CQRS and Event Sourcing

d i g i t a l c r a f t s m a n s h i p

CQRS and

Event sourcing!

!stefan van Raaphorst

Disclaimer - the experts Greg Young - http://goodenoughsoftware.net/

Udi Dahan - http://www.udidahan.com/ (The Software Simplist)

Disclaimer by the experts NO it's not a silver bullet nor a Golden Hammer

Yes a lot of best practices and well known patterns are involved

CQS - Command-query separation

Definition

Separate command methods that change state from query methods that read state

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A principle

Source: http://en.wikipedia.org/wiki/Command-query_separation [Bertrand Meyer]

Command-Query Responsibility Segregation DefinedDefinition

Separate command messages that change state from query messages that read state.

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As Design - Architecture - Pattern from Domain Driven Design

Source: http://codebetter.com/gregyoung/2009/08/13/command-query-separation/

Single model

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!Source: http://martinfowler.com/bliki/CQRS.html

CQRS model

Mindset: Eventual Consistency

Data is out-of-date/stale the moment a user sees it on the screen

in a collaborative environment.

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Write and Read model

Mindset

This brings the notion of write models are different to read models

and querying "search engine style" is a whole separate thing.

Commands and events

Mindset: Intent in action

Are you correcting an address or do you move someone to a different address

Event sourcing

State History

Event sourcingCapture meta-data

Intent - "Update" address was this Correcting a address or did this person move (so we can do some marketing)

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History/behavior - Shopping cart with two items or added 3 items and removed one

Event sourcing

Mindset:

Creating new reports and views for historical data

So what is the use? Capture business changes in a lossless manner

Audit trail

It makes it easy to test, replay (business logic) failures in from the field

What are the benefits? The ability to partition on a domain level and keep the model simple

Optimize for a single model (read/write)

To have choice in technology used (e.g. not one DB to tackle all issues)

d i g i t a l c r a f t s m a n s h i p

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!stefan van Raaphorst

http://www.thedeveloperday.com/tag/cqrs/