When building an Event Sourcing system, can we change the past? Can we also forecast “events” in the future? If we want to provide various projections/visions of things that hapenned over the time, what are the consequences and trade-offs on our code? All these questions will be answered during this talk. But moreover, I'll tell you our learning story about Bi-temporal Event Sourcing. Yet another illustration of the power of modeling, sketches and concrete examples when we need to understand each others.

1. As time goes by…
Episode 2: technical challenges of bi-temporal Event Sourcing
@tpierrain
t

2. If you want
to win…
Tweet like hell on…
#newCrafts

4. REQUIREMENTS
—

5. REQUIREMENTS
—
Understand their former decisions

6. REQUIREMENTS
—
Understand their former decisions
Fix their perception of the world
retroactively

7. REQUIREMENTS
—
Understand their former decisions
Fix their perception of the world
retroactively
(Re)produce reports in and from the past

8. REQUIREMENTS
—
Understand their former decisions
Fix their perception of the world
retroactively
(Re)produce reports in and from the past
Forecast ”events” in the future

9. REQUIREMENTS
—
Understand their former decisions
Fix their perception of the world
retroactively
(Re)produce reports in and from the past
Forecast ”events” in the future
Strong audit trail

10. They needed a time machine

11. Event Sourcing baby!

12. Event Sourcing 101

13. Remember your banking account?
Event Sourcing

14. Event Sourcing

15. Final state of the world
All changes since
the beginning

16. All changes since the beginning
=
A list of events

17. Event [ɪˈvent]
Describes a fact, something that have happened.
Usually expressed in a past-tense form.
(e.g.: TalkStarted)

18. public class Fund : EventSourcedAggregate
{
public Guid Id { get; private set; }
public string Name { get; private set; }
public Fund(IEnumerable<Event> events)
{
foreach(var evt in events)
{
Apply(evt);
}
}
private Apply(FundRenamed event)
{
this.Name = event.FundNewName;
}
public IEnumerable<Event> Rename(string newName)
{
!// business logic and invariants checks HERE
!// Raise event(s)
return new[] { new FundRenamed(this.Id, newName) };
}
}
Event-Sourced business objects

19. public class Fund : EventSourcedAggregate
{
public Guid Id { get; private set; }
public string Name { get; private set; }
public Fund(IEnumerable<Event> events)
{
foreach(var evt in events)
{
Apply(evt);
}
}
private Apply(FundRenamed event)
{
this.Name = event.FundNewName;
}
public IEnumerable<Event> Rename(string newName)
{
!// business logic and invariants checks HERE
!// Raise event(s)
return new[] { new FundRenamed(this.Id, newName) };
}
}
Properties / Members
Event-Sourced business objects

20. public class Fund : EventSourcedAggregate
{
public Guid Id { get; private set; }
public string Name { get; private set; }
public Fund(IEnumerable<Event> events)
{
foreach(var evt in events)
{
Apply(evt);
}
}
private Apply(FundRenamed event)
{
this.Name = event.FundNewName;
}
public IEnumerable<Event> Rename(string newName)
{
!// business logic and invariants checks HERE
!// Raise event(s)
return new[] { new FundRenamed(this.Id, newName) };
}
}
Evolution functions
Change state
Event-Sourced business objects

21. public class Fund : EventSourcedAggregate
{
public Guid Id { get; private set; }
public string Name { get; private set; }
public Fund(IEnumerable<Event> events)
{
foreach(var evt in events)
{
Apply(evt);
}
}
private Apply(FundRenamed event)
{
this.Name = event.FundNewName;
}
public IEnumerable<Event> Rename(string newName)
{
!// business logic and invariants checks HERE
!// Raise event(s)
return new[] { new FundRenamed(this.Id, newName) };
}
}
Constructor
Change state
Event-Sourced business objects

22. public class Fund : EventSourcedAggregate
{
public Guid Id { get; private set; }
public string Name { get; private set; }
public Fund(IEnumerable<Event> events)
{
foreach(var evt in events)
{
Apply(evt);
}
}
private Apply(FundRenamed event)
{
this.Name = event.FundNewName;
}
public IEnumerable<Event> Rename(string newName)
{
!// business logic and invariants checks HERE
!// Raise event(s)
return new[] { new FundRenamed(this.Id, newName) };
}
}
Decision functions
Don’t change state
Event-Sourced business objects

23. public class Fund : EventSourcedAggregate
{
public Guid Id { get; private set; }
public string Name { get; private set; }
public Fund(IEnumerable<Event> events)
{
foreach(var evt in events)
{
Apply(evt);
}
}
private Apply(FundRenamed event)
{
this.Name = event.FundNewName;
}
public IEnumerable<Event> Rename(string newName)
{
!// business logic and invariants checks HERE
!// Raise event(s)
return new[] { new FundRenamed(this.Id, newName) };
}
}
Event-Sourced business objects

24. public class Fund : EventSourcedAggregate
{
public Guid Id { get; private set; }
public string Name { get; private set; }
public Fund(IEnumerable<Event> events)
{
foreach(var evt in events)
{
Apply(evt);
}
}
private Apply(FundRenamed event)
{
this.Name = event.FundNewName;
}
public IEnumerable<Event> Rename(string newName)
{
!// business logic and invariants checks HERE
!// Raise event(s)
return new[] { new FundRenamed(this.Id, newName) };
}
}
Decision functions
Don’t change state
Event-Sourced business objects

25. Event-Sourced business objects
public class Fund : EventSourcedAggregate
{
public Guid Id { get; private set; }
public string Name { get; private set; }
public Fund(IEnumerable<Event> events)
{
foreach(var evt in events)
{
Apply(evt);
}
}
private Apply(FundRenamed event)
{
this.Name = event.FundNewName;
}
public IEnumerable<Event> Rename(string newName)
{
!// business logic and invariants checks HERE
!// Raise event(s)
return new[] { new FundRenamed(this.Id, newName) };
}
}
Decision functions
Don’t change state
Evolution functions
Change state

26. Projection [prəˈjekSHən]
Deriving a current state from a stream of events

27. Fund myFund = fundRepository.GetById(fundId, projectedAt);
Event-Sourcing in motion
Guid
timestamp

28. Fund myFund = fundRepository.GetById(fundId, projectedAt);
Now
b7f3ddfb-2cb5….
Event-Sourcing in motion

29. t
Fund
id:
Name:
Creation date:
Management Company:
Shares:
Event-Sourcing in motion

30. t
FundCreated
“super yield ”
Jan 1st
2016
Fund
id: b7f3ddfb -2cb5….
Name: super yield
Creation date: Jan 1st
2016
Management Company:
Shares:
Event-Sourcing in motion

31. t
Jan 1st
2016
ManagementCompanyAssigned
“BNP”
Jan 30th
2016
Fund
id: b7f3ddfb -2cb5….
Name: super yield
Creation date: Jan 1st
2016
Management Company:
BNP
Shares:
Event-Sourcing in motion

32. t
Jan 1st
2016
Jan 30th
2016
FundRenamed
“Yield 2k22”
Apr 1st
2016
Fund
id: b7f3ddfb -2cb5….
Name: Yield 2k22
Creation date: Jan 1st
2016
Management Company:
BNP
Shares:
Event-Sourcing in motion

33. t
Jan 1st
2016
Jan 30th
2016
Apr 1st
2016
ShareAdded
“Part D”
Jun 2nd
2016
Fund
id: b7f3ddfb -2cb5….
Name: Yield 2k22
Creation date: Jan 1st
2016
Management Company:
BNP
Shares:
Part D
Event-Sourcing in motion

34. Fund
id: b7f3ddfb -2cb5….
Name: Yield 2k22
Creation date: Jan 1st
2016
Management Company:
BNP
Shares:
Part D
End of
Stream
>> <<t
Jan 1st
2016
Jan 30th
2016
Apr 1st
2016
Jun 2nd
2016
Event-Sourcing in motion

35. Projected at now
Ready to work
Fund
id: b7f3ddfb -2cb5….
Name: Yield 2k22
Creation date: Jan 1st
2016
Management Company:
BNP
Shares:
Part D

36. The beauty of it
Pick the projection date you want ;-)

37. Project same Fund at Apr 1st 2016
(same stream of events)
Event-Sourcing in motion

38. t
Fund
id:
Name:
Creation date:
Management Company:
Shares:
Event-Sourcing in motion

39. t
FundCreated
“super yield ”
Jan 1st
2016
Fund
id: b7f3ddfb -2cb5….
Name: super yield
Creation date: Jan 1st
2016
Management Company:
Shares:
Event-Sourcing in motion

40. t
Jan 1st
2016
ManagementCompanyAssigned
“BNP”
Jan 30th
2016
Fund
id: b7f3ddfb -2cb5….
Name: super yield
Creation date: Jan 1st
2016
Management Company:
BNP
Shares:
Event-Sourcing in motion

41. t
Jan 1st
2016
Jan 30th
2016
FundRenamed
“Yield 2k22”
Apr 1st
2016
Fund
id: b7f3ddfb -2cb5….
Name: Yield 2k22
Creation date: Jan 1st
2016
Management Company:
BNP
Shares:
Event-Sourcing in motion

42. t
Jan 1st
2016
Jan 30th
2016
Apr 1st
2016
Fund
id: b7f3ddfb -2cb5….
Name: Yield 2k22
Creation date: Jan 1st
2016
Management Company:
BNP
Shares:
We’re
done here
;-)
>> <<
Event-Sourcing in motion

43. Command [kəˈmɑːnd]
Triggers an action. Modify the state of the system
e.g.: RenameFund

44. Commands are used to take decisions and to raise events.

45. Commands are used to take decisions and to raise events.
Command
(RenameFund)
Aggregate
(Fund X)

46. Commands are used to take decisions and to raise events.
Command
(RenameFund)
Events
(FundRenamed)
Events are used to restore/change the state of our aggregates.
Aggregate
(Fund X)

47. so far so good?
😉

48. Specificities
of bi-temporal
event sourcing
& Challenges

49. Challenge #1
How to change what can’t be changed?

50. Challenge #1
- Events are immutable (facts)
- We save them in append-only streams (Event Store)
t
Q: How to retroactively change former mistaken events?
Q: How to insert forgotten events in the past (in the middle of
a stream)?

51. we don’t change events
we just add
more events

52. we don’t change events
we just add
more events

53. “One more thing…”

54. “One more thing…
All our events have…”

55. createdAt
validFrom !-> when it applies
2 timestamps

56. createdAt
validFrom !-> when it applies
2 timestamps
( bi-temporal)

57. Types of events
validFrom
createdAt
Classic event

58. Types of events
validFrom
createdAt
Classic event
validFrom createdAt
Retroactive event

59. Thus we can hide former events

60. Thus we can hide former events
t
Feb 1st
2016
Feb 3rd
2016
Feb 5th
2016
“new naaame”
“initial name”

61. Thus we can hide former events
t
Feb 1st
2016
Feb 3rd
2016
Feb 5th
2016
“new naaame”
“initial name”
“fixed name ;-)”

62. Thus we can hide former events
t
Feb 1st
2016
Feb 3rd
2016
Feb 5th
2016
“new naaame”
“initial name”
“fixed name ;-)”

64. Challenge #2
Forecasting “facts”?!?

65. Challenge #2
- Event Store stores “Events” ;-)
- Events are things that have already happened (facts)
t
Q: How to store things that have to happen in an Event Store?

67. Let’s store ”a Command (X) has just been Scheduled” events

68. validFromcreatedAt
“Command Scheduled” event“Command Scheduled” event
RenameFund
(Cmd)

69. validFromcreatedAt
CommandScheduled<RenameFund> event
RenameFund
(Cmd)

70. 3 Types of events

71. 3 Types of events
validFrom
createdAt
Classic event

72. 3 Types of events
validFrom
createdAt
Classic event
validFrom createdAt
Retroactive event

73. 3 Types of events
validFrom
createdAt
Classic event
validFrom createdAt
Retroactive event
validFromcreatedAt
“Command Scheduled” event
C

74. Our model

76. Challenge #3
How to project unsorted streams of
events?

77. Challenge #3
- With retroactive events, we have unsorted streams of events
- Some events may hide others
- Past may change at any time. Future too
t
Q: How to implement projections in such environment?

78. Let’s review some options

79. 1 Forward-only projections with filters

80. 1 Forward-only projections with filters
2 Sort - then - Apply events

81. 1 Forward-only projections with filters
2 Sort - then - Apply events
3 Rebased Timelines with branches

82. 1 Forward-only projections with filters
2 Sort - then - Apply events
3 Rebased Timelines with branches
too complicated

83. 1 Forward-only projections with filters
2 Sort - then - Apply events
3 Rebased Timelines with branches
too complicated
not compliant with bigstreams of events

84. 2 Sort - then - Apply events
3 Rebased Timelines with branches
too complicated
not compliant with bigstreams of events
1 Forward-only projections with filters

86. Challenge #4
How to provide multiple visions of the
same reality?
(operational - audit-trail)

87. Challenge #4
- 1 stream of events per aggregate instance
- We need to provide both an operational mode and an audit mode
t
Q: How to provide various points of view in our reality?

88. Thank you Greg!

89. 2 projection modes

90. / As AtAs Of "

91. / As AtAs Of
As we should
have known
"

92. / As AtAs Of
As we should
have known
Patches inside
"

93. /As Of
As we should
have known
Patches inside
As At
As we knew
(at the time)
"

94. / As At
As we knew
(at the time)
Audit trails
As Of
As we should
have known
Patches inside
"

95. / As At
As we knew
(at the time)
Audit trails
As Of
As we should
have known
Patches inside
"

96. Since we have a model,
let’s find some exemples

98. Our main usages (patches included)
As Of - as we should have known

99. As Of - as we should have known
Imagine a stream of 3 events for a Fund

100. 1.Let’s see how those events have been
saved over the time

101. As Of - as we should have known
t
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017

102. As Of - as we should have known
t
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
“First name”

103. As Of - as we should have known
t
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
“First name” “official name”

104. As Of - as we should have known
t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
“First name” “official name”
“patched name”

105. 2. Let’s do some As Of projections

106. event?
👎
Do NOT
Apply
👍
Apply
or

107. As Of - as we should have known
Name:
???
Projection date
Jun 1st 2017

108. As Of - as we should have known
t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
Name:
???
Projection date
Jun 1st 2017

109. As Of - as we should have known
t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
“First name” “official name”
“patched name”
Name:
???
Projection date
Jun 1st 2017

110. As Of - as we should have known
t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
“First name”
???
Name:
Projection date
Jun 1st 2017

111. As Of - as we should have known
t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
“First name”
👍
Apply
Name:
“First name”
-pending-
Projection date
Jun 1st 2017

112. As Of - as we should have known
t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
“official name”“First name”
Projection date
Jun 1st 2017
???
Name:
“First name”
-pending-

113. As Of - as we should have known
t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
👍
Apply
Name:
“Official name”
-pending-
“official name”“First name”
Projection date
Jun 1st 2017

114. As Of - as we should have known
t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
Name:
“Official name”
-pending-
“official name”“First name”
“patched name”
Projection date
Jun 1st 2017
???

115. As Of - as we should have known
t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
👎
Do NOT
Apply
Name:
“Official name”
-pending-
“official name”“First name”
“patched name”
Projection date
Jun 1st 2017

116. As Of - as we should have known
t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
“First name” “official name”
“patched name”
Name:
“official name”
Projection date
Jun 1st 2017

117. Same events / another projection date

118. As Of - as we should have known
t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
Name:
???
Projection date
Apr 1st 2016

119. As Of - as we should have known
t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
“First name” “official name”
“patched name”
Name:
???
Projection date
Apr 1st 2016

120. As Of - as we should have known
t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
“First name”
Name:
“First name”
-pending-
Projection date
Apr 1st 2016
???

121. As Of - as we should have known
t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
“First name”
👍
Apply
Name:
“First name”
-pending-
Projection date
Apr 1st 2016

122. As Of - as we should have known
t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
Name:
“First name”
-pending-
“official name”“First name”
Projection date
Apr 1st 2016
???

123. As Of - as we should have known
t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
Name:
“First name”
-pending-
“official name”“First name”
👎
Do NOT
Apply
Projection date
Apr 1st 2016

124. As Of - as we should have known
t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
Name:
“patched name”
-pending-
“official name”“First name”
“patched name”
Projection date
Apr 1st 2016
???

125. As Of - as we should have known
t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
Name:
“patched name”
-pending-
“official name”“First name”
“patched name”
👍
Apply
Projection date
Apr 1st 2016

126. As Of - as we should have known
t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
“First name” “official name”
“patched name”
Name:
“patched name”
Projection date
Apr 1st 2016

127. "

128. As At - as we knew (at the time) "
audit-trail use cases

129. t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
Name:
???
Projection date
Apr 1st 2016
As At - as we knew (at the time) "

130. t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
“First name” “official name”
“patched name”
Name:
???
Projection date
Apr 1st 2016
As At - as we knew (at the time) "

131. t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
“First name”
Name:
Projection date
Apr 1st 2016
As At - as we knew (at the time) "
???

132. t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
“First name”
👍
Apply
Name:
“First name”
-pending-
Projection date
Apr 1st 2016
As At - as we knew (at the time) "

133. t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
Name:
“First name”
-pending-
“official name”“First name”
Projection date
Apr 1st 2016
As At - as we knew (at the time) "
???

134. t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
Name:
“First name”
-pending-
“official name”“First name”
👎
Do NOT
Apply
Projection date
Apr 1st 2016
As At - as we knew (at the time) "

135. t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
“official name”“First name”
Projection date
Apr 1st 2016
As At - as we knew (at the time) "
“patched name”
Name:
“First name”
-pending-
???

136. t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
“official name”“First name”
Projection date
Apr 1st 2016
As At - as we knew (at the time) "
👎
Do NOT
Apply
“patched name”
Name:
“First name”
-pending-

137. t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
“First name” “official name”
“patched name”
Name:
“First name”
Projection date
Apr 1st 2016
As At - as we knew (at the time) "

138. Same events / another projection date

139. t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
Name:
???
Projection date
Jun 1st 2017
As At - as we knew (at the time) "

140. t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
“First name” “official name”
“patched name”
Name:
???
As At - as we knew (at the time) "
Projection date
Jun 1st 2017

141. t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
“First name”
Name:
As At - as we knew (at the time) "
???
Projection date
Jun 1st 2017

142. t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
“First name”
👍
Apply
Name:
“First name”
-pending-
As At - as we knew (at the time) "
Projection date
Jun 1st 2017

143. t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
Name:
“First name”
-pending-
“official name”“First name”
As At - as we knew (at the time) "
???
Projection date
Jun 1st 2017

144. t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
Name:
“official name”
-pending-
“official name”“First name”
As At - as we knew (at the time) "
Projection date
Jun 1st 2017
👍
Apply

145. t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
“official name”“First name”
As At - as we knew (at the time) "
“patched name”
???
Projection date
Jun 1st 2017
Name:
“official name”
-pending-

146. t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
“official name”“First name”
As At - as we knew (at the time) "
👎
Do NOT
Apply
“patched name”
Projection date
Jun 1st 2017
Name:
“official name”
-pending-

147. As At - as we knew (at the time) "
Projection date
Jun 1st 2017
Name:
“official name”t
Apr 1st
2016
Jan 1st
2016
Jan 1st
2017
Jun 1st
2017
“First name” “official name”
“patched name”

148. Still with me?

150. Challenge #5
How to produce a report for March 1st
like if we had made it June 1st?

151. Challenge #5
- We don’t want to suffer from hindsight bias
- We need to understand how we had perceived the world in the past
- We should not see some events that happened after a viewpoint
t

152. Q: How to produce a report for March 1st, like if we had
made it June 1st?

153. Name:
???
t
Q: How to produce a report for March 1st, like if we had
made it June 1st?
???
Feb 2nd
2016
Feb 1st
2016
Feb 28th
2016
March 1st
2016
Feb 3rd
2016
Jul 1st
2016
Jun 1st
2016

154. t
“name 1”
“name 2”
“name 5”
Feb 2nd
2016
Feb 1st
2016
Feb 28th
2016
March 1st
2016
Feb 3rd
2016
Jul 1st
2016
Jun 1st
2016
Q: How to produce a report for March 1st, like if we had
made it June 1st? Name:
???
???

155. t
Feb 2nd
2016
Feb 1st
2016
Feb 28th
2016
March 1st
2016
“name 1”
“name 2”
“name 5”
Feb 3rd
2016
Jul 1st
2016
Jun 1st
2016
Q: How to produce a report for March 1st, like if we had
made it June 1st? Name:
???
Date of the
report
Report
target date

156. t
Feb 2nd
2016
Feb 1st
2016
Feb 28th
2016
March 1st
2016
“name 1”
“name 2”
“name 5”
Feb 3rd
2016
Jul 1st
2016
Jun 1st
2016
Q: How to produce a report for March 1st, like if we had
made it June 1st?
Date of the
report
Report
target date
Apply all events created
before June 1st (viewPoint)
Don’t apply events created
after June 1st (viewPoint)

157. t
Feb 2nd
2016
Feb 1st
2016
Feb 28th
2016
March 1st
2016
“name 5”
Feb 3rd
2016
Jul 1st
2016
Jun 1st
2016
Q: How to produce a report for March 1st, like if we had
made it June 1st?
Date of the
report
Report
target date
Apply all events created
before June 1st (viewPoint)
“name 1”
“name 2”
Don’t apply events created
after June 1st (viewPoint)

158. t
Feb 2nd
2016
Feb 1st
2016
Feb 28th
2016
March 1st
2016
“name 5”
Feb 3rd
2016
Jul 1st
2016
Jun 1st
2016
Q: How to produce a report for March 1st, like if we had
made it June 1st?
Date of the
report
Report
target date
Apply all events created
before June 1st (viewPoint)
“name 1”
“name 2”
Don’t apply events created
after June 1st (viewPoint)

159. which bring us
2 new concepts

160. Viewpoint AsOfUntil
projection mode

161. Fund myFund = fundRepository.GetAsOUntil(fundId, projectedAt, viewPoint);
AsOf Until
Guid
March 1st
2016
June 1st
2016

162. In code we trust

169. ?
Name1?
Name2?
Name5?

172. Challenge #6
Retroactive commands
vs. Retroactive events?

173. Challenge #6
- Events are facts
- Commands are used to take decisions and to raise events
- Command may fail
t
Q: What should we do with ’retroactive’?

175. a) Apply a retroactive command to a aggregate
projected in the past? (may fail)

176. a) Apply a retroactive command to a aggregate
projected in the past? (may fail)
b) Create an event in the past for an aggregate?

177. a) Apply a retroactive command to a aggregate
projected in the past? (may fail)
b) Create an event in the past for an aggregate?
c) The answer C

178. c) There is no silver bullet

179. But we’ve mostly picked the option
a) Apply a retroactive command to a aggregate
projected in the past? (may fail)
so far

181. Challenge #7
Is U.T.C. enough?

182. tChallenge #7
- We store instants in our Event Store (createdAt, validFrom timestamps)
- Some events are applied on an entire “day” (e.g. June 8th)

183. t
Q: Which timestamps should we pick to describe a “day”?
Challenge #7
- We store instants in our Event Store (createdAt, validFrom timestamps)
- Some events are applied on an entire “day” (e.g. June 8th)

184. t
Q: Which timestamps should we pick to describe a “day”?
Q: Is UTC enough to store all instants?
Challenge #7
- We store instants in our Event Store (createdAt, validFrom timestamps)
- Some events are applied on an entire “day” (e.g. June 8th)

185. UTC [Utéçé;-)]
Universal Time Coordinated is the primary time standard by which the world
regulates clocks and time. It does not observe daylight saving time.

186. Instant [ˈÇaAriiiv]
Describes “when something happened”. Can then be interpreted in a
particular time zone and calendar system.
e.g.: 2018-05-18T16:40:35Z (Z means UTC, i.e. +00h)

187. Istanbul (+03)
What does June 8th even mean?

188. What does June 8th even mean?
Paris (+02)
Istanbul (+03)

189. UTC (+00)
Paris (+02)
Istanbul (+03)
What does June 8th even mean?

190. UTC (+00)
Jun 8th 2018
00h (+ 03h)
Paris (+02)
Istanbul (+03)
Jun 8th 2018
23h59 (+ 03h)
What does June 8th even mean?

191. UTC (+00)
Jun 8th 2018
00h (+ 03h)
Jun 7th 2018
21h (+ 00h)
Paris (+02)
Istanbul (+03)
Jun 8th 2018
23h59 (+ 03h)
Jun 8th 2018
21h59 (+ 00h)
What does June 8th even mean?

192. UTC (+00)
Paris (+02)
Istanbul (+03)
Jun 8th 2018
00h (+ 02h)
Jun 8th 2018
23h59 (+ 02h)
Jun 8th 2018
23h59 (+ 03h)
Jun 8th 2018
21h59 (+ 00h)
Jun 8th 2018
00h (+ 03h)
Jun 7th 2018
21h (+ 00h)
What does June 8th even mean?

193. UTC (+00)
Paris (+02)
Istanbul (+03)
Jun 8th 2018
00h (+ 02h)
Jun 7th 2018
22h (+ 00h)
Jun 8th 2018
23h59 (+ 02h)
Jun 8th 2018
21h59 (+ 02h)
Jun 8th 2018
23h59 (+ 03h)
Jun 8th 2018
21h59 (+ 00h)
Jun 8th 2018
00h (+ 03h)
Jun 7th 2018
21h (+ 00h)
What does June 8th even mean?

194. UTC (+00)
Paris (+02)
Istanbul (+03)
Jun 7th 2018
22h (+ 00h)
Jun 8th 2018
21h59 (+ 02h)
Jun 8th 2018
21h59 (+ 00h)
Jun 7th 2018
21h (+ 00h)
Event 1 ?
Should event 1 be involved in a June 8th projection?

195. UTC (+00)
Paris (+02)
Istanbul (+03)
Jun 7th 2018
22h (+ 00h)
Jun 8th 2018
21h59 (+ 02h)
Jun 8th 2018
21h59 (+ 00h)
Jun 7th 2018
21h (+ 00h)
Event 1?
only if it’s June 8th - Paris time

196. June 8th Paris !!= June 8th Istanbul

197. UTC (+00)
Paris (+02)
Istanbul (+03)
Jun 7th 2018
22h (+ 00h)
Jun 8th 2018
21h59 (+ 02h)
Jun 8th 2018
21h59 (+ 00h)
Jun 7th 2018
21h (+ 00h)
June 8th Paris time !!= June 8th Istanbul time

198. UTC Time is mandatory to store & compare events
UTC (+00)
Paris (+02)
Istanbul (+03)
Jun 7th 2018
22h (+ 00h)
Jun 8th 2018
21h59 (+ 02h)
Jun 8th 2018
21h59 (+ 00h)
Jun 7th 2018
21h (+ 00h)

199. UTC is mandatory but not enough
UTC (+00)
Paris (+02)
Istanbul (+03)
Jun 7th 2018
22h (+ 00h)
Jun 8th 2018
21h59 (+
02h)
Jun 8th 2018
21h59 (+
00h)
Jun 7th 2018
21h (+ 00h)

200. We need UTC Time + time zone of the event

201. time zone of the event?

202. Time zone of the action
that originated the event?
time zone of the event?

203. Time zone of the action
that originated the event?
Time zone associated with
the topic (i.e. a Fund)?
time zone of the event?

205. - Wrap up -

206. Discovering the unknown

207. Without concrete examples and a
model, you’re dead.

208. And more than ever…

209. y.a.g.n.i
you ain’t gonna need it

211. Would you
d a r e t o
destroy it?

212. OVER
ENGINEERING
is an embarrassment
to ourselves

213. Get prepared
to Design
BREAK
THROUGH

214. Thank you!

215. (White paper ongoing)
@tpierrain (use case driven)
Thank you!

216. And the
winner is…

217. ?

218. 7 SPEAKERS, 5 TALKS ORIGINAUX DE 15-20 MIN
EN QUOI NOUS INTÉRESSER AUX PROBLÈMES DU
MÉTIER NOUS A RENDU PLUS EFFICACE
LE MARDI 26 JUIN, CHEZ
DOMAIN FROM THE TRENCHES

219. Appendix

220. As At #2

221. As At #2 - as we knew at the time

222. As At #2 - as we knew at the time

223. As At #2 - as we knew at the time
“Patched name” will be <<discarded>> by the
following FundRenamed event

224. As Of #2

225. As Of #2 - as we should have known
“Patched name” is not <<discarded>> by the
following FundRenamed event (“official name”)

226. As Of #2 - as we should have known
But then it is replaced by the new one
( “OfficialName”)

229. You want to be ahead of what is done in software?
You understand there are bigger issues than just coding
We are looking for people who are daring, curious, sharing,
but who especially like team work
recruits