CQRS and Event Sourcing

CQRS and Event Sourcing

Nikolay Vasilev

CQRS and Event Sourcing •  Applica0on Architecture •  CQRS and Event Sourcing •  Code / Example •  Ques0ons •  Resources

Bulgarian Java User Group Nikolay Vasilev, CC BY-SA 3.0 CQRS + ES Page 2

Bulgarian Java User Group Nikolay Vasilev, CC BY-SA 3.0

Application Architecture

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 Nikolay Vasilev, CC BY-SA 3.0 CQRS + ES Page 4

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.


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



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.


Event Driven Architecture (EDA) •  Event Sourcing

•  Promotes, detect, consume and react to events

•  Pipes and Filters


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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



Event Driven Architecture (EDA) •  Events as mechanism for Storage



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


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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? ;)


Read Service

Write Service

CQRS •  Stereotypical vs. CQRS Architecture


Source: [Young, CQRS]

Stereotypical Architecture

CQRS •  Stereotypical vs. CQRS Architecture



CQRS

CQRS and Event Sourcing



CQRS + ES •  CQRS + EDA with

Domain Events


CQRS + ES




Code / Example

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/



Questions?


Thank you for your aGen0on!

Resources

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


CQRS and Event Sourcing

Technology