CQRS & EVENT SOURCING
IN THE WILD
MICHIEL ROOK

➤ Java, PHP & Scala
developer
➤ Consultant, trainer,
speaker
➤ Dutch Web Alliance
➤ make.io
➤ Maintainer of Phing
➤ @michieltcs

YOU

RAISE YOUR HAND
IF YOU HAVE

heard about CQRS / Event Sourcing
RAISE YOUR HAND
IF YOU HAVE

heard about CQRS / Event Sourcing
followed a tutorial, built a hobby project
RAISE YOUR HAND
IF YOU HAVE

heard about CQRS / Event Sourcing
followed a tutorial, built a hobby project
used it in production
RAISE YOUR HAND
IF YOU HAVE

TOPICS
➤ Quick recap
➤ Replays and rebuilds
➤ Event versioning
➤ Concurrency
➤ Scale

QUICK RECAP

' Event Sourcing ensures that all
changes to application state are stored
as a sequence of events.
-Martin Fowler

ACTIVE RECORD VS. EVENT SOURCING
Account
number
Balance
12345678 €€ 50,00
... ...
Money Withdrawn
Account number 12345678
Amount €€ 50,00
Money Deposited
Account number 12345678
Amount €€ 100,00
Account Created
Account number 12345678

COMMANDS TO EVENTS
Deposit Money
Account number 12345678
Amount €€ 100,00
Money Deposited
Account number 12345678
Amount €€ 100,00
class DepositMoney {
public $accountNumber;
public $amount;
}
class MoneyDeposited {
public $accountNumber;
public $amount;
public $timestamp;
}
function depositMoney(DepositMoney $command) {
$this->apply(new MoneyDeposited($command->accountNumber,
$command->amount, date()));
}

AGGREGATES
class BankAccount {
public $accountNumber;
public $balance;
// command handlers...
public function applyAccountCreated(AccountCreated $event) {
$this->accountNumber = $event->accountNumber;
$this->balance = 0;
}
public function applyMoneyDeposited(MoneyDeposited $event) {
$this->balance += $event->amount;
}
public function applyMoneyWithdrawn(MoneyWithdrawn $event) {
$this->balance -= $event->amount;
}
}

AGGREGATE STATE
Account
number
Balance
12345678 €€ 0,00
Money Withdrawn
Account number 12345678
Amount €€ 50,00
Money Deposited
Account number 12345678
Amount €€ 100,00
Account Created
Account number 56789012
Account
number
Balance
12345678 €€ 100,00
Account
number
Balance
12345678 €€ 50,00

CQRS
➤ Command Query Responsibility Segregation
➤ Separate writes (events) and reads (UI)
➤ Optimize for reads!

Domain
Event Bus
Event Handlers
Command
Repository
Data Layer
Database Database
Event
Store
commands
events
events
queries DTOs
Aggregates

PROS AND CONS
➤ Domain fit
➤ Testing
➤ Audit trail
➤ Scalability
➤ Complexity
➤ Library support / maturity

DISCLAIMER

REPLAYS AND REBUILDS

PROJECTIONS AND READ MODELS
User Registered
User Registered
User Unregistered Number of active users?

PROJECTIONS AND READ MODELS
User Registered
User Deactivated
User Reactivated
Number of active users?
User Registered
User Unregistered

PROJECTIONS AND READ MODELS
User Registered Event Handler
Number of
active users
+1
User Unregistered Event Handler
Number of
active users
-1

PROJECTIONS AND READ MODELS
Events Event Handler(s) Storage

ELASTICSEARCH
class CompanyRegistered {
public $companyId;
public $name;
public $street;
public $city;
}
function handleCompanyRegistered(CompanyRegistered $event) {
$this->elasticsearch->index([
'index' => 'companies',
'type' => 'company',
'id' => $event->companyId,
'body' => [
'name' => $event->name,
'address' => [
'street' => $event->street,
'city' => $event->city
]
]
]);
}

READ MODEL UPDATES
➤ New type
➤ New structure
➤ Based on existing events
➤ Generate from scratch?

REBUILDING
Stop
application
Remove
old read
model
Loop over
events
Apply to
read model
Start
application

ZERO DOWNTIME
Loop over
existing
events
Apply to
new read
model
Apply
queued
events
Start using
new read
model
New events Queue

ELASTICSEARCH: MAPPING CHANGE
Create
index with
new
mapping
Reindex
existing
documents
Apply
queued
events
Start using
new index
New events Queue

CHALLENGE: LONG RUNNING REBUILDS
➤ Alternatives
➤ Distributed
➤ In memory
➤ Partial
➤ Background

CHALLENGE: SIDE EFFECTS
User Registered
User Id 123abc
Email Address test@example.net
Event Handler
Exclude during replays!

CHALLENGE: TRANSACTIONS
Event Handler
Event Handler
Event
Event Handler
Event Handler ?

CHALLENGE: EVENTUAL CONSISTENCY
➤ Asynchronous event handlers
➤ Reads eventually return the same value
➤ Compare with ACID

EVENT VERSIONING

DILEMMA
➤ New business requirements
➤ Refactoring
➤ New view on events

NEW EVENTS / VERSIONS
➤ No longer relevant
➤ Renamed
➤ Additional or renamed field(s)
➤ Too coarse, too fine

SUPPORT YOUR LEGACY?
➤ Events are immutable
➤ Correct (incorrect) old events with new events

UPCASTING
Event Store
UserRegistered_V1
Upcaster
UserRegistered_V2
Event Handler

UPCASTING
class UserRegistered_V1 {
public $userId;
public $name;
public $timestamp;
}
class UserRegistered_V2 {
public $userId;
public $name;
public $date;
}
function upcast($event): array {
if (!$event instanceof UserRegistered_V1) {
return [];
}
return [
new UserRegistered_V2($event->userId, $event->name,
$event->timestamp->format("Y-m-d"))
];
}

REWRITING HISTORY
Load (subset of) events
Deserialize
Modify
Serialize
Save/replace

THINGS TO BE AWARE OF
Upcasting
➤ Performance
➤ Complexity
➤ Existing projections not
automatically updated
Rewriting events
➤ Running code that
depends on old structure
➤ Breaking serialization
➤ Changing wrong events

CONCURRENCY

CONCURRENT COMMANDS
Withdraw Money
Account number 12345678
Amount €€ 50,00
Deposit Money
Account number 12345678
Amount €€ 100,00
?

PESSIMISTIC LOCKING
Withdraw Money
Account number 12345678
Amount €€ 50,00
Deposit Money
Account number 12345678
Amount €€ 100,00
Account
number
Balance
12345678 €€ 100,00
Account
number
Balance
12345678 €€ 50,00
wait for lock
lock

OPTIMISTIC LOCKING
Withdraw Money
Account number 12345678
Amount €€ 50,00
version 1
Deposit Money
Account number 12345678
Amount €€ 100,00
version 1
Account
number
Balance
12345678 €€ 100,00
version 2
ConcurrencyException

SCALE

PERFORMANCE
➤ Server
➤ Database
➤ Framework
➤ Language

STORAGE
➤ #events
➤ #aggregates
➤ #events_per_aggregate
➤ Serializer
➤ Speed
➤ Payload size
➤ Costs

SNAPSHOTS
Events
1 Account Created
2 Money Deposited
3 Money Withdrawn
4 Money Deposited
SNAPSHOT
5 Money Withdrawn
Events
1 Account Created
2 Money Deposited
3 Money Withdrawn
4 Money Deposited
5 Money Withdrawn

SHARDING
➤ Aggregate Id, Type, Event Timestamp, ...
➤ Rebalancing
➤ Distribution

ARCHIVING EVENTS
➤ Reduce working set
➤ Inactive / deleted aggregates
➤ Historic / irrelevant events
➤ Cheaper storage

FRAMEWORK COMPARISON
Framework Upcasting Snapshots Replaying
Broadway
(PHP)
No (PR) No (PR) Not in core
Prooph
(PHP)
MessageFactory
Yes, triggers on
event count
Example code,
off line
Axon
(Java/Scala)
Upcaster /
UpcasterChain
Yes, triggers on
event count
Yes,
ReplayingCluster
Akka
Persistence
(Java/Scala)
Event Adapter
Yes, decided by
actor
Yes

BROADWAY VS PROOPH
class TodoItem extends EventSourcedAggregateRoot {
private $itemId;
public function __construct(PostTodo $command) {
$this->apply(new TodoPosted($command->getItemId()));
}
public function getAggregateRootId()
{
return $this->itemId;
}
public function applyTodoPosted(TodoPosted $event) {
echo "Posted: " . $event->getItemId() . "n";
}
}

BROADWAY VS PROOPH
class TodoItem extends AggregateRoot {
private $itemId;
public function __construct(PostTodo $command) {
$this->recordThat(new TodoPosted($command->getItemId()));
}
protected function aggregateId() {
return $this->itemId;
}
public function whenTodoPosted(TodoPosted $event) {
echo "Posted: " . $event->getItemId() . "n";
}
}

THANK YOU!