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Fighting
Bottlenecks 	

with CQRS/ES

Mathias Verraes	

@mathiasverraes	

verraes.net

ResearchGate	

Berlin 	

Nov 30, 2013
Student 	

of 	

Systems

Labourer	

of	

Legacy

Meddler 	

of	

Models
Podcast	

elephantintheroom.io

Blog	

verraes.net

DDDBE	

domaindriven.be
Business Problem	

A Simplified Model	

Command/Query Responsibility Segregation	

Event Sourcing	

Read Model Projections	

Eventual Consistency
Business Problem
Sell a gazillion concert tickets	

to desperate fans

… in a hurry!
Don’t sell more 	

than we have

Don’t crash	

during peaks

Don’t annoy	

our customers
A Simplified Model
Model invariants as a	

Presale Aggregate?
Presale # Tickets
#availableTickets

#reservedTickets

#soldTickets
Model invariants as 	

individual Ticket Aggregates
Individual Ticket status
available

reserved

sold
Command/Query 	

Responsibility Segregation
Client

Remote Façade
Application Services
Domain Model
ORM
Database
Highly cohesive	

Read + Write
Model?
Bottleneck
Client

Remote Façade
Application Services
Domain Model

onsistent
entually C
Ev
Database

ORM
Database
CQRS challenges the assumption	

that reading and writing are 	

supposed to share the same
abstractions.
CQRS challenges the assumption	

that reading and writing are 	

supposed to share the same
models.
CQRS challenges the assumption	

that reading and writing are 	

supposed to share the same
databases.
CQRS challenges the assumption	

that reading and writing are 	

supposed to share the same
applications.
Segregation of read and write	

is a radical form of decoupling.
Client

Remote Façade

Remote Façade

Application Svcs

Application Svcs

Domain Model

Domain Model

ORM

ORM

Database

Database
Radical!
Client

Read
Model

Write
Model
Client
DT
O
o
C

m
m

Write
Model
Events

Read
Model

ds
an
Interact	

with State

Project
State

Guard	

Invariants
Task Based
UI

Polyglot	

Persistence

Event
Store
Event Sourcing
TicketWasReserved [ ticketId, forUser, onDate ]
Ticket state = Ticket history	

!

TicketWasSold [ toUser ]	

TicketWasReserved [ forUser, onDate ]	

ReservationWasReleased [ ]	

TicketWasReserved [ forUser, onDate ]	

TicketWasPrinted [ inPresale ]
Mature domains already work this way	

!

AccountWasDebited [ 20 ]	

AccountWasDebited [ 10 ]	

AccountWasCredited [ 100 ]	

AccountWasOpened
BankAccount 	

{	

debit(amount) {	

guardThatAccountIsNotOverdraft(amount)	

recordThat(	

new AccountWasDebited(amount)	

)	

} 	

}
BankAccount 	

{	

apply(AccountWasDebited event) {	

amount = amount - event.amount	

} 	

}
BankAccount 	

{	

getRecordedEvents() {}	

reconstituteFromHistory(eventHistory) {}	

}
Read Model Projections
TicketWasSold	

TicketWasReserved	

ReservationWasReleased	

TicketWasReserved	

TicketWasPrinted

Ticket	

!
!
TicketWasSold	

TicketWasReserved	

ReservationWasReleased	

TicketWasReserved	

TicketWasPrinted

AvailableTicket	

TotalNumberOfAvailableTickets	

AverageNumberOfTicketsPerBuyer	

AverageNumberOfReleasesPerTicket
Eventual Consistency
Writes are immediately consistent	

Reads are eventually consistent
Many issues stemming from eventual consistency	

can be solved in the user interface
A Ticket can only be sold once. 	

How do we know it’s available?
To buy x tickets:
Get a batch of x*2 PossiblyAvailableTickets	

!

ReserveTicket 	

ack/nack	

loop while acks < 3
More?	

!

Testing	

Context Mapping	

Event Storming	

Process Managers	

Occasionally Connected Systems	

Reporting	

Legacy Integrations	

…
Questions?

@mathiasverraes

verraes.net

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Fighting Bottlencks with CQRS - ResearchGate