Time Keeps On Slipping

a brief introduction into Event Sourcing

Jeroen v.d. Gulik

• CTO / Co-Founder Isset (http://isset.nl)

• Señor Developer

• Builder of Artificial Stupidity

• @n0x13

“Those who cannot remember the past are condemned to repeat it”

- George Santayana

DroomPress CMS

As a Content owner I want to know which Content has been

edited more than X times

As a Content owner I want to know which Content has been edited more than X times and by whom

As an Editor I want to see all the changes that have

been made

RDBMS: Store data as the current representation

What if …

• Multiple authors

• Retract all changes from 1 editor

• Move content to different author

• Move content to different creator / updater

History is lost

Data evolves

Store all the Changes (Δ delta’s),that have

happened

Event Sourcing!

Don't save the current state of objects

Save the changes that lead to the current State

Domain Events

Something that has happened in the past

Example

• ContentWasCreated

• ContentWasEdited

• ContentChangedAuthor

Advantages

• Complete log of every state change ever

• Complete trace/audit trail

• Debugging heaven

• Very good performance (append only)

• No more mapping objects to tables

Event Stream

Replay all the Events to get the Current State

What about performance?

Computers are fastMany thousands of events replayed per second

SnapshotStore your state at a certain point in time

CQRSCommand Query Responsibility Segregation

Disclaimer: One doesn’t imply the other

Best of both worlds

Why use Event Sourcing?

• Easy to go from Event Sourcing to RDBMS

• From RDBMS to Event Sourcing, you start from scratch

Questions?

Learn more!• Implementing Domain-Driven Design by Vaughn

Vernon

• http://docs.geteventstore.com/introduction/event-sourcing-basics/

• http://www.martinfowler.com/eaaDev/EventSourcing.html

• http://verraes.net/2014/03/practical-event-sourcing/

• https://www.youtube.com/watch?v=JHGkaShoyNs