Event sourcing anug 20190227

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This document discusses event sourcing and the Marten library. Event sourcing uses events to represent state changes, allowing for concurrency and projections. Events are persisted and describe all state changes to an aggregate. Projections transform events into read models. Marten is a .NET library that uses PostgreSQL as an event store for event sourcing applications. The document also discusses evolving aggregates over time by adding new events and features while maintaining backwards compatibility.

Software

ABOUT
Christian Horsdal Gammelgaard
Independent Consultant
www.horsdal-consult.dk
c.horsdal@gmail.com
@chr_horsdal

EVENT SOURCING
• Events as representation of state
• Concurrency
• Projections
• Evolution
• Domain events and integration events

EVENTS AS THE REPRESENTATION OF STATE
UserAggregate v1
UserAggregate v2
UserAggregate v3
UserAggregate v4
UserCreatedEve
nt
UsernameAddedEv
ent
AddressAddedEve
nt
UsernameUpdatedEve
nt

ALL STATE CHANGES PRODUCE A DOMAIN
EVENT
…so the collection of events describe all state changes

UserCreatedEve
nt
UsernameAddedEv
ent
AddressAddedEve
nt
UsernameUpdatedEve
nt
AggregateId
AggregateId
Username
AggregateId
Address
AggregateId
Username
UserAggregate

CONCURRENCY
• Two threads updating the same aggregate …
• …Version of aggregate is number of applied events
• Optimistic concurrency:
• Only store events with sequence number > aggregate version

PROJECTIONS
• Persisted
• Or on demand
• Immediate
• Or eventual

Aggregate
Event
Event
Event
Event
Event
Event
Query
side
Command
side

EVOLUTION: NEW FEATURES
• New logic in aggregate
• New events
• Projections?

NEW FEATURE: MULTIPLE ADDRESSES
UserAggregate v1
UserAggregate v2
UserAggregate v3
UserAggregate v4
UserCreatedEve
nt
UsernameAddedEv
ent
AddressAddedEve
nt
AddressAddedEvent

UserCreatedEve
nt
UsernameAddedEv
ent
AddressAddedEve
nt
AggregateId
AggregateId
Username
AggregateId
Address
UserAggregate
AddressAddedEve
nt
AggregateId
Address

UserCreatedEve
nt
UsernameAddedEv
ent
AddressAddedEve
nt
AggregateId
AggregateId
Username
AggregateId
Address
User id by address
AddressAddedEve
nt
AggregateId
Address

NEW FEATURE: CONNECT TO FACEBOOK
UserAggregate v1
UserAggregate v2
UserAggregate v3
UserAggregate v4
UserCreatedEve
nt
UsernameAddedEv
ent
AddressAddedEve
nt
ConnectedToFaceboo
k

UserCreatedEve
nt
UsernameAddedEv
ent
AddressAddedEve
nt
ConnectedToFaceboo
k
AggregateId
AggregateId
Username
AggregateId
Address
Facebook id
All users with Facebook

ADDING FIELDS TO EVENTS
• In case more data is given in command
• More context becomes relevant
• Tighter business rules
• New feature: Address type

EVENTS ARE PERSISTED
…code must be backwards compatible

NOT ALL EVENTS ARE EQUAL
DOMAIN EVENTS
• Fine grained
• All state changes
• Internal to a service
INTEGRATION EVENTS
• Course grained
• Signifies business events
• Meant for other services

MARTEN
• .NET event store library
• Uses PostgreSQL

ABOUT ME
Christian Horsdal
Independent Consultant
www.horsdal-consult.dk
c.horsdal@gmail.com
@chr_horsdal

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Event sourcing anug 20190227

1. EVENT SOURCING AND MARTEN

2. ABOUT Christian Horsdal Gammelgaard Independent Consultant www.horsdal-consult.dk c.horsdal@gmail.com @chr_horsdal

3. EVENT SOURCING • Events as representation of state • Concurrency • Projections • Evolution • Domain events and integration events

4. EVENTS AS THE REPRESENTATION OF STATE UserAggregate v1 UserAggregate v2 UserAggregate v3 UserAggregate v4 UserCreatedEve nt UsernameAddedEv ent AddressAddedEve nt UsernameUpdatedEve nt

5. ALL STATE CHANGES PRODUCE A DOMAIN EVENT …so the collection of events describe all state changes

6. UserCreatedEve nt UsernameAddedEv ent AddressAddedEve nt UsernameUpdatedEve nt AggregateId AggregateId Username AggregateId Address AggregateId Username UserAggregate

7. CONCURRENCY • Two threads updating the same aggregate … • …Version of aggregate is number of applied events • Optimistic concurrency: • Only store events with sequence number > aggregate version

8. PROJECTIONS

9. UserCreatedEve nt UsernameAddedEv ent AddressAddedEve nt UsernameUpdatedEve nt AggregateId AggregateId Username AggregateId Address AggregateId Username Username to aggregate id

10. PROJECTIONS • Persisted • Or on demand • Immediate • Or eventual

11. Aggregate Event Event Event Event Event Event Query side Command side

12. EVOLUTION

13. EVOLUTION: NEW FEATURES • New logic in aggregate • New events • Projections?

14. NEW FEATURE: MULTIPLE ADDRESSES UserAggregate v1 UserAggregate v2 UserAggregate v3 UserAggregate v4 UserCreatedEve nt UsernameAddedEv ent AddressAddedEve nt AddressAddedEvent

15. EVENTS ARE PERSISTED

16. UserCreatedEve nt UsernameAddedEv ent AddressAddedEve nt AggregateId AggregateId Username AggregateId Address UserAggregate AddressAddedEve nt AggregateId Address

17. UserCreatedEve nt UsernameAddedEv ent AddressAddedEve nt AggregateId AggregateId Username AggregateId Address User id by address AddressAddedEve nt AggregateId Address

18. NEW FEATURE: CONNECT TO FACEBOOK UserAggregate v1 UserAggregate v2 UserAggregate v3 UserAggregate v4 UserCreatedEve nt UsernameAddedEv ent AddressAddedEve nt ConnectedToFaceboo k

19. ALL STATE CHANGES PRODUCE A DOMAIN EVENT …so the collection of events describe all state changes

20. UserCreatedEve nt UsernameAddedEv ent AddressAddedEve nt ConnectedToFaceboo k AggregateId AggregateId Username AggregateId Address Facebook id All users with Facebook

21. ADDING FIELDS TO EVENTS • In case more data is given in command • More context becomes relevant • Tighter business rules • New feature: Address type

22. EVENTS ARE PERSISTED …code must be backwards compatible

23. UserCreatedEve nt UsernameAddedEv ent AddressAddedEve nt AggregateId AggregateId Username AggregateId Address UserAggregate AddressAddedEve nt AggregateId Address AddressType

24. IGNORING EVENTS UPGRADING EVENTS

25. DOMAIN EVENTS AND INTEGRATION EVENTS

26. NOT ALL EVENTS ARE EQUAL DOMAIN EVENTS • Fine grained • All state changes • Internal to a service INTEGRATION EVENTS • Course grained • Signifies business events • Meant for other services

27. BREAK

28. MARTEN • .NET event store library • Uses PostgreSQL

29. ABOUT ME Christian Horsdal Independent Consultant www.horsdal-consult.dk c.horsdal@gmail.com @chr_horsdal

Event sourcing anug 20190227

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