DDD, CQRS, ES lessons learned

DDD, CQRS & ES: Lessons Learned

(and walls bumped into)

@GitteTitterproq.blogspot.com

One does not simply

Use DDD, CQRS & ES

UI

Commands

Command Bus

Command

Command Handler

Domain Model

Domain Model

Domain Model

Repository

Event Store

Aggregates

Events

Event Bus Event Handler

Data

Thin Data Layer

Query DTO

Events

https://github.com/MarkNijhof/Fohjin

Problem 1

“Every ‘dossier’, ‘scan’, ‘decision’, ... Should have a unique incremental number”

UI

Commands

Command Bus

Command

Command Handler

Domain Model

Domain Model

Domain Model

Repository

Event Store

Aggregates

Events

Event Bus Event Handler

Data

Thin Data Layer

Query DTO

Events Use an autoincrement

column?

Use a ‘DossierNumberGenerator’

DossierNumberGenerator

_highestNumber = 01. GenerateNext

2. DossierNumberGeneratedEvent

3. onDossierNumberGenerated_highestNumber ++;

DossierNumberGenerator

_highestNumber = 0

onDossierNumberGenerated_highestNumber ++;

DossierNumberGeneratedEvent

DossierNumberGeneratedEvent

DossierNumberGeneratedEvent

Replay

Repository.GetById<DossierNumberGenerator>(id);

Problem 2

DomainEvent

Id

EventProviderId

Version

Raised

repo.GetById<EventProvider>(eventproviderId);

Get events based on type, not on eventproviderId?

Migrate those events

WIP

AnotherNewRefactoredAggregate

ANewRefactoredAggregate

anAggregate

SomeEvent

SomeOtherEvent

AndSomeMoreEvent

Migration Process

SomeEvent

SomeOtherEvent

AndSomeMoreEvent

AndAFunkyEvent

https://github.com/vermegi/Eventstream.Migratorhttps://github.com/NEventStore/NEventStore.Migrations

Event Store Event Store

Read the entire event stream Save the entire event stream

UI

Commands

Command Bus

Command

Command Handler

Domain Model

Domain Model

Domain Model

Repository

Event Store

Aggregates

Events

Event Bus Event Handler

Data

Thin Data Layer

Query DTO

Events

Replay needed!

Event Versioning with an EventUpdater

AnAggregate

_somePrivateStuff = 0

onSomeEvent:_somePriveStuff = somevalue;

SomeEvent

AnotherEvent

AndAnotherEvent

Replay

Repository.GetById<AnAggregate>(aggregateId);

SomeEvent

AnotherEvent

AndAnotherEvent

Update

Repository.GetById<AnAggregate>(aggregateId);

SomeUpdatedEvent

AnotherEvent

AndAnotherEvent

AndAnotherEvent

Problem 3

AnAggregateSnapshot

Repository.GetById<AnAggregate>(aggregateId);

_version = 666

SomeEventVersion = 667

AnotherEventVersion = 668

AndAnotherEventVersion = 669

AnAggregate

_somePrivateStuff = 0

onSomeEvent:_somePriveStuff = somevalue;

Replay

Problem 4

Hard to use in debugging

Hard to do data updates

Hard to query

Problem 5

AnAggregate

_somePrivateValue = 0DoSomething

onSomethingHappened_somePrivateValue = something;

DoSomething

SomethingHappenedEvent

WIPhttps://github.com/vermegi/ReplayDomain

UI

Commands

Command Bus

Command

Command Handler

Domain Model

Domain Model

Domain Model

Repository

Event Store

Aggregates

Events

Event Bus Event Handler

Data

Thin Data Layer

Query DTO

Events

Hard to do data updates

Reuse the same logic

Do it functional

Problem 6

And for the good news?

Some final thoughts ...

Thank you!