ddding tools = akka persistence

DDDing with Akka Persistence !

Konrad 'ktoso' Malawski GeeCON 2014 @ Kraków, PL

Konrad `@ktosopl` Malawski

hAkker @

Konrad `@ktosopl` Malawski

typesafe.com geecon.org

Java.pl / KrakowScala.pl sckrk.com / meetup.com/Paper-Cup @ London

GDGKrakow.pl meetup.com/Lambda-Lounge-Krakow

hAkker @

mainly by Martin Krasser !!(as contractor for Typesafe) !inspired by:

https://github.com/krasserm

https://github.com/eligosource/eventsourced

akka-persistence

Dependencies

libraryDependencies ++= Seq(! "com.typesafe.akka" %% “akka-actor" % "2.3.1",! "com.typesafe.akka" %% "akka-persistence-experimental" % "2.3.1"!)

Show of hands!

sourcing styles

Command Sourcing Event Sourcing

msg: DoThing

msg persisted before receive

imperative, “do the thing”

business logic change, can be reflected in reaction

Processor

sourcing styles

Command Sourcing Event Sourcing

msg: DoThing msg: ThingDone

msg persisted before receive commands converted to events, must be manually persisted

imperative, “do the thing” past tense, “happened”

business logic change, can be reflected in reaction

business logic change, won’t change previous events

Processor EventsourcedProcessor

Compared to “Good ol’ CRUD Model”

“Mutable Record”

state =

apply(es)

“Series of Events”

Actors

An Actor that keeps count of messages it processed !

Let’s send 2 messages to it (it’s “commands”)

!!class Counter extends Actor {! var count = 0! def receive = {! case _ => count += 1! }!}

crash!

restart

restart

restarted

restarted

!wrong!

expected count == 2!

restarted

Consistency Boundary

Consistency Boundary equals Async Boundary

Boundaries

aggregate

“eventual”

A.K.A. async

Business rules

aggregate

Any rule that spans AGGREGATES will not be expected to be up-to-date at all times. Through event processing, batch processing, or

other update mechanisms, other dependencies can be resolved within some specific time. [Evans, p. 128]

aggregate

consistentconsistent

eventually consistent

Let’s open the toolbox

Processor

Processor (command sourcing)

var count = 0 !

def processorId = “a” !

Journal (DB)

Processor

Journal (DB)

Processor

var count = 0 !

Journal (DB)

Processor

var count = 0 !

Journal (DB)

Processor

var count = 0 !

Journal (DB)

Processor

var count = 1 !

Journal (DB)

Processor

var count = 1 !

crash!

Journal (DB)

Processor

restart

Journal (DB)

Processor

var count = 0 !

restart

Journal (DB)

Processor

var count = 0 !

replay!

restart

Journal (DB)

Processor

var count = 0 !

replay!

Journal (DB)

Processor

var count = 1 !

replay!

Journal (DB)

Processor

var count = 1 !

Journal (DB)

Processor

var count = 2 !

Processor

import akka.persistence._!!class CounterProcessor extends Processor {! var count = 0! override val processorId = "counter"!! def receive = {! case Persistent(payload, seqNr) =>! // payload already persisted! count += 1! }!}

Processor

counter ! Persistent(payload)

Processor

sequenceNr (generated by akka)

is already persisted!

Processor

import akka.persistence._!!class CounterProcessor extends Processor {! var count = 0! override val processorId = "counter"!! def receive = {! case notPersisted: Event =>! // will not replay this msg!! count += 1! }!}

counter ! payload

won’t persist

won’t replay

Processorimport akka.persistence._!!class CounterProcessor extends Processor {! var count = 0! override val processorId = "counter"!! def receive = {! case Persistent(payload, seqNr) =>! // payload already persisted! count += 1!! case notPersistentMsg =>! // msg not persisted - like in normal Actor! count += 1! }!}

Processor

Upsides

• Persistent Command Sourcing “out of the box”

• Pretty simple, persist handled for you

• Once you get the msg, it’s persisted

• Pluggable Journals (HBase, Cassandra, Mongo, …)

• Can replay to given seqNr (post-mortem etc!)

Processor

Downsides

• Exposes Persistent() to Actors who talk to you

• No room for validation before persisting

• There’s one Model, we act on the incoming msg

• Lower throughput than plain Actor (limited by DB)

Eventsourced Processor

Eventsourced Processor (event sourcing)

super quick domain modeling!

sealed trait Command!case class GiveMe(geeCoins: Int) extends Command!case class TakeMy(geeCoins: Int) extends Command

Commands - what others “tell” us; not persisted

case class Wallet(geeCoins: Int) {! def updated(diff: Int) = State(geeCoins + diff)!}

State - reflection of a series of events

sealed trait Event!case class BalanceChangedBy(geeCoins: Int) extends Event!

Events - reflect effects, past tense; persisted

var state = S0 !

EventsourcedProcessor

Command

Journal

var state = S0 !

Journal

Generate Events

var state = S0 !

Journal

Generate Events

ACK “persisted”

Journal

var state = S0 !

“Apply” event

Journal

var state = S0 !

Journal

var state = S0 !

Journal

var state = S0 !

Journal

var state = S0 !

Ok, he got my $.

class GeeCoinWallet extends EventsourcedProcessor {!! var state = Wallet(geeCoins = 0)!! def updateState(e: Event): State = {! case BalanceChangedBy(geeCoins) => state updated geeCoins! }! ! // API:!! def receiveCommand = ??? // TODO!! def receiveRecover = ??? // TODO!!}!

def receiveCommand = {!! case TakeMy(geeCoins) =>! persist(BalanceChangedBy(geeCoins)) { changed =>! state = updateState(changed) ! }!!!!!!!}

async callback

def receiveCommand = {!!!!!!! case GiveMe(geeCoins) if geeCoins <= state.geeCoins =>! persist(BalanceChangedBy(-geeCoins)) { changed =>! state = updateState(changed) ! sender() ! TakeMy(-geeCoins)! }!}

Safe to access sender here

async callback

def receiveCommand = {!!!!!!! case GiveMe(geeCoins) if geeCoins <= state.geeCoins =>! persist(BalanceChangedBy(-geeCoins)) { changed =>! state = updateState(changed) ! sender() ! TakeMy(-geeCoins)! }!}

Is this safe? It’s async! Races?!

Ordering guarantees

Journal

var state = S0 !

Ordering guarantees

Journal

var state = S0 !

Commands get “stashed” until processing C1’s events are acted upon.

Journal

Ordering guarantees

var state = S0 !

events get applied in-order

Journal

Ordering guarantees

var state = S0 !

C3 E1 E2

and the cycle repeats

Eventsourced, recovery

/** MUST NOT SIDE-EFFECT! */!def receiveRecover = {! case replayedEvent: Event => ! updateState(replayedEvent)!}

exact same code for all events!

Snapshots

Snapshots (in SnapshotStore)

Eventsourced, snapshots

def receiveCommand = {! case command: Command =>! saveSnapshot(state) // async!!}

/** MUST NOT SIDE-EFFECT! */!def receiveRecover = {! case SnapshotOffer(meta, snapshot: State) => ! this.state = state!! case replayedEvent: Event => ! updateState(replayedEvent)!}

snapshot!? how?

…sum of states…

Snapshots

Journal

E1 E2 E3 E4

E5 E6 E7 E8

state until [E8]

Snapshots

Snapshot Store

snapshot!

Journal

E1 E2 E3 E4

E5 E6 E7 E8

state until [E8]

Snapshots

Snapshot Store

Journal

E1 E2 E3 E4

E5 E6 E7 E8

crash!

Snapshots

Snapshot Store

Journal

E1 E2 E3 E4

E5 E6 E7 E8

crash!

“bring me up-to-date!”

Snapshots

Snapshot Store

Journal

E1 E2 E3 E4

E5 E6 E7 E8

restart!replay!

“bring me up-to-date!”

Snapshots

Snapshot Store

restart!replay!

Journal

E1 E2 E3 E4

E5 E6 E7 E8

state until [E8]

Snapshots

Snapshot Store

restart!replay!

Journal

E1 E2 E3 E4

E5 E6 E7 E8

state until [E8]

Snapshots

Snapshot Store

Journal

E1 E2 E3 E4

E5 E6 E7 E8

We could delete these!

trait MySummer extends Processor {! var nums: List[Int]! var total: Int! ! def receive = {! case "snap" => saveSnapshot(total)! case SaveSnapshotSuccess(metadata) => // ...! case SaveSnapshotFailure(metadata, reason) => // ...! }!}!

Snapshots, save

Async!

Snapshots, success

final case class SnapshotMetadata(! processorId: String, sequenceNr: Long, ! timestamp: Long)

Snapshots, success

Snapshot Recovery

class Counter extends Processor {! var total = 0! ! def receive = {! case SnapshotOffer(metadata, snap: Int) => ! total = snap!! case Persistent(payload, sequenceNr) => // ...! }!}

Snapshots

Upsides

• Simple!

• Faster recovery (!)

• Allows to delete “older” events

• “known state at point in time”

Snapshots

Downsides

• More logic to write

• Maybe not needed if events replay “fast enough”

• Possibly “yet another database” to pick

• snapshots are different than events, may be big!

Journal (DB)

!Processor

!def processorId = “a”

pooling

Journal (DB)

!Processor

pooling

different ActorPath, same processorId

class DoublingCounterProcessor extends View {! var state = 0! override val processorId = "counter"!! def receive = {! case Persistent(payload, seqNr) =>! // “state += 2 * payload” !! }!}

Akka Persistence Plugins

Plugins are Community maintained! (“not sure how production ready”)

http://akka.io/community/#journal_plugins

• Journals - Cassandra, HBase, Mongo …

• SnapshotStores - Cassandra, HDFS, HBase, Mongo …

SnapshotStore Plugins!

http://akka.io/community/#journal_plugins

Extra: Channels

Extras: Channel

Features:

• de-duplication

Extras: PersistentChannel

Features:

• “at least once delivery”

Try it now

Try it now() // !

typesafe.com/activator

akka-sample-persistence-scala

• Official docs: http://doc.akka.io/docs/akka/2.3.0/scala/persistence.html

• Patrik’s Slides & Webinar: http://www.slideshare.net/patriknw/akka-persistence-webinar

• Papers:

• Sagas http://www.cs.cornell.edu/andru/cs711/2002fa/reading/sagas.pdf

• Life beyond Distributed Transactions: http://www-db.cs.wisc.edu/cidr/cidr2007/papers/cidr07p15.pdf

• Pics:

• http://misaspuppy.deviantart.com/art/Messenger-s-Cutie-Mark-A-Flying-Envelope-291040459

Mailing List

groups.google.com/forum/#!forum/akka-user

Eric Evans: "The DDD book”

Talk: “Acknowledging CAP at the Root” !!!!!!

Vaughn Vernon’s Book and Talk !!

ktoso @ typesafe.com twitter: ktosopl github: ktoso blog: project13.pl team blog: letitcrash.com GeeCON 2014 @ Kraków, PL

Dzięki! Thanks! ありがとう！ !

ping me:

ddding tools = akka persistence

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