You Attended Talk: an introduction to event sourcing

Imagine for a moment you work for a large online retailer specialising in household goods. One morning, the head of marketing comes to you and says "I've had this great idea we're going to send discount vouchers to anyone who's changed their address in the past 3 months; people who've recently moved are more likely to be buying new furniture. Could you retrieve a list of all these customers?" You explain to him your systems only store a customers current address and doesn't record when it was last changed, a new feature is added to the backlog and the head of marketing leaves a little disappointed.

What if you could build a system which was able to answer this kind of question without knowing it up front? A possible solution is to use event sourcing, being able to go back to any previous state of your data is just one advantage of using event sourcing in your application. Event sourcing is a total paradigm shift from the more traditional model of storing an application's current state and can appear to be a very unnatural way to think about and build a system. In this talk I'm going to show you examples of why event sourcing can be a superior model and how to go about building an event sourced system in PHP.

Example code: https://github.com/carnage/broadway-bowling

Transcript

1. You Attended Talk: An Introduction To Event Sourcing Christopher Riley PHP North West, 2015 1
2. Introduction
3. A familiar model 3
4. Deleting data LineItem table id order id product code quantity 1 1 PK00001 3 2 1 PK00002 1 3 2 PK00002 1 4 3 PK00007 4 5 4 PK00003 2 6 4 PK00005 6 4
5. Deleting data LineItem table id order id product code quantity 1 1 PK00001 3 3 2 PK00002 1 4 3 PK00007 4 5 4 PK00003 2 6 4 PK00005 6 5
6. Deleting data LineItem table id order id product code quantity deleted 1 1 PK00001 3 0 2 1 PK00002 1 0 3 2 PK00002 1 0 4 3 PK00007 4 0 5 4 PK00003 2 0 6 4 PK00005 6 0 6
7. Deleting data LineItem table id order id product code quantity deleted 1 1 PK00001 3 0 2 1 PK00002 1 1 3 2 PK00002 1 0 4 3 PK00007 4 0 5 4 PK00003 2 0 6 4 PK00005 6 0 7
8. Editing data LineItem table id order id product code quantity deleted 1 1 PK00001 3 0 2 1 PK00002 1 1 3 2 PK00002 1 0 4 3 PK00007 4 0 5 4 PK00003 2 0 6 4 PK00005 6 0 8
9. Editing data LineItem table id order id product code quantity deleted 1 1 PK00001 2 0 2 1 PK00002 1 1 3 2 PK00002 1 0 4 3 PK00007 4 0 5 4 PK00003 2 0 6 4 PK00005 6 0 9
10. Editing data LineItem table id order id product code quantity deleted version 1 1 PK00001 3 0 1 2 1 PK00002 1 1 1 3 2 PK00002 1 0 1 4 3 PK00007 4 0 1 5 4 PK00003 2 0 1 6 4 PK00005 6 0 1 10
11. Editing data LineItem table id order id product code quantity deleted version 1 1 PK00001 3 0 1 2 1 PK00002 1 1 1 3 2 PK00002 1 0 1 4 3 PK00007 4 0 1 5 4 PK00003 2 0 1 6 4 PK00005 6 0 1 1 1 PK00001 2 0 2 11
12. A familiar model (again) 12
13. Event sourcing
14. Event sourcing example 14
15. Advantages
16. Another Example
17. Technical advantages • Debugging 17
18. Technical advantages • Debugging • Scalabilty 18
19. Technical advantages • Debugging • Scalabilty • Auditing 19
20. Summary
21. Retrieving data
22. Performance Problems?
23. CQRS
24. MVC Application 24
25. CQRS Application 25
26. Technical advantages • Scalabilty 26
27. Technical advantages • Scalabilty • Performance 27
28. Technical advantages • Scalabilty • Performance • Multiple read models 28
29. CQRS + Event sourcing Application 29
30. Code - Stage 1
31. Bowling Game Entity 31
32. Game Started Event 32
33. Bowling Game Entity 33
34. Command Handler 34
35. Code - Stage 2
36. Command Handler 36
37. Validation of Throw 37
38. Handling Throw Score Recorded Event 38
39. Code - Stage 3
40. Event Sourced Repository 40
41. Projection 41
42. Read Model 42
43. Code - Stage 4
44. Projection 2 44
45. Should I use Event sourcing? • Complex to implement 45
46. Should I use Event sourcing? • Complex to implement • Development overhead 46
47. Should I use Event sourcing? • Complex to implement • Development overhead • Less understanding 47
48. Should I use Event sourcing? • Complex to implement • Development overhead • Less understanding • Hard to retofit 48
49. Conclusions
50. Further reading • Greg Young • https://github.com/qandidate-labs/broadway • https://github.com/carnage/broadway-bowling 50
51. Thanks • https://joind.in/talk/view/15440 • @giveupalready • https://github.com/carnage 51