Designing Events-first Microservices by Jonas Boner In this talk, we will explore the nature of events, what it means to be event-driven, and how we can unleash the power of events and commands by applying an events-first domain-driven design to microservices-based architectures. We will start by developing a solid theoretical understanding of how to design systems of event-driven microservices. Then we will discuss the practical tools and techniques you can use to reap the most benefit from that design, as well as, most importantly, what to avoid along the way. We’ll discuss how an events-first design approach to building microservices can improve the following characteristics over competing techniques: - increase certainty - increase resilience - increase scalability - increase traceability - increase loose coupling - reduce risk Skeptics should definitely attend.

1. Designing
Events First
Microservices
Jonas Bonér
@jboner

2. So, you want to do
microservices?

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

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

5. Events First
Domain Driven
Design

6. “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)

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

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

9. What is an
Event?

10. The Nature of Events

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

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

13. ✴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

14. ✴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

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)
✴ 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

16. Mine the
Facts

18. Event Storming

19. Event Driven Design

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

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

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

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

24. Event Driven Design

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

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

27. ✴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

28. Commands Eventsvs

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

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

31. 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

32. 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)

33. 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

34. 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

35. Let the Events Define the
Bounded Context

36. Event Driven
Services

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

38. 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

39. 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

40. 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

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

42. Publish Facts
To Outside World

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

44. Event Driven
Services

45. Event Driven
Services

46. Event Driven
Services
Command

47. Event Driven
Services
Command

48. Event Driven
Services
Command

49. Event Driven
Services
Command
Event
Event Stream

50. Event Driven
Services
Command
Event
EventEvent
Event Stream

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

56. Event
Stream
Use The

57. Event
Stream
Use The
as the communication fabric

58. Event
Stream
Use The

59. Event
Stream
Use The
as the integration fabric

60. Event
Stream
Use The

61. Event
Stream
Use The
as the replication fabric

62. Event
Stream
Use The

63. Event
Stream
Use The
as the consensus fabric

64. Event
Stream
Use The

65. Event
Stream
Use The
as the Persistence fabric

66. The Problem With
CRUD Services

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

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

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

70. ✴ 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

71. ✴ 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

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

73. STRONG
Consistency
Is the wrong default
In distributed systems

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

75. Eventual
Consistency
We have to rely on

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

77. Embrace
Reality
We Need to

78. Embrace
Reality
We Need to
Not Fight it

79. Information
Has Latency

80. Information Is Always
From the Past

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

82. “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

83. Events Can Lead To Greater
Certainty

84. “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

85. Events Can Help Us Craft
Autonomous Islands
Of Determinism

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

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

88. 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

89. 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

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

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

92. Event Based
Persistence

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

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

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

96. 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

97. 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

98. 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

99. 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

100. 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

101. 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

102. 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

103. 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

104. 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

105. “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)

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

107. Event Logging
The Bedrock

108. Event Sourcing
A Cure For the Cardinal Sin

109. Event
Sourced
Services

110. Happy Path
Event
Sourced
Services

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

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

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

114. 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

115. 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

116. 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

117. 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

118. 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

119. 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

120. Event Sourcing

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

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

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

124. 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

125. 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.)

126. Untangle Your
Read and Write Models
With CQRS

127. CQRS

128. CQRS

129. CQRS
Write Side Model
Commands

130. CQRS
Write Side Model
Write to Event Log
Commands

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

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

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

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

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

136. EventsAllow Us To Manage
Time

137. “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)

138. ✴ 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

139. Event Sourcing
Allows For
Time Travel

140. Event Sourcing
Allows For
Time Travel

141. 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

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

143. Key Takeaways

144. 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

145. 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

146. http://akka.io

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