Future is a Thing of the Past

Traditional approaches to data persistence record results rather than intentions. Event Sourcing provides us a way to do the exact opposite. In this talk, we explore the freedoms gained by throwing off the shackles of state persistence.

Transcript

1. Future is a Thing of the Past
2. my name is Mitchell
4. I’m an event sourcerer I’m an event sourcerer
5. introduction to our domain
6. an online board game shop
7. the processes we’ll handle
8. selling a board game
9. restocking a board game
10. these processes are very domain oriented
11. Domain-Driven Design
12. DDD
13. for the younger crowd
14. D to the Double D
15. DDD
16. understand the business
17. ties in extremely well with Domain Events
18. Domain Events
19. they describe the purpose of a change
20. Board Game Stock Was Replenished
21. all about the language
22. the words you use are very important
23. they capture a change in time
24. events are ...
25. events are ... a log
26. events are ... history of facts
27. events are ... set in stone
28. events are the core concept behind Event Sourcing
29. Event Sourcing doesn’t work without them
30. what is a domain event’s anatomy?
31. name
32. name payload
33. Board Game Stock Was Replenished { “boardGameId”: “52b6f6ef-62cd-4c29-b34b-562eeda1884f” “restocked”: 666 }
34. domain events are supposed to be concise
35. conventions for Domain Events
36. disclaimer don’t consider these to be absolute
37. the name
38. use past-tense
39. make sentences
40. leave out the technical stuff
41. the payload the payload
42. again, leave out the technical stuff
43. keep an eye out for the amount of properties
44. try using simple objects
45. instead of a Member entity, use MemberId value object
46. Domain Event example
47. class BoardGameStockWasReplenished implements DomainEvent { private $boardGameId; private $amount; function __constructor(BoardGameId $boardGameId, $amount) { $this->boardGameId = $boardGameId; $this->amount = $amount; } // getters... }
48. Event Sourcing in a nutshell
49. instead of changing values
50. we append events
51. instead of modifying columns in rows
52. we insert a new row with the change
53. we only store events with Event Sourcing
54. why would we do this?
55. we won’t lose information
56. implicit information
57. UPDATE board_games SET stock = 8 WHERE ...
58. we know what and when something happened
59. we have a bunch of stored events now..
60. Colt Express Quantum Condottiere StartedSellingBoardGame BoardGameStockWasReplenished StartedSellingBoardGame StartedSellingBoardGame BoardGameStockWasReplenished BoardGameManufacturingStopped
61. Colt Express Quantum Condottiere BoardGameStockWas Replenished StartedSellingBoardGame BoardGameManufacturing Stopped StartedSellingBoardGame StartedSellingBoardGame BoardGameStockWas Replenished BoardGameStockWas Replenished BoardGameStockWas Replenished
62. an event stream
63. BoardGame [ StartedSellingBoardGame BoardGameStockWasReplenished ] ,
64. BoardGame StartedSellingBoardGame BoardGameStockWasReplenished time
65. lifecycle of a domain concept
66. how do we record events?
67. through our object
68. $boardGame = BoardGame::startSelling('Cold Express', 10);
69. $boardGame = new BoardGame('Cold Express', 10);
70. $boardGame = BoardGame::startSelling('Cold Express', 10);
71. public static function startSelling($name, $price) { $id = Uuid::v4(); $boardGame = new BoardGame; $boardGame->record(new StartedSellingBoardGame($id, $name, $price)); $boardGame->applyStartedSellingBoardGame($event); return $boardGame; }
72. public static function startSelling($name, $price) { $id = Uuid::v4(); $boardGame = new BoardGame; $boardGame->record(new StartedSellingBoardGame($id, $name, $price)); $boardGame->applyStartedSellingBoardGame($event); return $boardGame; }
73. public static function startSelling($name, $price) { $id = Uuid::v4(); $boardGame = new BoardGame; $boardGame->record(new StartedSellingBoardGame($id, $name, $price)); $boardGame->applyStartedSellingBoardGame($event); return $boardGame; }
74. public static function startSelling($name, $price) { $id = Uuid::v4(); $boardGame = new BoardGame; $boardGame->record(new StartedSellingBoardGame($id, $name, $price)); $boardGame->applyStartedSellingBoardGame($event); return $boardGame; }
75. public static function startSelling($name, $price) { $id = Uuid::v4(); $boardGame = new BoardGame; $boardGame->record(new StartedSellingBoardGame($id, $name, $price)); $boardGame->applyStartedSellingBoardGame($event); return $boardGame; }
76. public static function startSelling($name, $price) { $id = Uuid::v4(); $boardGame = new BoardGame; $boardGame->record(new StartedSellingBoardGame($id, $name, $price)); $boardGame->applyStartedSellingBoardGame($event); return $boardGame; }
77. public static function startSelling($name, $price) { $id = Uuid::v4(); $boardGame = new BoardGame; $boardGame->record(new StartedSellingBoardGame($id, $name, $price)); $boardGame->applyStartedSellingBoardGame($event); return $boardGame; }
78. public static function startSelling($name, $price) { $id = Uuid::v4(); $boardGame = new BoardGame; $boardGame->record(new StartedSellingBoardGame($id, $name, $price)); $boardGame->applyStartedSellingBoardGame($event); return $boardGame; }
79. BoardGame will hold StartedSellingBoardGame
80. then what?
81. $events = $boardGame->pendingEvents();
82. write them in an event store
83. basically one large table
84. event_store stream_id integer event_id varchar version integer payload json metadata text event_name text recorded_on string
85. $eventStore->write($boardGame->id(), $events);
86. Colt Express is out of stock..
87. how do we retrieve the object with it’s state?
88. $boardGame = $entityManager->find('BoardGame', $id); or $boardGame = BoardGame::find($id);
89. $events = $eventStore->load($boardGameId); $boardGame = BoardGame::rebuildFrom($events);
90. from the start
91. $events = $eventStore->load($boardGameId); $boardGame = BoardGame::rebuildFrom($events);
92. public static function rebuildFrom(Events $events) { $object = new BoardGame; /** @var Event $event */ foreach ($events as $event) $object->applyEvent($event); return $object; }
93. protected function applyEvent(Event $event) { if ($event instanceof StartedSellingBoardGame) $this->applyStartedSellingBoardGame($event); elseif ($event instanceof BoardGameStockWasReplenished) $this->applyBoardGameStockWasReplenished($event); // more stuff }
94. private function applyStartedSellingBoardGame(StartedSellingBoardGame $event) { $this->name = $event->name(); $this->price = $event->price(); }
95. $boardGame->restock(666);
96. $events = $boardGame->pendingEvents();
97. $eventStore->write($boardGame->id(), $events);
98. we need to list our board games
99. replaying events is cheap and fast
100. but ... at some point it’ll be expensive
101. this is where read models come into play
102. projection # name price stock discount 1 Machi Koro €21 14 0 2 Exploding Kittens €15 0 0 ... ... ... ... ... 13 Quantum €50 3 10%
103. a projection can be ... several things
104. a projection can be ... mysql table
105. a projection can be ... elasticsearch doc
106. a projection can be ... plain text file
107. how do we create a projection?
108. a projector
109. it listens to dispatched events … and projects
110. projection StartedSellingBoardGame StartedSellingBoardGame BoardGameStockWasReplenished StartedSellingBoardGame BoardGameWasDiscounted # name price stock discount events 1 Colt Express €21 14 0 2 Exploding Kittens €15 5 0 3 Quantum €50 3 10%
111. modify the projection with each event
112. it’s disposable
113. throw the projection away and reproject it
114. similar banking to our bank balance
115. our balance is a sum the transactions
116. transactions
117. transactions credited €300
118. transactions credited €300 credited €150
119. transactions credited €300 credited €150 debited €72
120. transactions credited €300 credited €150 debited €72 credited €300
121. transactions credited €300 credited €150 debited €72 credited €300 debited €600
122. 300 + 150 - 72 + 300 - 600
123. 300 + 150 - 72 + 300 - 600 =
124. 300 + 150 - 72 + 300 - 600 = 78
125. 300 + 150 - 72 + 300 - 600 = 78
126. our balance can be recalculated
128. so with events..
129. since we’re recording everything*
130. we can react to everything*
131. * our entire app is typically not event sourced
132. we replenished Colt Express’ stock
133. how do our customers know about new stock?
134. we notify them
135. we can listen for BoardGameStockWasReplenished
136. message bus listening to events
137. message bus listening to events
138. message bus listening to events the event
139. message bus listening to events
140. message bus listening to events
141. message bus listening to events
142. message bus listening to events
143. message bus listening to events NotifySubscribedThatBoardGameStockIsReplenished
144. message bus listening to events
145. message bus listening to events
146. message bus listening to events
147. message bus listening to events
148. and all is good
150. listeners can have state
151. we call them process managers
152. we have an administration app
153. trial ends when 10 invoices are sent
154. trial ends at 10 invoices sent invoices 0
155. trial ends at 10 invoices sent invoices 1 InvoiceW asSent
156. trial ends at 10 invoices sent invoices 2 InvoiceW asSent InvoiceW asSent
157. trial ends at 10 invoices sent invoices 3 InvoiceW asSent InvoiceW asSent InvoiceW asSent
158. trial ends at 10 invoices sent invoices 4 InvoiceW asSent InvoiceW asSent InvoiceW asSent InvoiceW asSent
159. trial ends at 10 invoices sent invoices 8 InvoiceW asSent InvoiceW asSent InvoiceW asSent InvoiceW asSent ...
160. trial ends at 10 invoices sent invoices 9 InvoiceW asSent InvoiceW asSent InvoiceW asSent InvoiceW asSent ... InvoiceW asSent
161. trial ends at 10 invoices InvoiceW asSent InvoiceW asSent InvoiceW asSent InvoiceW asSent ... InvoiceW asSent InvoiceW asSent sent invoices 10
162. $account->endTrial();
163. let’s talk about one last thing
164. who has used a database before?
165. do multiple system have access to the same table?
166. shared state
167. shared mutable state
168. database
169. database app
170. database app
171. database app insert … into ...
172. database app
173. database app service
174. database app service
175. database app service service service
176. database app service service service app app
177. everything has access
178. consistency will be tough
179. BoardGame ABC BoardGame Table ABC Service 1
180. BoardGame ABC BoardGame Table ABC Service 2
181. BoardGame ABC Service 2 BoardGame ABC Service 1
182. BoardGame ABCD BoardGame Table ABC Service 1
183. BoardGame ABX BoardGame Table ABC Service 2
184. BoardGame ABX BoardGame Table ABX Service 2
185. BoardGame ABCD BoardGame Table ABX Service 1
186. BoardGame ABCD BoardGame Table ABCD Service 1
187. BoardGame ABX BoardGame Table ABCD Service 2
188. BoardGame ABX BoardGame Table ABCD Service 2 ?
190. Event Sourcing helps with consistency
191. what crazy things are possible?
192. what amazing things are possible?
193. 1. have a time machine at your disposal
195. Great Scott!
196. 2. projections as html pages
197. 3. project same data to multiple things
198. mysql for viewing elasticsearch for searching
199. boardgame_listing boardgame_listing_v2
200. 4. have the best audit log ...
201. 4. have the best audit log ... ever
202. 5. fix database schema bugs
203. 6. post to twitter when something happens
204. and much more ...
205. thanks for listening
206. EVENT SOURCERY INTRODUCTION TO DDD, CQRS, AND EVENT SOURCING eventsourcery.com
207. again, thanks so much
208. ❤ Nick Spelt
209. ❤ Nick Spelt For helping with one small detail
210. questions? @mitchellvanw veryfast#21807 joind.in/talk/8300c