State, when state?
answering the (some/mes) unanswerable

The “default” layered architecture
(simplified)
API
Applicationservices
Repository/ORM
DomainModels
Client

The “default” layered architecture
CQRSified
Client
QueryAPI
Queryservices
Repository/ORM
CommandAPI
Commandservices
DomainModels

– 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.”

“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/

The “default” layered architecture
CQRSified
Client
Repository/ORM
CommandAPI
Commandservices
Command
Model
QueryAPI
Queryservices
QueryModel
QueryAPI
Queryservices
QueryModel
QueryAPI
Queryservices
QueryModel
QueryAPI
Queryservices
QueryModel

What is in fact “current state”?

Eventual consistency

Distributed architectureDistributed architecture
QueryAPI
Queryservices
QueryModel
QueryAPI
Queryservices
QueryModel
QueryAPI
Queryservices
QueryModel
QueryAPI
Queryservices
QueryModel
ORM/Repository
customerOnboarded: {
aggregate_id: “b766f9d2”,
payload: [name, email]
}
Client
Cursor
Cursor
ORM/RepositoryORM/Repository
customerEmailChanged: {
aggregate_id: “b766f9d2”,
payload: [email]
}
AggregateAggregateAggregate
CommandAPI
Commandservices
Command
Model
CommandAPI
Commandservices
Command
Model
CommandAPI
Commandservices
Command
Model

–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.”

What have we gained?
(1) System can be put it any prior state
(2) Temporal queries
(3) Future needs
(4) TesDng is easier as there is no update and delete
(5) Distributed transacDons possible
(6) Complete state reconstrucDon at any Dme!
(7) Easier replicaDon/backups
(8) Regulatory compliance

Thank you!
Armin Pašalić / @krule

!
Ques=ons?