Video and slides synchronized, mp3 and slide download available at URL https://bit.ly/2pC2qbb. Jonas Bonér explores the nature of events, what it means to be event-driven, and how to unleash the power of events and commands by applying an events-first domain-driven design to microservices-based architectures. Filmed at qconnewyork.com. Jonas Bonér is the founder and CTO of Lightbend, inventor of the Akka project, initiator and co-author of the Reactive Manifesto and a Java Champion.

1. Designing
Events First
Microservices
Jonas Bonér
@jboner

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microservices-events-first-design

3. Presented at QCon New York
www.qconnewyork.com
Purpose of QCon
- to empower software development by facilitating the spread of
knowledge and innovation
Strategy
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Highlights
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- held in 9 cities worldwide

4. So, you want to do
microservices?

5. Make sure you don’t end up with
Microliths

6. Make sure you don’t end up with
Microliths
We can do better than this

7. Events First
Domain Driven
Design

8. “When you start modeling events, it
forces you to think about the behaviour
of the system. As opposed to thinking
about the structure of the system.”
- Greg Young
A Decade of DDD, CQRS, Event Sourcing, Greg Young (Presentation from 2016)

9. ✴ Don’t focus on the things
The Nouns
The Domain Objects

10. ✴ Don’t focus on the things
The Nouns
The Domain Objects
✴ Focus on what happens
The Verbs
The Events

11. What is an
Event?

12. The Nature of Events

13. ✴Events represent Facts of information
➡ Facts are immutable
➡ Facts Accrue - Knowledge can only grow
The Nature of Events

14. ✴Events represent Facts of information
➡ Facts are immutable
➡ Facts Accrue - Knowledge can only grow
✴ Events/Facts can be disregarded/Ignored
The Nature of Events

15. ✴Events represent Facts of information
➡ Facts are immutable
➡ Facts Accrue - Knowledge can only grow
✴ Events/Facts can be disregarded/Ignored
✴ Events/Facts Can not be retracted (once accepted)
The Nature of Events

16. ✴Events represent Facts of information
➡ Facts are immutable
➡ Facts Accrue - Knowledge can only grow
✴ Events/Facts can be disregarded/Ignored
✴ Events/Facts Can not be retracted (once accepted)
✴ Events/Facts Can not be deleted (once accepted)
➡ Might be needed for legal or moral reasons
The Nature of Events

17. ✴Events represent Facts of information
➡ Facts are immutable
➡ Facts Accrue - Knowledge can only grow
✴ Events/Facts can be disregarded/Ignored
✴ Events/Facts Can not be retracted (once accepted)
✴ Events/Facts Can not be deleted (once accepted)
➡ Might be needed for legal or moral reasons
✴ Events/Facts (new) can invalidate existing Facts
The Nature of Events

18. Mine the
Facts

20. Event Storming

21. Event Driven Design

22. ✴ IntentS
➡ Communication
➡ Conversations
➡ Expectations
➡ Contracts
➡ Control Transfer
Event Driven Design

23. ✴ IntentS
➡ Communication
➡ Conversations
➡ Expectations
➡ Contracts
➡ Control Transfer
Event Driven Design
✴ Facts
➡ State
➡ History
➡ Causality
➡ Notifications
➡ State Transfer

24. ✴ IntentS
➡ Communication
➡ Conversations
➡ Expectations
➡ Contracts
➡ Control Transfer
Event Driven Design
✴ Facts
➡ State
➡ History
➡ Causality
➡ Notifications
➡ State Transfer
Commands

25. ✴ IntentS
➡ Communication
➡ Conversations
➡ Expectations
➡ Contracts
➡ Control Transfer
Event Driven Design
✴ Facts
➡ State
➡ History
➡ Causality
➡ Notifications
➡ State Transfer
Commands Events

26. Event Driven Design

27. ✴Commands
➡ Object form of method/Action request
➡ Imperative: CreateOrder, ShipProduct
Event Driven Design

28. ✴Commands
➡ Object form of method/Action request
➡ Imperative: CreateOrder, ShipProduct
✴Reactions
➡ Represents side-effects
Event Driven Design

29. ✴Commands
➡ Object form of method/Action request
➡ Imperative: CreateOrder, ShipProduct
✴Reactions
➡ Represents side-effects
✴Events
➡ Represents something that has happened
➡ Past-tense: OrderCreated, ProductShipped
Event Driven Design

30. Commands Eventsvs

31. Commands Eventsvs
1. All about intent 1. Intentless

32. Commands Eventsvs
1. All about intent
2. Directed
1. Intentless
2. Anonymous

33. Commands Eventsvs
1. All about intent
2. Directed
3. Single addressable
destination
1. Intentless
2. Anonymous
3. Just happens - for
others (0-N) to observe

34. Commands Eventsvs
1. All about intent
2. Directed
3. Single addressable
destination
4. Models personal
communication
1. Intentless
2. Anonymous
3. Just happens - for
others (0-N) to observe
4. Models broadcast
(speakers corner)

35. Commands Eventsvs
1. All about intent
2. Directed
3. Single addressable
destination
4. Models personal
communication
5. Distributed focus
1. Intentless
2. Anonymous
3. Just happens - for
others (0-N) to observe
4. Models broadcast
(speakers corner)
5. Local focus

36. Commands Eventsvs
1. All about intent
2. Directed
3. Single addressable
destination
4. Models personal
communication
5. Distributed focus
6. Command & Control
1. Intentless
2. Anonymous
3. Just happens - for
others (0-N) to observe
4. Models broadcast
(speakers corner)
5. Local focus
6. Autonomy

37. Let the Events Define the
Bounded Context

38. Event Driven
Services

39. 1. REceive & react (or not)
to facts* that are coming its way
Event Driven
Services

40. 1. REceive & react (or not)
to facts* that are coming its way
2. Publish new facts* asynchronously
to the rest of the world
Event Driven
Services

41. 1. REceive & react (or not)
to facts* that are coming its way
2. Publish new facts* asynchronously
to the rest of the world
3. Invert the control flow
to minimize coupling and increase autonomy
Event Driven
Services

42. 1. REceive & react (or not)
to facts* that are coming its way
2. Publish new facts* asynchronously
to the rest of the world
3. Invert the control flow
to minimize coupling and increase autonomy
Event Driven
Services
*Facts == Immutable Events

43. Mutable State Is Fine
But Needs To Be
Contained
And Non Observable

44. Publish Facts
To Outside World

45. ✴ Maintains Integrity & consistency
✴ Is our Unit of Consistency
✴ Is our Unit of Failure
✴ Is our Unit of Determinism
✴ Is fully Autonomous
The Aggregate

46. Event Driven
Services

47. Event Driven
Services

48. Event Driven
Services
Command

49. Event Driven
Services
Command

50. Event Driven
Services
Command

51. Event Driven
Services
Command
Event
Event Stream

52. Event Driven
Services
Command
Event
EventEvent
Event Stream

53. Event Driven
Services
Command
Event
EventEvent
Event Stream

54. Event Driven
Services
Command
Event
EventEvent
Event Stream

55. Event Driven
Services
Command
Event
EventEvent
Event Stream

56. Event Driven
Services
Command
Event
EventEvent
Event Stream

57. Event Driven
Services
Eventual
Consistency
Command
Event
EventEvent
Event Stream

58. Event
Stream
Use The

59. Event
Stream
Use The
as the communication fabric

60. Event
Stream
Use The

61. Event
Stream
Use The
as the integration fabric

62. Event
Stream
Use The

63. Event
Stream
Use The
as the replication fabric

64. Event
Stream
Use The

65. Event
Stream
Use The
as the consensus fabric

66. Event
Stream
Use The

67. Event
Stream
Use The
as the Persistence fabric

68. The Problem With
CRUD Services

69. ✴ CRUD is fine for totally isolated data
The Problem With
CRUD Services

70. ✴ CRUD is fine for totally isolated data
✴ But, cross CRUD services consistency
The Problem With
CRUD Services

71. ✴ CRUD is fine for totally isolated data
✴ But, cross CRUD services consistency
➡ Is hard ⇨ No Joins
The Problem With
CRUD Services

72. ✴ CRUD is fine for totally isolated data
✴ But, cross CRUD services consistency
➡ Is hard ⇨ No Joins
➡ Has ad-hoc & weak guarantees
The Problem With
CRUD Services

73. ✴ CRUD is fine for totally isolated data
✴ But, cross CRUD services consistency
➡ Is hard ⇨ No Joins
➡ Has ad-hoc & weak guarantees
The Problem With
CRUD Services

74. “Two-phase commit is the
anti-availability protocol.”
- Pat Helland
Standing on Distributed Shoulders of Giants - Pat Helland

75. STRONG
Consistency
Is the wrong default
In distributed systems

76. STRONG
Consistency
Is the wrong default
In distributed systems
What can we do?

77. Eventual
Consistency
We have to rely on

78. Eventual
Consistency
We have to rely on
But relax—it’s how the world works

79. Embrace
Reality
We Need to

80. Embrace
Reality
We Need to
Not Fight it

81. Information
Has Latency

82. Information Is Always
From the Past

83. Welcome To The Wild Ocean Of
Non Determinism
Distributed Systems

84. “In a system which cannot count on
distributed transactions, the management
of uncertainty must be implemented in the
business logic.”
- Pat Helland
Life Beyond Distributed Transactions, Pat Helland (2007)
We Need To Model
Uncertainty

85. Events Can Lead To Greater
Certainty

86. “An autonomus component can only
promise its own behavior.”
“Autonomy makes information local,
leading to greater certainty and stability.”
- Mark Burgess
In Search of Certainty, Thinking in Promises - Mark Burgess

87. Events Can Help Us Craft
Autonomous Islands
Of Determinism

88. Data on the inside vs Data on the outside - Pat Helland

89. Data on the inside vs Data on the outside - Pat Helland
Inside Data
Our current present ⇨ state

90. Data on the inside vs Data on the outside - Pat Helland
Inside Data
Our current present ⇨ state
Outside Data
Blast from the past ⇨ Events/facts

91. Data on the inside vs Data on the outside - Pat Helland
Inside Data
Our current present ⇨ state
Outside Data
Blast from the past ⇨ Events/facts
Between Services
Hope for the future ⇨ commands

92. A system of microservices is a
never ending stream towards convergence

93. There Is No Now
A system of microservices is a
never ending stream towards convergence

94. Resilience
is by
Design
Photo courtesy of FEMA/Joselyne Augustino

95. Events Can Help Us
Manage
Failure
Instead Of Trying To Avoid It

96. Requirements for a
Sane Failure Model
1. Contained—Avoid cascading failures
2. Reified—as Events
3. Signalled—Asynchronously
4. Observed—by 1-N
5. Managed—Outside failed Context
Failures need to be

97. Event Based
Persistence

98. You can use CRUD
Together with Event Streams
To get an internally consistent Materialized View

99. Service B
Service A
You can use CRUD
Together with Event Streams
To get an internally consistent Materialized View

100. Service B
Service A
CRUD
You can use CRUD
Together with Event Streams
To get an internally consistent Materialized View
CRUD

101. Service B
Service A
TABLE A
CRUD
TABLE B
You can use CRUD
Together with Event Streams
To get an internally consistent Materialized View
CRUD

102. Service B
Service A
TABLE A
CRUD
TABLE B
You can use CRUD
Together with Event Streams
To get an internally consistent Materialized View
CRUD

103. Service B
Service A
TABLE A
CRUD
TABLE B
You can use CRUD
Together with Event Streams
To get an internally consistent Materialized View
CRUD
Atomic Update & event

104. Service B
Service A
TABLE A
CRUD
TABLE B
You can use CRUD
Together with Event Streams
To get an internally consistent Materialized View
CRUD
Atomic Update & event

105. Service C
Service B
Service A
TABLE A
CRUD
TABLE B
You can use CRUD
Together with Event Streams
To get an internally consistent Materialized View
CRUD
Atomic Update & event

106. Service C
Service B
Service A
TABLE A
CRUD
TABLE B
You can use CRUD
Together with Event Streams
To get an internally consistent Materialized View
CRUD
Atomic Update & event

107. Service C
Service B
Service A
TABLE A
CRUD
TABLE B
JOINS
Table A
Table B
You can use CRUD
Together with Event Streams
To get an internally consistent Materialized View
CRUD
Atomic Update & event

108. Service C
Service B
Service A
TABLE A
CRUD
TABLE B
JOINS
Table A
Table B
You can use CRUD
Together with Event Streams
To get an internally consistent Materialized View
CRUD
Read
Only
Atomic Update & event

109. Service C
Service B
Service A
Eventual
Consistency
TABLE A
CRUD
TABLE B
JOINS
Table A
Table B
You can use CRUD
Together with Event Streams
To get an internally consistent Materialized View
CRUD
Read
Only
Atomic Update & event

110. “Update-in-place strikes systems
designers as a cardinal sin: it violates
traditional accounting practices that have
been observed for hundreds of years.”
- jim Gray
The Transaction Concept, Jim Gray (1981)

111. “The truth is the log.
The database is a cache
of a subset of the log.”
- Pat Helland
Immutability Changes Everything, Pat Helland (2015)

112. Event Logging
The Bedrock

113. Event Sourcing
A Cure For the Cardinal Sin

114. Event
Sourced
Services

115. Happy Path
Event
Sourced
Services

116. Happy Path
Event
Sourced
Services
1) Receive and verify Command
(“ApprovePayment”)

117. Happy Path
Event
Sourced
Services
2) Create new Event
(“PaymentApproved”)
1) Receive and verify Command
(“ApprovePayment”)

118. Happy Path
Event
Sourced
Services
2) Create new Event
(“PaymentApproved”)
1) Receive and verify Command
(“ApprovePayment”)
3) Append Event
to Event Log

119. Happy Path
Event
Sourced
Services
2) Create new Event
(“PaymentApproved”)
1) Receive and verify Command
(“ApprovePayment”)
3) Append Event
to Event Log
4) Update internal
component state

120. Happy Path
Event
Sourced
Services
5) Run side-effects
(approve the payment)
2) Create new Event
(“PaymentApproved”)
1) Receive and verify Command
(“ApprovePayment”)
3) Append Event
to Event Log
4) Update internal
component state

121. SAD Path - recover from failure
Happy Path
Event
Sourced
Services
5) Run side-effects
(approve the payment)
2) Create new Event
(“PaymentApproved”)
1) Receive and verify Command
(“ApprovePayment”)
3) Append Event
to Event Log
4) Update internal
component state

122. SAD Path - recover from failure
Happy Path
Event
Sourced
Services
5) Run side-effects
(approve the payment)
2) Create new Event
(“PaymentApproved”)
1) Receive and verify Command
(“ApprovePayment”)
3) Append Event
to Event Log
4) Update internal
component state
1) Rehydrate Events
from Event Log

123. SAD Path - recover from failure
Happy Path
Event
Sourced
Services
5) Run side-effects
(approve the payment)
2) Create new Event
(“PaymentApproved”)
1) Receive and verify Command
(“ApprovePayment”)
3) Append Event
to Event Log
4) Update internal
component state
1) Rehydrate Events
from Event Log
2) Update internal
component state

124. SAD Path - recover from failure
Happy Path
Event
Sourced
Services
5) Run side-effects
(approve the payment)
2) Create new Event
(“PaymentApproved”)
1) Receive and verify Command
(“ApprovePayment”)
3) Append Event
to Event Log
4) Update internal
component state
1) Rehydrate Events
from Event Log
2) Update internal
component state
Memory Image

125. Event Sourcing

126. Event Sourcing
✴ One single Source of Truth with All history

127. Event Sourcing
✴ One single Source of Truth with All history
✴ Allows for Memory Image (Durable In-Memory State)

128. Event Sourcing
✴ One single Source of Truth with All history
✴ Allows for Memory Image (Durable In-Memory State)
✴ Avoids the Object-relational mismatch

129. Event Sourcing
✴ One single Source of Truth with All history
✴ Allows for Memory Image (Durable In-Memory State)
✴ Avoids the Object-relational mismatch
✴ Allows others to Subscribe to state changes

130. Event Sourcing
✴ One single Source of Truth with All history
✴ Allows for Memory Image (Durable In-Memory State)
✴ Avoids the Object-relational mismatch
✴ Allows others to Subscribe to state changes
✴ Has good Mechanical sympathy
(Single Writer Principle etc.)

131. Untangle Your
Read and Write Models
With CQRS

132. CQRS

133. CQRS

134. CQRS
Write Side Model
Commands

135. CQRS
Write Side Model
Write to Event Log
Commands

136. CQRS
Write Side Model
Events
Write to Event Log
Commands

137. CQRS
Write Side Model
Events
Events
Write to Event Log
Commands

138. CQRS
Read Side Model
Write Side Model
Events
Events
Read Data Store
Write to Event Log
Commands

139. CQRS
Read Side Model
Write Side Model
Events
Queries
Events
Read Data Store
Write to Event Log
Commands

140. CQRS
Read Side Model
Write Side Model
Events
Queries
Events
Read Data Store
Write to Event Log
Eventual
Consistency
Commands

141. EventsAllow Us To Manage
Time

142. “Modelling events forces you to have a
temporal focus on what’s going on in the system.
Time becomes a crucial factor of the system.”
- Greg Young
A Decade of DDD, CQRS, Event Sourcing, Greg Young (Presentation from 2016)

143. ✴ Event is a snapshot in time
✴ Event ID is an index for time
✴ Event Log is our full history
The database of Our past
The path to Our present
Event Sourcing
Allows Us To
Model Time

144. Event Sourcing
Allows For
Time Travel

145. Event Sourcing
Allows For
Time Travel

146. Event Sourcing
Allows For
Time Travel
✴Replay the log for historic debugging
✴Replay the log for auditing & traceability
✴Replay the log on failure
✴Replay the log for replication

147. We Can Even Fork the Past
...Or Join Two Distinct Pasts

148. Key Takeaways

149. Key Takeaways
Events-First design helps you to:
✴ reduce risk when modernizing applications
✴ Move Faster towards a Resilient and Scalable architecture
✴ Design autonomous services
✴ Balance Certainty and Uncertainty

150. Key Takeaways
Events-First design helps you to:
✴ reduce risk when modernizing applications
✴ Move Faster towards a Resilient and Scalable architecture
✴ Design autonomous services
✴ Balance Certainty and Uncertainty
Event Logging allows you to:
✴ AVOID CRUD and ORM
✴ take control of your system’s history
✴ time-travel
✴Balance Strong and eventual consistency

151. http://akka.io

152. Learn More
Download my latest book for free at:
bit.ly/reactive-microsystems

153. Watch the video with slide
synchronization on InfoQ.com!
https://www.infoq.com/presentations/
microservices-events-first-design