Let's write some history - DPC 2015

1/92
Event Sourcing
Let's write some history
https://joind.in/14226

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Willem-Jan Zijderveld
@willemjanz
github.com/wjzijderveld



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Rotterdam
http://labs.qandidate.com

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Broadway
Event Sourcing and CQRS library for PHP
github.com/qandidate-labs/broadway

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Event Sourcing

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You are throwing away data!

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Only most
recent state
gets stored

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What was the previous
state?

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Why did the state
change?

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It's hard to keep track of
all the data

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A company has one or
more connected accounts

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A company has one or
more enabled applications

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An account has access to
one or more applications

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Complicated solutions to store and
retrieve the data

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But we manage
Until...

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Boss: Which accounts
received access to
application X in
December?

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"We'll add a timestamp
and we will know"
From that point in time

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With CRUD you lose
information
You only store the last known information

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Revoking access to an application for
an account

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You could lose when the access was
revoked

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You could lose why it got revoked

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You could lose for which application
it was revoked

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You wouldn't know anything anymore

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Let's assume you have
a soft-delete in place

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Boss: I want to know for which
applications access got revoked more
than once
in the last year

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Keep track of revocations
account_id app_id datetime
75623 8 2015-01-01T00:00:00+0000
75623 8 2015-01-27T18:54:18+0000
Past revocations are gone

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How does Event Sourcing
help me with that?

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Store your data in
a different way

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Record what has changed
The resulting state becomes a natural effect

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EventStream

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A serie of facts
CompanyRegistered
AppEnabled
AccountConnected
AccessGrantedToApp

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Been there, done that

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Single source of truth
One source to rule all state
The events cannot lie, it happened, deal with it

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Your version control is
event sourced

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Sure thing, but how?
There isn't one true answer

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Event Sourcing
DDD
CQRS
All 3 are optional, but work very nice together

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Domain Driven Design
The business should be reflected in your code

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Entity
An model with an identity

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Aggregate root
Responsible for keeping a group of entities consistent

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I want to update my
model
How do I do that?

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Record the change

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You normally might do it like this
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But we want to record an
event

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Record the events
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Just one possible implementation

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Save your events in an
eventstore

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Reloading your model

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Domain Message
A message to tell your application what happened

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DomainMessage
Identifier
Sequencenumber
Payload
Timestamp
Metadata

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Identifier + sequence
number

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Payload
The event itself, it tells you what
happened

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Should contain everything
It should only depend on previous events
f
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/
/ c
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r + g
e
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r
s
}

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Timestamp
It tells you when it happened

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Metadata
Descriptive, not structural

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Let's look at some other
events
CompanyRegistered
AppEnabled
AccountConnected
AccessGrantedToApp

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CompanyRegistered
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(
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;
}

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AccountConnected
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}

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AppEnabled
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;
}

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AccessGrantedToApp
/
/ C
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}

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We can retrieve all
AccessGranted events for
December

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Won't this be slow?

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CQRS
Command Query Responsibility Segregation

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Separate your writes and
your reads

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Read Models
Create them from events using projections
Specific read models for specific views in your application

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CompanyRegistration
c
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s C
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/
/ .
.
}
}

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Company Registration Projector
c
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s C
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p
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p
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)
;
}
}

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Use the right tool for the
right job

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Possibilities are endless

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The ability to create multiple read
models
List of company registrations
Graph of all connections between companies and accounts
Creating reports about the amount of revocations

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So.. how does this flow
work in an application?
We'll use Commands

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The write part of CQRS

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Command & CommandHandler
A CommandHandler deals with Commands and
communicates with the Aggregate root

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From Controller
to CommandHandler
c
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'
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;
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View Slide

From CommandHandler
to Aggregate
c
l
a
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s C
o
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f
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$
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:
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e
(
$
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)
;
}
}

View Slide

From Aggregate to Event
p
u
b
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c s
t
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c f
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n
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, $
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w C
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)
;
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r
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p
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b
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p
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d
(
)
;
}

View Slide

From Aggregate
Repository to EventStore
/
/ C
o
m
p
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.
p
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f
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$
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)
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(
)
;
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, $
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p
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h
(
$
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)
;
}

View Slide

From event to read model
c
l
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s C
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r
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c
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p
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n a
p
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p
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v
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(
C
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t
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r
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t $
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,
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e
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s
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e $
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e
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) {
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(
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d
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O
n
(
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;
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v
e
(
$
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o
m
p
a
n
y
)
;
}
}

View Slide

Controller -> CommandBus -> CommandHandler ->
AggregateRoot -> Event -> EventStore -> EventBus -> Projectors
-> Read Model

View Slide

Quite a bit to chew
Try not to reinvent everything yourself

View Slide

Testing
Scenario based testing
Given - When - Then

View Slide

Test your Command
Handler
$
t
h
i
s
-
>
s
c
e
n
a
r
i
o
-
>
g
i
v
e
n
(
[
n
e
w C
o
m
p
a
n
y
R
e
g
i
s
t
e
r
e
d
E
v
e
n
t
(
1
2
3
)
]
)
-
>
w
h
e
n
(
n
e
w E
n
a
b
l
e
A
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F
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r
C
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y
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m
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d
(
4
2
, 1
2
3
)
)
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>
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h
e
n
(
[
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w A
p
p
E
n
a
b
l
e
d
E
v
e
n
t
(
4
2
, 1
2
3
)
]
)
;

View Slide

Or your aggregate
$
t
h
i
s
-
>
s
c
e
n
a
r
i
o
-
>
g
i
v
e
n
(
[
n
e
w C
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m
p
a
n
y
R
e
g
i
s
t
e
r
e
d
E
v
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n
t
(
1
2
3
)
]
)
-
>
w
h
e
n
(
f
u
n
c
t
i
o
n (
$
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o
m
p
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n
y
) {
$
c
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m
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y
-
>
e
n
a
b
l
e
A
p
p
(
4
2
)
;
}
)
-
>
t
h
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n
(
[
n
e
w A
p
p
E
n
a
b
l
e
d
E
v
e
n
t
(
4
2
, 1
2
3
)
]
)
;

View Slide

Don't forget your read
models
$
t
h
i
s
-
>
s
c
e
n
a
r
i
o
-
>
g
i
v
e
n
(
[
]
)
-
>
w
h
e
n
(
n
e
w C
o
m
p
a
n
y
R
e
g
i
s
t
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r
e
d
E
v
e
n
t
(
1
2
3
, '
A
c
m
e I
n
c
'
)
)
-
>
t
h
e
n
(
[
n
e
w C
o
m
p
a
n
y
R
e
g
i
s
t
r
a
t
i
o
n
(
1
2
3
, '
A
c
m
e I
n
c
'
)
]
)
;

View Slide

Time travel is possible!
Use events you recorded to create a new report
multiple years after the fact

View Slide

You made a mistake in a projection?
So what? Correct your projector and recreate your read model
from your event stream

View Slide

Questions?
joind.in/14226
@willemjanz
Freenode: #qandidate

View Slide

More information
http://codebetter.com/gregyoung/2010/02/20/why-use-
event-sourcing/
http://codebetter.com/gregyoung/2010/02/13/cqrs-and-
event-sourcing/
http://martinfowler.com/eaaDev/EventSourcing.html
http://martinfowler.com/bliki/CQRS.html
http://www.axonframework.org/docs/2.3/domain-
modeling.html
http://labs.qandidate.com/

View Slide

Let's write some history - DPC 2015

Let's write some history - DPC 2015

Willem-Jan Zijderveld

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