The document discusses storing a sequence of events instead of state in a system. It proposes an "event store" approach where commands are logged as events and the current state can be reconstructed by replaying the sequence of events from the beginning. This allows an audit log to be maintained and for multiple "read models" to be generated from the event stream to serve different queries or views of the data.

1. WHAT IF WE STORED EVENTS
INSTEAD OF STATE?

2. @JefClaes - jefclaes.be

5. SCENARIO’S
• The user can put up a card.
• The user can move the card to the doing section.
• …

6. COMMANDS
• Put up a card.
• Move card to doing section.
• …

11. State + Behavior

15. Traditional systems store current state.

16. We have no idea what has happened in our system.

17. Audit logs anyone?

18. State Audit log

20. Deposit 500 € 500 €
Deposit 200 € 700 €
Withdraw 300 € 400 €
Deposit 1000 € 1400 €
Withdraw 400 € 1000 €
Withdraw 200 € 800 €
… …

21. Deposit 500 € Deposited 500 € 500 €
Deposit 200 € Deposited 200 € 700 €
Withdraw 300 € Withdrawn 300 € 400 €
Deposit 300 € Deposited 1000 € 1400 €
Withdraw 400 € Withdrawn 400 € 1000 €
Withdraw 200 € Withdrawn 200 € 800 €
… …

22. Command - Event - State

23. Something that happened - past tense.

24. Store the sequence of events that led up to the
current state, instead of the current state.

25. Command State
Put up a new card
“Reset Password”
Card
“Reset Password”
“Todo”
Move card to doing
Card
“Reset Password”
“Doing”
… …

26. Command Event State
Put up a new card
“Reset Password”
Card put up
“Reset Password”
“Todo”
Card
“Reset Password”
“Todo”
Move card to doing
Moved card to section
“Doing”
Card
“Reset Password”
“Doing”
… … …

42. thinkbeforecoding.com

46. An event store

48. EventId StreamId Type Payload MetaData
1 card/1
CardPutUp { … } { … }
2 card/1
MovedCard
ToSection
{ … } { … }
3 card/1
MovedCard
ToSection
{ … }
{ … }
… … … …

50. You can’t undo the past.

51. Append-only.

52. Forever cacheable.

53. What about queries?

54. Model
Writes Reads

55. Write model
Writes

56. Write model
Writes
Read model
Reads

57. Write model
Writes
Read model
Reads
Events

59. We can build a read model
from events committed to the write model.

60. Events Dispatcher Projection Read model

61. Events Dispatcher Projection Read model
Projection Read model

62. Events Dispatcher Projection Read model
Projection Read model
Projection Read model

63. We can build read models
from events committed to the write model.

64. More than one way to look at your data.

65. Behavior
Search
Reporting
Views
…

66. Complex
Slow
Fragile

79. What if I mess up my read models?
What if I need to update my read models?
What if I need a new read model?

80. Temporal queries.

81. Tell me which day most work gets done.

82. Monday Tuesday Wednesday …
CardMoved
CardMoved
CardMoved
CardMoved
CardMoved
CardMoved
CardMoved
CardMoved
CardMoved
CardMoved

84. Not just for read models and queries.

86. Events Dispatcher Projection Read model
Projection Read model
Projection Read model
Integration

87. Pub-Sub

88. Events Dispatcher
Projection
Projection
Projection
Transaction

89. Events Dispatcher
Projection
Projection
Projection

90. Events
Projection
Projection
Projection

91. Events Dispatcher (Queue)
Projection (Queue)
Projection (Queue)
Projection (Queue)

92. When?

93. When you can’t afford to lose data.

94. Business Intelligence
Auditing
Reproducing issues
Challenging/changing business models

95. Decouple reads from writes

96. Conceptual purity
Simplicity (no monolith)
Polyglot persistance
Performance
High availability

97. Integration with other systems
Testing
…

98. No silver bullet

99. In conclusion

100. Event sourcing, for when the
what and the when are important.

101. … but really, it’s a lot more than that.

102. Experiment!

103. http://neventstore.org/
http://geteventstore.com/

104. https://groups.google.com/forum/#!forum/dddcqrs

105. @gregyoung
@abdullin
@randompunter
@jonathan_oliver
@jen20
@yreynhout
@mathiasverraes
@thinkb4coding
@eulerfx
@ziobrando
@tojans

106. Thank you!
@JefClaes
http://jefclaes.be