The document discusses functional programming and event sourcing. It begins with an overview of functional programming principles like avoiding side effects and variable mutation. It then provides examples of modeling user data retrieval in a functional way using classes like Cache, UserRepo, and UserFinder. The examples demonstrate functional patterns like avoiding stateful objects and embracing immutable and recursive functions. The document argues that functional programming and event sourcing are well-aligned due to their shared emphasis on immutable data models and avoidance of shared state.

1. Functional Programming
& Event Sourcing
A pair made in heaven
twitter: @rabbitonweb, email: paul.szulc@gmail.com

2. Functional Programming

3. ● no assignment statements

4. ● no assignment statements
● no variables

5. ● no assignment statements
● no variables
● once given a value, never
change

6. ● no assignment statements
● no variables
● once given a value, never
change
● no side-effects at all

7. ● no assignment statements
● no variables
● once given a value, never
change
● no side-effects at all

8. “The functional programmer
sounds rather like a mediæval
monk,

9. “The functional programmer
sounds rather like a mediæval
monk, denying himself the
pleasures of life

10. “The functional programmer
sounds rather like a mediæval
monk, denying himself the
pleasures of life in the hope
that it will make him virtuous.”

11. Functional Programming

12. case class User(id: Long, fn: String, ln: String)

13. case class User(id: Long, fn: String, ln: String)
class Cache { def check(id: Long): Option[User] = ??? }

14. case class User(id: Long, fn: String, ln: String)
class Cache { def check(id: Long): Option[User] = ??? }
case class UserRepo(cache: Cache) {
def retrieve(id: Long): User = ???
}

15. case class User(id: Long, fn: String, ln: String)
class Cache { def check(id: Long): Option[User] = ??? }
case class UserRepo(cache: Cache) {
def retrieve(id: Long): User = ???
}
class UserFinder(cache: Cache, repo: UserRepo) {
}

16. case class User(id: Long, fn: String, ln: String)
class Cache { def check(id: Long): Option[User] = ??? }
case class UserRepo(cache: Cache) {
def retrieve(id: Long): User = ???
}
class UserFinder(cache: Cache, repo: UserRepo) {
def findUser(id: Long): User = {
}
}

17. case class User(id: Long, fn: String, ln: String)
class Cache { def check(id: Long): Option[User] = ??? }
case class UserRepo(cache: Cache) {
def retrieve(id: Long): User = ???
}
class UserFinder(cache: Cache, repo: UserRepo) {
def findUser(id: Long): User = {
val maybeUser: Option[User] = cache.check(id)
}
}

18. case class User(id: Long, fn: String, ln: String)
class Cache { def check(id: Long): Option[User] = ??? }
case class UserRepo(cache: Cache) {
def retrieve(id: Long): User = ???
}
class UserFinder(cache: Cache, repo: UserRepo) {
def findUser(id: Long): User = {
val maybeUser: Option[User] = cache.check(id)
if (maybeUser.isDefined) {
maybeUser.get
}
}
}

19. case class User(id: Long, fn: String, ln: String)
class Cache { def check(id: Long): Option[User] = ??? }
case class UserRepo(cache: Cache) {
def retrieve(id: Long): User = ???
}
class UserFinder(cache: Cache, repo: UserRepo) {
def findUser(id: Long): User = {
val maybeUser: Option[User] = cache.check(id)
if (maybeUser.isDefined) {
maybeUser.get
} else {
val user: User = repo.retrieve(id)
cache.insert(user.id, user)
}
}
}

20. No concept of ‘time’

21. f(input): output

22. input f(input): output

23. input f(input): output output

24. input f(input): output output g(input): output

25. input f(input): output output g(input): output output

26. input f(input): output output g(input): output output

27. h = g o f
input f(input): output output g(input): output output

28. Modularity & composition

29. h = g o f
input f(input): output output g(input): output output

30. /**
* This function returns a reversed list
* @param list A list to be reversed
* @return A reversed list
*/
public List<T> reverse(List<t> list) { ??? }

32. /**
* This function returns a reversed list
* @param list A list to be reversed
* @return A reversed list
*/
public List<T> reverse(List<t> list) { ??? }

33. /**
* This function returns a reversed list
* @param list A list to be reversed
public List<T> reverse(List<t> list) { ??? }

34. /**
* This function returns a reversed list
public List<T> reverse(List<t> list) { ??? }

35. /**
public List<T> reverse(List<t> list) { ??? }

36. public List<T> reverse(List<t> list) { ??? }

37. public List<T> reverse(List<t> list) {
return list.sort();
}

38. def smdfknmsdfp[A](a: A): A = ???

39. def smdfknmsdfp[A](a: A): A = a

40. def identity[A](a: A): A = a

41. def smdfknmsdfp[A](a: A): A = a

42. def smdfknmsdfp(a: Int): Int = ???

43. def smdfknmsdfp(a: Int): Int = a
= a + 10
= 10

44. “Why Functional Programming Matters”
J. Hughes
http://comjnl.oxfordjournals.org/content/32/2/98.
full.pdf

45. “Why Functional Programming Matters”
J. Hughes, Nov. 1988
http://comjnl.oxfordjournals.org/content/32/2/98.
full.pdf

46. Soft introduction
“Why Functional Programming Matters”
J. Hughes, Nov. 1988
http://comjnl.oxfordjournals.org/content/32/2/98.
full.pdf

47. “Program Design by Calculation”
J.N. Oliveira
http://www4.di.uminho.pt/~jno/ps/pdbc_part.pdf

48. “Program Design by Calculation”
J.N. Oliveira, Draft
http://www4.di.uminho.pt/~jno/ps/pdbc_part.pdf

49. Patterns in FP World
“Program Design by Calculation”
J.N. Oliveira, Draft
http://www4.di.uminho.pt/~jno/ps/pdbc_part.pdf

50. Patterns in FP World Math Matters
“Program Design by Calculation”
J.N. Oliveira, Draft
http://www4.di.uminho.pt/~jno/ps/pdbc_part.pdf

51. “A lengthy approach to Haskell fundamentals”
http://www.davesquared.net/2012/05/lengthy-
approach-to-haskell.html

52. Functional Programming

53. Functional Programming

57. How to do something
useful?

58. How to do something
useful?

59. case class User(id: Long, fn: String, ln: String)
class Cache { def check(id: Long): Option[User] = ??? }
case class UserRepo(cache: Cache) {
def retrieve(id: Long): User = ???
}
class UserFinder(cache: Cache, repo: UserRepo) {
def findUser(id: Long): User = {
val maybeUser: Option[User] = cache.check(id)
if (maybeUser.isDefined) {
maybeUser.get
} else {
val user: User = repo.retrieve(id)
cache.insert(user.id, user)
}
}
}

60. case class User(id: Long, fn: String, ln: String)

61. case class User(id: Long, fn: String, ln: String)
class Cache {}

62. case class User(id: Long, fn: String, ln: String)
class Cache {}
def check(id: Long)(cache: Cache): (Cache, Option[User]) = ...

63. case class User(id: Long, fn: String, ln: String)
class Cache {}
def check(id: Long)(cache: Cache): (Cache, Option[User]) = ...
def retrieve(id: Long)(cache: Cache): (Cache, User) = ...

64. case class User(id: Long, fn: String, ln: String)
class Cache {}
def check(id: Long)(cache: Cache): (Cache, Option[User]) = ...
def retrieve(id: Long)(cache: Cache): (Cache, User) = ...
def findUser(id: Long)(cache: Cache): (Cache, User) = {
}
}

65. case class User(id: Long, fn: String, ln: String)
class Cache {}
def check(id: Long)(cache: Cache): (Cache, Option[User]) = ...
def retrieve(id: Long)(cache: Cache): (Cache, User) = ...
def findUser(id: Long)(cache: Cache): (Cache, User) = {
val (c, mu) = check(id)(cache)
}
}

66. case class User(id: Long, fn: String, ln: String)
class Cache {}
def check(id: Long)(cache: Cache): (Cache, Option[User]) = ...
def retrieve(id: Long)(cache: Cache): (Cache, User) = ...
def findUser(id: Long)(cache: Cache): (Cache, User) = {
val (c, mu) = check(id)(cache)
mu match {
case Some(u) => (c, u)
}
}
}

67. case class User(id: Long, fn: String, ln: String)
class Cache {}
def check(id: Long)(cache: Cache): (Cache, Option[User]) = ...
def retrieve(id: Long)(cache: Cache): (Cache, User) = ...
def findUser(id: Long)(cache: Cache): (Cache, User) = {
val (c, mu) = check(id)(cache)
mu match {
case Some(u) => (c, u)
case None => retrieve(id)(c)
}
}
}

68. case class User(id: Long, fn: String, ln: String)
class Cache {}
def check(id: Long)(cache: Cache): (Cache, Option[User]) = ...
def retrieve(id: Long)(cache: Cache): (Cache, User) = ...
def findUser(id: Long)(cache: Cache): (Cache, User) = {
val (c, mu) = check(id)(cache)
mu match {
case Some(u) => (c, u)
case None => retrieve(id)(c)
}
}
}

69. S => (S, A)

70. State[S, A]
S => (S, A)

71. State[S, A]
S => (S, A)
.run(S)

72. State[S, A]
S => (S, A)
.map(A => B): State[S, B]

73. State[S, A]
S => (S, A)
.flatMap(A => State[S, B]): State[S,B]

74. object State {
def apply[S, A] (f: S => (S,A)): State[S, A] =
}

75. object State {
def apply[S, A] (f: S => (S,A)): State[S, A] =
new State[S, A] {
def run(s: S) = f(s)
}
}

76. object State {
def apply[S, A] (f: S => (S,A)): State[S, A] =
new State[S, A] {
def run(s: S) = f(s)
}
}
def check(id: String) =

77. object State {
def apply[S, A] (f: S => (S,A)): State[S, A] =
new State[S, A] {
def run(s: S) = f(s)
}
}
def check(id: String) =
(c: Cache) => (c, c.get(id))

78. object State {
def apply[S, A] (f: S => (S,A)): State[S, A] =
new State[S, A] {
def run(s: S) = f(s)
}
}
def check(id: String) = State[Cache, Option[User]].apply {
(c: Cache) => (c, c.get(id))
}

79. object State {
def apply[S, A] (f: S => (S,A)): State[S, A] =
new State[S, A] {
def run(s: S) = f(s(
}
}
def check(id: String) = State[Cache, Option[User]]{
(c: Cache) => (c, c.get(id))
}

80. trait State[S, +A] {
}

81. trait State[S, +A] {
def run(initial: S): (S, A)
}

82. trait State[S, +A] {
def run(initial: S): (S, A)
def map[B](f: A => B): State[S, B] =
}
}

83. trait State[S, +A] {
def run(initial: S): (S, A)
def map[B](f: A => B): State[S, B] =
State {
}
}
}

84. trait State[S, +A] {
def run(initial: S): (S, A)
def map[B](f: A => B): State[S, B] =
State { s0 =>
(_, _ )
}
}
}

85. trait State[S, +A] {
def run(initial: S): (S, A)
def map[B](f: A => B): State[S, B] =
State { s0 =>
(_, f(a))
}
}
}

86. trait State[S, +A] {
def run(initial: S): (S, A)
def map[B](f: A => B): State[S, B] =
State { s0 =>
val (s, a) = run(s0)
(_, f(a))
}
}
}

87. trait State[S, +A] {
def run(initial: S): (S, A)
def map[B](f: A => B): State[S, B] =
State { s0 =>
val (s, a) = run(s0)
(s, f(a))
}
}
}

88. trait State[S, +A] {
def run(initial: S): (S, A)
def map[B](f: A => B): State[S, B] =
State { s0 =>
val (s, a) = run(s0)
(s, f(a))
}
}
def flatMap[B](f: A => State[S,B]): State[S, B] =
}
}

89. trait State[S, +A] {
def run(initial: S): (S, A)
def map[B](f: A => B): State[S, B] =
State { s0 =>
val (s, a) = run(s0)
(s, f(a))
}
}
def flatMap[B](f: A => State[S,B]): State[S, B] =
f(a)
}
}

90. trait State[S, +A] {
def run(initial: S): (S, A)
def map[B](f: A => B): State[S, B] =
State { s0 =>
val (s, a) = run(s0)
(s, f(a))
}
}
def flatMap[B](f: A => State[S,B]): State[S, B] =
val (s, a) = run(s0)
f(a)
}
}

91. trait State[S, +A] {
def run(initial: S): (S, A)
def map[B](f: A => B): State[S, B] =
State { s0 =>
val (s, a) = run(s0)
(s, f(a))
}
}
def flatMap[B](f: A => State[S,B]): State[S, B] =
State { s0 =>
val (s, a) = run(s0)
f(a)
}
}
}

92. trait State[S, +A] {
def run(initial: S): (S, A)
def map[B](f: A => B): State[S, B] =
State { s0 =>
val (s, a) = run(s0)
(s, f(a))
}
}
def flatMap[B](f: A => State[S,B]): State[S, B] =
State { s0 =>
val (s, a) = run(s0)
f(a).run(s)
}
}
}

93. case class User(id: Long, fn: String, ln: String)
class Cache {}
def check(id: Long)(cache: Cache): (Cache, Option[User]) = ???
def retrieve(id: Long)(cache: Cache): (Cache, User) = ???
def findUser(id: Long)(cache: Cache): (Cache, User) = {
val (c, mu) = check(id)(cache)
mu match {
case Some(u) => (c, u)
case None => retrieve(id)(c)
}
}
}

94. case class User(id: Long, fn: String, ln: String)
class Cache {}
def check(id: Long)(cache: Cache): (Cache, Option[User]) = ???
def retrieve(id: Long)(cache: Cache): (Cache, User) = ???
def findUser(id: Long)(cache: Cache): (Cache, User) = {
val (c, mu) = check(id)(cache)
mu match {
case Some(u) => (c, u)
case None => retrieve(id)(c)
}
}
}

95. case class User(id: Long, fn: String, ln: String)
class Cache {}
def check(id: Long): State[Cache, Option[User]] = ???
def retrieve(id: Long): State[Cache, User] = ???
def findUser(id: Long): State[Cache, User] = {
for {
maybeUser <- check(id)
user <- maybeUser match {
case Some(u) => State { c => (c, u)}
case None => retrieve(id)
}
} yield (user)
}

96. Event Sourcing

97. Event Sourcing
driven by business

98. Bloggers Conf App

99. Bloggers Conf App
● Can create an account

100. Bloggers Conf App
● Can create an account
● List all bloggers already using the app

101. Bloggers Conf App
● Can create an account
● List all bloggers already using the app
● Mark/unmark other blogger as a friend

102. Bloggers Conf App
● Can create an account
● List all bloggers already using the app
● Mark/unmark other blogger as a friend
● Mark/unmark other blogger as an enemy

103. Bloggers Conf App
● Can create an account
● List all bloggers already using the app
● Mark/unmark other blogger as a friend
● Mark/unmark other blogger as an enemy
● Deactivate its account

104. Bloggers Conf App
Bloggers
id first_name last_name active
1 Jan Kowalski T
2 Krystian Nowak T
3 Malgorzata Kucharska T

105. Bloggers Conf App
Bloggers
id first_name last_name active
1 Jan Kowalski T
2 Krystian Nowak T
3 Malgorzata Kucharska T
Friends
id friend_id
3 1

106. Bloggers Conf App
Bloggers
id first_name last_name active
1 Jan Kowalski T
2 Krystian Nowak T
3 Malgorzata Kucharska T
Friends
id friend_id
3 1
Enemies
id enemy_id
3 2

107. Bloggers Conf App
Bloggers
id first_name last_name active
1 Jan Kowalski T
2 Krystian Nowak T
3 Malgorzata Kucharska T
Friends
id friend_id
3 1
Enemies
id enemy_id
3 2
The Structure

108. Structure is not that
important

109. Structure is not that
important
Changes more often than behaviour

110. Start thinking about facts
occurring

111. Start thinking about facts
occurring
Derive structure from them

112. Blogger
Account
Created
(id=3)

113. Blogger
Account
Created
(id=3)
Befriended
Blogger
id=1

114. Blogger
Account
Created
(id=3)
Befriended
Blogger
id=1
Made
Enemy of
Blogger
id=2

115. Blogger
Account
Created
(id=3)
Befriended
Blogger
id=1
Made
Enemy of
Blogger
id=2
Bloggers
id first_name last_name active
1 Jan Kowalski T
2 Krystian Nowak T
Friends
id friend_id
Enemies
id enemy_id

116. Blogger
Account
Created
(id=3)
Befriended
Blogger
id=1
Made
Enemy of
Blogger
id=2
Bloggers
id first_name last_name active
1 Jan Kowalski T
2 Krystian Nowak T
3 Malgorzata Kucharska T
Friends
id friend_id
Enemies
id enemy_id

117. Blogger
Account
Created
(id=3)
Befriended
Blogger
id=1
Made
Enemy of
Blogger
id=2
Bloggers
id first_name last_name active
1 Jan Kowalski T
2 Krystian Nowak T
3 Malgorzata Kucharska T
Friends
id friend_id
3 1
Enemies
id enemy_id

118. Blogger
Account
Created
(id=3)
Befriended
Blogger
id=1
Made
Enemy of
Blogger
id=2
Bloggers
id first_name last_name active
1 Jan Kowalski T
2 Krystian Nowak T
3 Malgorzata Kucharska T
Friends
id friend_id
3 1
Enemies
id enemy_id
3 2

119. Blogger
Account
Created
(id=3)
Befriended
Blogger
id=1
Made
Enemy of
Blogger
id=2

120. Blogger
Account
Created
(id=3)
Befriended
Blogger
id=1
Made
Enemy of
Blogger
id=2
Befriended
Blogger
id=2
Unfriended
Blogger
id=2
Blogger
Account
Created
(id=3)
Befriended
Blogger
id=1
Made
Enemy of
Blogger
id=2

121. Event Sourcing
driven by business

122. Event Sourcing
The only model that does not lose data

123. “Enemy of my enemy is
my friend”

124. Bloggers
id first_name last_name active
1 Jan Kowalski T
2 Krystian Nowak T
3 Malgorzata Kucharska T
Friends
id friend_id
3 1
Enemies
id enemy_id
3 2

125. Bloggers
id first_name last_name active
1 Jan Kowalski T
2 Krystian Nowak T
3 Malgorzata Kucharska T
4 Tomasz Młynarski T
Friends
id friend_id
3 1
Enemies
id enemy_id
3 2
4 2

126. Bloggers
id first_name last_name active
1 Jan Kowalski T
2 Krystian Nowak T
3 Malgorzata Kucharska T
4 Tomasz Młynarski T
5 Monika Jagoda T
Friends
id friend_id
3 1
Enemies
id enemy_id
3 2
4 2
2 5

127. id = 3
id = 2
id = 1
id = 4
id = 5
id = 6

128. id = 3
id = 2
id = 1
id = 4
id = 5
id = 6

129. id = 3
id = 2
id = 1
id = 4
id = 5
id = 6

130. id = 3
id = 2
id = 1
id = 4
id = 5
id = 6

131. id = 3
id = 2
id = 1
id = 4
id = 5
id = 6

132. id = 3
id = 2
id = 1
id = 4
id = 5
id = 6

133. id = 3
id = 2
id = 1
id = 4
id = 5
id = 6

134. id = 3
id = 2
id = 1
id = 4
id = 5
id = 6

135. Benefits of Event Sourcing

136. Benefits of Event Sourcing
● Ability to go in time and figure out exactly what have
happened

137. Benefits of Event Sourcing
● Ability to go in time and figure out exactly what have
happened
● Scientific measurements over time, compare time
periods

138. Benefits of Event Sourcing
● Ability to go in time and figure out exactly what have
happened
● Scientific measurements over time, compare time
periods
● Built-in audit log

139. Benefits of Event Sourcing
● Ability to go in time and figure out exactly what have
happened
● Scientific measurements over time, compare time
periods
● Built-in audit log
● Enables temporal querying

140. Benefits of Event Sourcing
● Ability to go in time and figure out exactly what have
happened
● Scientific measurements over time, compare time
periods
● Built-in audit log
● Enables temporal querying
● Fits well with machine learning

141. Benefits of Event Sourcing
● Ability to go in time and figure out exactly what have
happened
● Scientific measurements over time, compare time
periods
● Built-in audit log
● Enables temporal querying
● Fits well with machine learning
● Preserves history - question not yet asked

142. Benefits of Event Sourcing
● Ability to go in time and figure out exactly what have
happened
● Scientific measurements over time, compare time
periods
● Built-in audit log
● Enables temporal querying
● Fits well with machine learning
● Preserves history - question not yet asked
● Writing regression tests is easy

143. Benefits of Event Sourcing
● Ability to go in time and figure out exactly what have
happened
● Scientific measurements over time, compare time
periods
● Built-in audit log
● Enables temporal querying
● Fits well with machine learning
● Preserves history - question not yet asked
● Writing regression tests is easy
● Polyglot data

144. Drawbacks of Event Sourcing

145. Drawbacks of Event Sourcing
● Historical record of your bad decisions

146. Drawbacks of Event Sourcing
● Historical record of your bad decisions
● Handling event duplicates

147. Drawbacks of Event Sourcing
● Historical record of your bad decisions
● Handling event duplicates
● Data eventually consistent

148. How to implement it?

149. Events vs Commands

150. Journal

151. Journal

152. Journal
val id = “”
val firstName: String = “”
val lastName: String = “”
val friends: List[String] =
List()

153. Journal
val id = “”
val firstName: String = “”
val lastName: String = “”
val friends: List[String] =
List()

154. Journal
val id = “”
val firstName: String = “”
val lastName: String = “”
val friends: List[String] =
List()

155. Journal
val id = “”
val firstName: String = “”
val lastName: String = “”
val friends: List[String] =
List()
Initialized(“1”, “Jan”, “Kowalski”

156. Journal
val id = “”
val firstName: String = “”
val lastName: String = “”
val friends: List[String] =
List()
Initialized(“1”, “Jan”, “Kowalski”

157. Journal
val id = “1”
val firstName: String =
“Jan”
val lastName: String =
“Kowalski”
val friends: List[String] =
List()

158. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] =
List()

159. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] =
List()

160. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] =
List()
Befriended(“10”)

161. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] =
List()
Befriended(“10”)

162. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] =
List(“10”)

163. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] = List
(“10”)

164. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] = List
(“10”)

165. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] = List
(“10”)

166. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] =
List(“10”, “31”)

167. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] = List
(“10”, “31”)

168. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] = List
(“10”, “31”)
Befriend(“31”)

169. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] = List
(“10”, “31”)
Befriend(“31”)

170. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] = List
(“10”, “31”)
Befriend(“31”)
validation

171. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] = List
(“10”, “31”)
Befriend(“31”)
validation

172. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] = List
(“10”, “31”)

173. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] = List
(“10”, “31”)
Befriend(“34”)

174. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] = List
(“10”, “31”)
Befriend(“34”)

175. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] = List
(“10”, “31”)
Befriend(“34”)
validation

176. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] = List
(“10”, “31”)
Befriend(“34”)
validation

177. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] = List
(“10”, “31”)
Befriend(“34”)
Befriended(“34”)

178. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] = List
(“10”, “31”)
Befriend(“34”)
Befriended(“34”)

179. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] = List
(“10”, “31”)
Befriend(“34”)
Befriended(“34”)

180. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] = List
(“10”, “31”)
Befriend(“34”)
Befriended(“34”)

181. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] = List
(“10”, “31”)
Befriend(“34”)

182. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] = List
(“10”, “31”)
Befriend(“34”)
ACK

183. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] =
List(“10”, “31”, “34”)
Befriend(“34”)

184. Journal
val id = “1”
val firstName: String = “Jan”
val lastName: String =
“Kowalski”
val friends: List[String] =
List(“10”, “31”, “34”)

185. Let’s see some code!
https://github.
com/rabbitonweb/es_cqrs_example

186. What we are missing?

187. What we are missing?
1. Read-model

188. What we are missing?
1. Read-model
2. Validation

189. And that’s all folks!

190. Paweł Szulc

191. Paweł Szulc
http://rabbitonweb.com

192. Paweł Szulc
http://rabbitonweb.com
@rabbitonweb

193. Paweł Szulc
http://rabbitonweb.com
@rabbitonweb
https://github.com/rabbitonweb/

194. Thank you!