Codemotion akka persistence, cqrs%2 fes y otras siglas del montón

MADRID · NOV 18-19 · 2016
Scala Programming @ Madrid
Akka persistence, CQRS/ES y otras
siglas del montón
Javier Santos

MADRID · NOV 18-19 · 2016
About us
Javier Santos @jpaniego
«Hay dos formas de programar sin
errores; sólo la tercera funciona»
Alan J Perlis

MADRID · NOV 18-19 · 2016
scalera.es
@scalerablog
About us

MADRID · NOV 18-19 · 2016
Problem to solve

MADRID · NOV 18-19 · 2016
DDD - Domain Driven Design
● Context - The setting in which a word or statement appears that
determines its meaning
● Domain (Ontology) - The subject area to which the user applies a
program is the domain of the software
● Model - A system of abstractions that describes selected aspects of a
domain and can be used to solve problems related to that domain
● Ubiquitous language - A language structured around the domain model
and used by all team members to connect all the activities of the team
with the software

MADRID · NOV 18-19 · 2016
Our domain model
case class User(
name: String,
address: Address,
credit: Double,
currentBike: Option[Id[Bike]])
case class Bike(
model: String,
battery: Bike.Battery.Status,
bikeStation: Option[Id[Station]])
case class Address(
street: String,
number: String,
zipCode: String)
case class Station(
address: Address,
maxBikeCapacity: Int,
currentCapacity: Int)

MADRID · NOV 18-19 · 2016
Anemic Domain Model

MADRID · NOV 18-19 · 2016
Anemic Domain Model
● Think about a domain class that only contains fields and no
methods.
● Anti-pattern against the main idea of the object-oriented
programming paradigm: combining data and process
together.
● Why not using C structs then? ¬¬

MADRID · NOV 18-19 · 2016
Anemic Domain Model
case class User(
name: String,
address: Address,
credit: Double,
currentBike: Option[Id[Bike]]) {
def startRental(bike: Id[Bike]): User = {
require(
currentBike.isEmpty,"There's another rental on course.")
require(
credit > 0, "Not enough credit to start a rental.")
this.copy(currentBike = Some(bike))
}
//...

MADRID · NOV 18-19 · 2016
Anemic Domain Model
//...
def finishRental(creditExpense: Double): User = {
require(
currentBike.isDefined, "There is not a rental on course.")
this.copy(
currentBike = None,
credit = credit - creditExpense)
}
}

MADRID · NOV 18-19 · 2016
Aggregate
● Cluster of domain objects
● The whole block of domain objects works as a sole entity.
User Bike
Address

MADRID · NOV 18-19 · 2016
Aggregate
trait Aggregate extends ...{
def id: Id[This]
}
case class Station(
address: Address,
maxBikeCapacity: Int,
currentCapacity: Int)

MADRID · NOV 18-19 · 2016
Event Sourcing
● “Every change to the state of an application is captured in an event
object, and that these event objects are themselves stored in the
sequence they were applied for the same lifetime as the application state
itself” - Martin Fowler

MADRID · NOV 18-19 · 2016
Event Sourcing
● The system state is the sum of the events

MADRID · NOV 18-19 · 2016
Stateful[T]
trait Stateful[S] {
type This <: Stateful[S]
type Action = scalaz.State[S, Event]
val state: S
def apply(s: S): This
def update(f: Action): (This, E) = {
val (newState, e) = f(state)
(apply(newState), e)
}
}

MADRID · NOV 18-19 · 2016
scalaz.State

MADRID · NOV 18-19 · 2016
scalaz.State
● Represents a state mutation that may generate some effect.
● type State[S, Effect] = StateT[Id, S, Effect]
● Monad = composable!

MADRID · NOV 18-19 · 2016
scalaz.State
type Action[E] = scalaz.State[User, E]
def addCredit(amount: Double): Action[Double] =
State(user =>
(user.copy(credit = user.credit + amount), amount))
def rentBike(bike: Id[Bike], timesUsed: Long): Action[Long] =
State(user =>
(user.copy(currentBike = Some(bike), timesUsed + 1))

MADRID · NOV 18-19 · 2016
scalaz.State
val topUpAndRent: Action[(Double, Long)] =
for {
newCredit <- addCredit(10)
timesUsed <- rentBike(Id(“bike-1”))
} yield (newCredit, timesUsed)
val (relaxingAnn, (newCredit, bikeAge)) =
topUpAndRent(User(“Ann Bottle”, Addres(...), 0.0, None)

MADRID · NOV 18-19 · 2016
Immutability perversion

MADRID · NOV 18-19 · 2016
StatefulAgg[T]
trait StateAgg[S] extends Stateful[S]{
_: PersistenActor =>
var aggState: S
def updateState(f: Action): Unit = {
val (newAgg, _) = update(f)
aggState = newAgg.state
}
}

MADRID · NOV 18-19 · 2016

MADRID · NOV 18-19 · 2016
CQRS
● Command Query Responsibility Segregation
● Main idea: “You can use a different model to update the
information than the model you use to read information”

MADRID · NOV 18-19 · 2016
CQRS

MADRID · NOV 18-19 · 2016
CQRS
trait Command[Agg] {
val to: Id[Agg]
}
trait Event

MADRID · NOV 18-19 · 2016
Akka persistence
● End of 2013
● Martin Krasser
● Main idea : store the internal state of an actor
● ...BUT not directly, only through the changes the actor has
suffered.

MADRID · NOV 18-19 · 2016
Akka persistence

MADRID · NOV 18-19 · 2016
Akka persistence - Failure recovery

MADRID · NOV 18-19 · 2016
Snapshotting

MADRID · NOV 18-19 · 2016
Persistent actors
trait PersistenceStuff extends PersistentActor{
def persistenceId: String
def receiveCommand: Receive
def receiveRecover: Receive
}

MADRID · NOV 18-19 · 2016
trait Reactive { _: Stateful[S] =>
val onEvent: Event => Action
val onSnapshot: Any => Action = {
case snapshot => State(s => (s, snapshot))
}
def asEvent(c: Command[This]): Event
}
Persistent actors

MADRID · NOV 18-19 · 2016
trait Aggregate[S] extends StateAgg[S] with Reactive with PersistentActor {
def persistenceId: String = id.id
def receiveRecover: Receive = {
case event: Event => update(onEvent(event))
case snapshot => update(onSnapshot(snapshot))
}
def receiveCommand: Receive = {
case cmd: Command[This@unchecked] =>
if (checkState(cmd))
persistAll(List(asEvent(cmd))){ event =>
update(onEvent(event))
sender ! event
}
else sender ! Nope
}
}
Persistent actors

MADRID · NOV 18-19 · 2016
case class UserAgg(state: User.Default) extends Aggregate[User] {
type This = UserAgg
def apply(s: User) = UserAgg(s)
override val onEvent = {
case e@AddedCredit(amount) =>
State(s => (s.copy(credit = s.credit+10),e))
}
def asEvent(c: Command[This]): Event = c match {
case AddCredit(_, amount) => AddedCredit(amount)
}
}
Persistent actors
boilerplate

MADRID · NOV 18-19 · 2016
Persistence Queries
● Like a PersistentActor but with no Command part
● peristenceId subscription or by tag
● refreshInterval in most plugins

MADRID · NOV 18-19 · 2016
Persistence Queries
val readJournal =
PersistenceQuery(system).readJournalFor[MyScaladslReadJournal](
"akka.persistence.query.my-read-journal")
// issue query to journal
val source: Source[EventEnvelope, NotUsed] =
readJournal.eventsByPersistenceId("user-1337")
// materialize stream, consuming events
implicit val mat = ActorMaterializer()
source.runForeach { event => println("Event: " + event) }

MADRID · NOV 18-19 · 2016
getOrCreate
● A new command is launched against user1...how to deliver it?
● Managers
class UserManager extends Actor {
override def receive = {
case cmd: Command[User@unchecked] =>
getOrCreate(cmd.addresse) forward cmd
}
}

MADRID · NOV 18-19 · 2016
getOrCreate
● Imagine there are 500k registered users …
● ...you create a persistent actor and deliver the just arrived
command…
● ...the actor keeps alive….

MADRID · NOV 18-19 · 2016
getOrCreate

MADRID · NOV 18-19 · 2016
Passivation
● Kill the actor if it’s iddle…
trait Passivation extends PersistentActor {
import ShardRegion.Passivate
context.setReceiveTimeout(60.seconds)
override def receiveCommand = {
//…
case ReceiveTimeout =>
context.parent ! Passivate(stopMessage = Stop)
}
}

MADRID · NOV 18-19 · 2016
Journal
● Different supported pluggable storages:
○ AndroidSQLite
○ BDB
○ Cassandra
○ Kafka
○ DynamoDB
○ HBase
○ MongoDB
○ …
● Why choosing a distributed database as Journal for High Availability?

MADRID · NOV 18-19 · 2016
HA : Clustering

MADRID · NOV 18-19 · 2016
HA : Clustering
● Send semantics
○ Send: The message is sent to an only cluster actor that fits in the
path. If several, the actor is chosen randomly.
clusterClient ! Send(«/user/handler», «hello»,
localAffinity = true)
○ SendToAll: The message is sent to all cluster actors that fit in the
path.
clusterClient ! SendToAll(«/user/handler», «hi»)
○ Publish: The message is sent to all topic subscribers
clusterClient ! Publish(«myTopic», «hello»)

MADRID · NOV 18-19 · 2016
Partitioning : Sharding
● Study case:
○ A command is sent (in a cluster)
to aggregate1, the actor is
created in node1, the command is
processed.
○ A second command is sent (in the
same cluster) to aggregate1, but
this time the command is
received by node2, so the actor is
created there.

MADRID · NOV 18-19 · 2016
● It provides
○ Load balancing: In case a new is added to the cluster or
another one crashes, the total actor amount is balanced.
○ Unique actor identification, so messages that are sent to the
same actor will be forwarded to the node that handles in that
moment that shard id (previous case won’t take place)

MADRID · NOV 18-19 · 2016
● Semantics
○ Entry: It represents a uniquely identified entity with
persistent state (Aggregate).
○ Shard: Entry group. Experts recommend
ShardAmount = 10 x MaxNodeAmount
○ ShardRegion: Same Entry shard group.

MADRID · NOV 18-19 · 2016

MADRID · NOV 18-19 · 2016
#graciasCarmena
#llevameEnTuBiciceta

MADRID · NOV 18-19 · 2016
Problem: eventual consistency
● Changes may take
some time
● If the user requires an
immediate update,
user views could be
updated when the
command part has
acknowledge the event
persistence (handle it
carefully...)

MADRID · NOV 18-19 · 2016
Problem: Distributed transactions
● Let’s suppose credit transfer is allowed among users
● Saga Pattern: A persistent actor that represents a
transaction among aggregates

MADRID · NOV 18-19 · 2016
Problem: Distributed transactions

MADRID · NOV 18-19 · 2016
Problem: cluster initialization

MADRID · NOV 18-19 · 2016
Problem: Testing a PersistentActor
TestActorRef + PersistentActor =

MADRID · NOV 18-19 · 2016
Problem : Schema evolution
● Imagine the following:
case class User(name: String, address: Address, amount: Double)
case class User(cardId: String, amount: Double, usualBike: Bike)
● Choosing proper serializers are the key (ProtoBuf, Avro, Java ser...no
way)
● Event migration (EventAdapter)
● How to:
○ add attribute
○ remove attribute
○ rename attribute
○ ...

MADRID · NOV 18-19 · 2016
Lightbend approach
● Runs on JRE 1.8
● Multiple service control
● Event stream abstraction
● Easy scalability
● Java SDK ...

MADRID · NOV 18-19 · 2016
scalera.es
@scalerablog

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