Łukasz Szymański "CQRS & Event Sourcing at OLX after year on produc...

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CQRS
Event Sourcing at

Łukasz Szymański
Development Team Lead at
@szymanskilukasz
http://szymanskilukasz.github.io/
https://www.linkedin.com/in/s...

CQRS

Command
Query
Responsibility
Segregation

you can use a diﬀerent
model to update
information than the
model you use to read
information
http://martinfowler.com/blik...

new AddCredits($userId, $amount);
Command

Command
new AddCredits($userId, $amount);
Query
new GetCredits($userId);

Event
Sourcing

http://www.martinfowler.com/eaaDev/EventSourcing.html
„
ensures that all
changes to
application state are
stored as a sequ...

Not just can we query
these events, we can
also use the event log
to reconstruct past
states
http://www.martinfowler.com/e...

Commad
Event Model
Event Storage Event Bus
Command Handler
Listeners
Read Storage
Read Model
Query

Commad
Event Model
Event Storage Event Bus
Command Handler
Listeners
Read Storage
Read Model
Query

Commad
Event Model
Event Storage Event Bus
Command Handler
Listeners
Read Storage
Read Model
Query
Critical section - use ...

Commad
Event Model
Event Storage Event Bus
Command Handler
Listeners
Read Storage
Read Model
Query
Critical section - use ...

That’s all

E-WalletCredits; Invoices; Refunds

Wallet Credits Bonus Credits Refund Credits
EXPIRE: X DAYS EXPIRE: X DAYS EXPIRE: X DAYS
INVOICE: YES/NO INVOICE: YES/NO I...

Wallet Credits Bonus Credits Refund Credits
EXPIRE: NEVER EXPIRE: 90 DAYS EXPIRE: 90 DAYS
INVOICE: YES INVOICE: NO INVOICE...

Opis walleta w Polsce
specyﬁkacja
komenty
enventy
Commands

Add Wallet Credits
Add Bonus Credits
Add Refund Credits

Sub Wallet Credits
Sub Bonus Credits
Sub Refund Credits
SubCredits

Opis walleta w Polsce
specyﬁkacja
komenty
enventy
Events

Wallet CreditsWasAdded
Bonus CreditsWasAdded
Refund CreditsWasAdded

Wallet CreditsWasSubtracted
Bonus CreditsWasSubtracted
Refund CreditsWasSubtracted

specyﬁkacja
komenty
enventy
7 commands
6 events

specyﬁkacja
komenty
enventy
WalletCredits BonusCredits RefundCredits
0 0 0

specyﬁkacja
komenty
enventy
WalletCreditsWasAdded +10
WalletCredits BonusCredits RefundCredits
0 0 0

specyﬁkacja
komenty
enventy
WalletCreditsWasAdded +10
WalletCredits BonusCredits RefundCredits
10 0 0

specyﬁkacja
komenty
enventy
WalletCreditsWasAdded +10
WalletCredits BonusCredits RefundCredits
10 0 0
BonusCreditsWasAdded...

specyﬁkacja
komenty
enventy
WalletCreditsWasAdded +10
WalletCredits BonusCredits RefundCredits
10 8 0
BonusCreditsWasAdded...

specyﬁkacja
komenty
enventy
WalletCreditsWasAdded +10
WalletCredits BonusCredits RefundCredits
10 8 0
BonusCreditsWasAdded...

specyﬁkacja
komenty
enventy
WalletCreditsWasAdded +10
WalletCredits BonusCredits RefundCredits
5 8 0
BonusCreditsWasAdded ...

specyﬁkacja
komenty
enventy
WalletCreditsWasAdded +10
WalletCredits BonusCredits RefundCredits
5 8 0
BonusCreditsWasAdded ...

specyﬁkacja
komenty
enventy
WalletCreditsWasAdded +10
WalletCredits BonusCredits RefundCredits
5 8 20
BonusCreditsWasAdded...

specyﬁkacja
komenty
enventy
WalletCreditsWasAdded +10
WalletCredits BonusCredits RefundCredits
5 8 20
BonusCreditsWasAdded...

specyﬁkacja
komenty
enventy
WalletCreditsWasAdded +10
WalletCredits BonusCredits RefundCredits
5 4 20
BonusCreditsWasAdded...

specyﬁkacja
komenty
enventy
WalletCreditsWasAdded +10
WalletCredits BonusCredits RefundCredits
5 4 20
BonusCreditsWasAdded...

specyﬁkacja
komenty
enventy
WalletCreditsWasAdded +10
WalletCredits BonusCredits RefundCredits
5 4 25
BonusCreditsWasAdded...

specyﬁkacja
komenty
enventy
WalletCreditsWasAdded +10
WalletCredits BonusCredits RefundCredits
5 4 25
BonusCreditsWasAdded...

specyﬁkacja
komenty
enventy
WalletCreditsWasAdded +10
WalletCredits BonusCredits RefundCredits
5 4 22
BonusCreditsWasAdded...

specyﬁkacja
komenty
enventy
WalletCreditsWasAdded +10
WalletCredits BonusCredits RefundCredits
5 4 22
BonusCreditsWasAdded...

specyﬁkacja
komenty
enventy
What about other
stuﬀ?

Commad
Event Model
Event Storage Event Bus
Command Handler
Listeners
Read Storage
Read Model
Query
?

Broadway
komponenty
Broadway
https://github.com/qandidate-labs/broadway

Auditing
log whether commands were
successful or not

CommandHandling
Provides the interface and a
CommandHandler base class for
handling events

Domain
domain abstractions of the Broadway
framework including the aggregate root,
domain messages and domain events.

EventDispatcher
The component provides an event
dispatcher interface and a simple
implementation

EventHandling
provides interfaces for an event bus and event
listeners, but also an implementation of a simple
event bus a...

EventSourcing
Provides base classes for event sourced
aggregate roots and entities, an event sourced
repository implementa...

EventStore
implementation based on doctrine/dbal to store
events in a relational database and an in-memory
implementation ...

ReadModel
provides storage for your read models, a
projector implementation to create read models
from event streams and t...

Repository
Component providing an abstraction of the
storage of aggregates.

Serializers
provides a simple serializer interface and a
serializer implementation based on "handwritten"
serializers

Commad
Event Model
Event Storage Event Bus
Command Handler
Listeners
Read Storage
Read Model
Query
Broadway

Commad
Event Model
Event Storage Event Bus
Command Handler
Listeners
Read Storage
Read Model
Query
Broadway
http://getproo...

Adding
Wallet
Credits

Commad
Event Model
Event Storage Event Bus
Command Handler
Listeners
Read Storage
Read Model
Query

Credits
Query

Commad
Event Model
Event Storage Event Bus
Command Handler
Listeners
Read Storage
Read Model
Query

Lessons Learned

Small StepsDon’t try to CQRS/EventSource everything

Keep It Simple
Avoid names like

BonusCreditsWasSubtractedBecauseAdminHitSubtractBonusCr
editsInAdminPanelEvent

Event Storming
is not that easy as it sounds

It’s not RDBM
Remember about analysts

Code changes
Events handle changes to data, what about changes to code?

Implement & Reprocess

SynchronizeRemember about synchronizing read layer - especially after fail

Commad
Event Model
Event Storage Event Bus
Command Handler
Listeners
Read Storage
Read Model
Query
Critical section - use ...

Performance
Snapshots

Performance
Snapshots

Łukasz Szymański
Development Team Lead at
@szymanskilukasz
http://szymanskilukasz.github.io/
https://www.linkedin.com/in/s...

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