The document discusses event sourcing, an approach where all changes to application state are stored as a sequence of events. This allows the state to be reconstructed from the event log and for retroactive changes to adjust the state automatically. Benefits include easier debugging by traveling back in time to exact states, simpler testing without deletes or updates, and backup of events instead of full data dumps. Caveats include a required mindset shift and training along with handling eventual consistency challenges with legacy systems. Real-world considerations include ensuring idempotent projectors and reactors, using aggregate IDs and sequence numbers, and employing SAGAs to handle errors during event application across microservices.

1. Beyond the “current” /ate…
… #me travel to the rescue!
WROCLOVE.RB 18.03.2018.
(and other superpowers)

2. Armin Pašalić / @krule

3. Photo by Inja Pavlić on Unsplash

4. Domain Model
API
Application services
Data Access
Request:
{
“email”:“foo@bar.com”,
“password”:“[SECRET]”
}
Response:
{
“id”: 42,
“email”:“foo@bar.com”
}

5. Domain Model
Command API
Command services
Data Access
Query API
Query services
Command Result: ACCEPTED
LOCATION: /
{
“id”:”0df814c1-2a9b-…“
}
Command: “Onboard Customer”
{
“email”:“foo@bar.com”,
“password”:“[SECRET]”
}
Query: Customer
{ “id”: ”0df814c1-2a9b-…“ }
Query Result: Success
{
“id”:”0df814c1-2a9b-…“,
“email”:“foo@bar.com”
}

6. – Bertrand Meyer / Object-oriented So5ware Construc:on (1988)
“Every method should either be a command that performs an ac7on, or a
query that returns data to the caller, but not both. In other words, asking a
ques7on should not change the answer.”

7. “One ques7on that has been hiAng me is whether or not this is something
that may work as a paBern at the object granularity as opposed to the
endpoint as many of these types of paBerns are relevant to both scopes.”
– Greg Young / hDp://codebeDer.com/gregyoung/2009/08/13/command-query-separa:on/

8. Command Model
Command API
Command services
Data Access
Query API
Query Model
Query API
Query Model
Query API
Query Model

9. roflscale
Super Power Unlocked
Photo by NASA on Unsplash

10. What is in fact “current state”?

11. Eventual consistency

12. Query API
Query Model
CustomerOnboarded: {
aggregate_id: “b766f9d2”,
payload: [name, email, uuid]
}
CustomerEmailChanged: {
aggregate_id: “b766f9d2”,
payload: [email]
}
Data AccessData AccessData Access
Command ModelCommand ModelCommand Model
Command servicesCommand servicesCommand services
Command APICommand APICommand API
Query API
Query Model
Query API
Query Model
73 73 73

13. Query API
Query Model
CustomerOnboarded: {
aggregate_id: “b766f9d2”,
payload: [name, email, uuid]
}
CustomerEmailChanged: {
aggregate_id: “b766f9d2”,
payload: [email]
}
Data AccessData AccessData Access
Command ModelCommand ModelCommand Model
Command servicesCommand servicesCommand services
Command APICommand APICommand API
Query API
Query Model
Query API
Query Model
73
31
73

14. –Mar:n Fowler
hDps://mar:nfowler.com/eaaDev/EventSourcing.html
“Event Sourcing ensures that all changes to applica7on state are stored as a
sequence of events. Not just can we query these events, we can also use the
event log to reconstruct past states, and as a founda7on to automa7cally
adjust the state to cope with retroac7ve changes.”

15. Mme travel
Super Power Unlocked
Photo by Brooke Campbell on Unsplash

16. precogniMon
Super Power Unlocked
Photo by Samuel Zeller on Unsplash

17. total self reconsMtuMon
Super Power Unlocked
Photo by Milivoj Kuhar on Unsplash

18. enchanted charisma
Super Power Unlocked
Photo by Berkay Gumustekin on Unsplash

19. Other benefits
• Debugging on exact state (via time travel)
• Testing w/o update and delete is quite a bit easier
• Backing up on event level is possible and not difficult

20. Caveats
• Mind shift
• Hiring/training
• Eventual consistency (especially dealing with legacy systems)

21. In PracMce / Seats
• Projectors / reactors should be idempotent
• Aggregate scoped sequence number
• Reuse command model aggregate for response when must
• Command uuid is good for deduplication
• Use SAGA to deal with rector synchronisation errors

22. QuesMons?

23. Thank you!
Armin Pašalić / @krule