Over the last couple of years, I have heard of Event sourcing but did't really know where to start until I did a tutorial at DPC '17. After having some basic information it was time to start a Hackathon and after that something production worty . In this talk I will Try to give the best information to get started and to know some of the problems you can face if you begin event-sourcing.

1. My first year with event-
sourcing
2018-05-03 DeventerPHP

2. Tim Huijzers
@Dragem
Developer @
drukwerkdeal.nl
Deventer NL

3. BeerWarehouse

4. Why use Event Sourcing

5. CRUD

6. We will save a new entry in our system because
we just bought it and will store it in the fridge for
later.

7. If we change the location the system only knows
about that location.

8. We drank it so it’s not in the system anymore

9. We want to keep a history of everything we
drank.

10. I want to know when I drank this in my history.

11. But that’s only for new beers.

12. Events

13. Same Information as before + Explicit action
about what happened

14. Make Small Events

15. Removed Location and changed name because
in the real world you might not know this yet.

16. When returning home I put the beer in my fridge

17. I need room in my fridge so I take it out. Using
the same Event

18. And at last a event about when I consumed it.

19. Crud
● I know what beer I have.
● I know when it was consumed.
● I know where it is.
Event-Sourcing
● I know what beer I have.
● I know when it was consumed.
● I know where it is.
● I know where it was before.
● I know when it was moved.
● I know where it was at any point in
time
● I know how many times it was moved.
● I know when it was added to the
system.
● I know what else was moved in that
day.

20. “Every software program relates to
some activity or interest of its user.”
Eric Evans, Domain-Driven Design: Tackling Complexity
in the Heart of Software

21. DDD

22. Some Principles
● Ubiquitous Language
● Entities
● Value Objects
● Aggregates
● Bounded Context
● Domain Events

23. “To communicate effectively, the
code must be based on the same
language used to write the
requirements - the same language
that the developers speak with each
other and with domain experts.”
Eric Evans, Domain-Driven Design: Tackling Complexity
in the Heart of Software

24. When To Use Event Sourcing

25. ● You need an audit log
● You like scalability
● You want to separate the read and write of an application
● You want to replay event on a dev machine to get an accurate
situation of what the state was at a point in time.
● You want reporting but don’t know what yet.
● You are done with mapping objects to tables

26. When NOT To Use Event Sourcing

27. ● You only need a simple CRUD system.
● You are processing a lot of personal data.
● You just want to query a lot of things on the DB
● You are starting on a big project for production

28. CQRS

29. “Every method should either be a
command that performs an action, or
a query that returns data to the caller,
but not both. In other words, Asking a
question should not change the
answer.”
Bertrand Meyer, Object-oriented Software Construction

30. Command

32. Query

34. Event Sourcing in code

35. Prooph

36. http://getprooph.org/

37. Command

42. Command Handler

45. Aggregate

47. Event

54. Back to the Aggregate

57. Think About Side Effects

60. Structuring your application

62. Understanding the DB

64. How many beers do I have?

65. How many different styles do I have?

66. How many beers have I drank last 30
days?

67. Projection

68. A Projection allows you to loop through all event (past and present) and build your
own views.
● Read Model
○ Define the data you would like to use.
● Projection
○ Loops through the events and applies that data to your view
● Finder
○ Helps you find data from that view.

71. Pitfalls

72. Refactoring is harder, think about
your architecture

73. Versioning
● Change an Event
● Make a new Event
● Make the Event right from the start

74. Something wrong with the event

75. Event are immutable, So don’t change them
● Try solving it another way first.
● Correct errors with new events
● Try a upcaster
● Make a new stream and fill it with mutated events (and test)
● Change the events in the database

76. What Now?

77. http://getprooph.org/

79. Snapshots

80. You Do Not Need Snapshots From
The start

81. Trigger on Event Count

82. Pure Event Sourcing Is Not A Holy
Grail

83. Do Not Save Personal Data In Events

84. Make Projections For All You Lists

85. Try It In A Hackathon First

86. Most Of The Time Your DB Is Not
Holy

87. But I want to start yesterday!
https://github.com/prooph/proophessor-do-symfony
http://getprooph.org/

88. Other Tools
● Broadway
○ No Upcaster,
○ No Snapshots,
○ No Replaying
● Axon
○ Upcasting by MessageFactory,
○ Snapshots by Trigger on event count,
○ Replaying by Example code for replay
● Akka
○ Upcasting by Event Adapter,
○ Snapshots decided by actor,
○ Replaying

89. Thanks, Any Questions?
Example code from talk on:
https://github.com/webbaard/BeerWarehouse