DDDing Tools = Akka Persistence
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The document presents a detailed discussion on implementing Akka persistence, focusing on command sourcing and event sourcing models. It covers aspects of actor state management, crash recovery, snapshots, and the practical benefits and downsides of using persistence in applications. Various coding examples and framework functionalities highlight how to effectively utilize Akka for stateful applications.
![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
consistent
consistent
eventually consistent](https://image.slidesharecdn.com/akka-persistence-geecon-2014-140516092453-phpapp02/85/DDDing-Tools-Akka-Persistence-29-320.jpg)
![state until [E8]
Snapshots
S8
!
!
Snapshot Store
snapshot!
!
!
Journal
E1 E2 E3 E4
E5 E6 E7 E8](https://image.slidesharecdn.com/akka-persistence-geecon-2014-140516092453-phpapp02/85/DDDing-Tools-Akka-Persistence-78-320.jpg)
![state until [E8]
Snapshots
S8
!
!
Snapshot Store
!
!
Journal
E1 E2 E3 E4
E5 E6 E7 E8
S8
crash!](https://image.slidesharecdn.com/akka-persistence-geecon-2014-140516092453-phpapp02/85/DDDing-Tools-Akka-Persistence-79-320.jpg)
![state until [E8]
Snapshots
!
!
Snapshot Store
S8
restart!
replay!
S8
!
!
Journal
E1 E2 E3 E4
E5 E6 E7 E8](https://image.slidesharecdn.com/akka-persistence-geecon-2014-140516092453-phpapp02/85/DDDing-Tools-Akka-Persistence-83-320.jpg)
![state until [E8]
Snapshots
!
!
Snapshot Store
S8
S8
!
!
Journal
E1 E2 E3 E4
E5 E6 E7 E8
We could delete these!](https://image.slidesharecdn.com/akka-persistence-geecon-2014-140516092453-phpapp02/85/DDDing-Tools-Akka-Persistence-84-320.jpg)
![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!](https://image.slidesharecdn.com/akka-persistence-geecon-2014-140516092453-phpapp02/85/DDDing-Tools-Akka-Persistence-85-320.jpg)
![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, success
final case class SnapshotMetadata(!
processorId: String, sequenceNr: Long, !
timestamp: Long)](https://image.slidesharecdn.com/akka-persistence-geecon-2014-140516092453-phpapp02/85/DDDing-Tools-Akka-Persistence-86-320.jpg)
![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, success](https://image.slidesharecdn.com/akka-persistence-geecon-2014-140516092453-phpapp02/85/DDDing-Tools-Akka-Persistence-87-320.jpg)
DDDing Tools = Akka Persistence
- 1. DDDing with Akka Persistence ! Konrad 'ktoso' Malawski GeeCON 2014 @ Kraków, PL Konrad `@ktosopl` Malawski
- 3. 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 @
- 4. mainly by Martin Krasser ! ! (as contractor for Typesafe) ! inspired by: https://github.com/krasserm https://github.com/eligosource/eventsourced akka-persistence
- 5. Dependencies libraryDependencies ++= Seq(! "com.typesafe.akka" %% “akka-actor" % "2.3.1",! "com.typesafe.akka" %% "akka-persistence-experimental" % "2.3.1"! )
- 10. 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
- 11. 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
- 12. Compared to “Good ol’ CRUD Model” state “Mutable Record” state = apply(es) “Series of Events”
- 13. Actors
- 14. count: 0 ! ! Actor An Actor that keeps count of messages it processed ! Let’s send 2 messages to it (it’s “commands”)
- 15. Actor ! ! class Counter extends Actor {! var count = 0! def receive = {! case _ => count += 1! }! }
- 19. Actor crash!
- 20. Actor restart
- 24. count: 1 ! ! wrong! expected count == 2! Actor restarted
- 29. 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 consistent consistent eventually consistent
- 30. Let’s open the toolbox
- 31. Processor
- 33. var count = 0 ! def processorId = “a” ! Journal (DB) ! ! ! Processor
- 34. Journal (DB) ! ! ! Processor var count = 0 ! def processorId = “a” !
- 35. Journal (DB) ! ! ! Processor var count = 0 ! def processorId = “a” !
- 36. Journal (DB) ! ! ! Processor var count = 0 ! def processorId = “a” !
- 37. Journal (DB) ! ! ! Processor var count = 1 ! def processorId = “a” !
- 38. Journal (DB) ! ! ! Processor var count = 1 ! def processorId = “a” ! crash!
- 40. Journal (DB) ! ! ! Processor var count = 0 ! def processorId = “a” ! restart
- 41. Journal (DB) ! ! ! Processor var count = 0 ! def processorId = “a” ! replay! restart
- 42. Journal (DB) ! ! ! Processor var count = 0 ! def processorId = “a” ! replay!
- 43. Journal (DB) ! ! ! Processor var count = 1 ! def processorId = “a” ! replay!
- 44. Journal (DB) ! ! ! Processor var count = 1 ! def processorId = “a” !
- 45. Journal (DB) ! ! ! Processor var count = 2 ! def processorId = “a” ! yay!
- 46. 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! }! }
- 47. 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! }! } counter ! Persistent(payload)
- 48. 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! }! } counter ! Persistent(payload)
- 49. 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! }! } counter ! Persistent(payload) sequenceNr (generated by akka) is already persisted!
- 50. 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
- 51. 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! ! case notPersistentMsg =>! // msg not persisted - like in normal Actor! count += 1! }! }
- 52. 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!)
- 53. 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)
- 56. 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
- 57. var state = S0 ! def processorId = “a” ! EventsourcedProcessor Command ! ! Journal
- 58. EventsourcedProcessor var state = S0 ! def processorId = “a” ! ! ! Journal Generate Events
- 59. EventsourcedProcessor var state = S0 ! def processorId = “a” ! ! ! Journal Generate Events E1
- 60. EventsourcedProcessor ACK “persisted” ! ! Journal E1 var state = S0 ! def processorId = “a” !
- 61. EventsourcedProcessor “Apply” event ! ! Journal E1 var state = S0 ! def processorId = “a” ! E1
- 62. EventsourcedProcessor ! ! Journal E1 var state = S0 ! def processorId = “a” ! E1 Okey!
- 63. EventsourcedProcessor ! ! Journal E1 var state = S0 ! def processorId = “a” ! E1 Okey!
- 64. EventsourcedProcessor ! ! Journal E1 var state = S0 ! def processorId = “a” ! E1 Ok, he got my $.
- 65. EventsourcedProcessor 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! ! }!
- 66. EventsourcedProcessor def receiveCommand = {! ! case TakeMy(geeCoins) =>! persist(BalanceChangedBy(geeCoins)) { changed =>! state = updateState(changed) ! }! ! ! ! ! ! ! } async callback
- 67. EventsourcedProcessor 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
- 68. EventsourcedProcessor 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?!
- 69. Ordering guarantees ! ! Journal E1 var state = S0 ! def processorId = “a” ! C1 C2 C3
- 70. Ordering guarantees ! ! Journal E1 var state = S0 ! def processorId = “a” ! C1 C2 C3 Commands get “stashed” until processing C1’s events are acted upon.
- 71. ! ! Journal Ordering guarantees var state = S0 ! def processorId = “a” ! C1 C2 C3 E1 E2 E2E1 events get applied in-order
- 72. C2 ! ! Journal Ordering guarantees var state = S0 ! def processorId = “a” ! C3 E1 E2 E2E1 and the cycle repeats
- 73. Eventsourced, recovery /** MUST NOT SIDE-EFFECT! */! def receiveRecover = {! case replayedEvent: Event => ! updateState(replayedEvent)! } exact same code for all events!
- 74. Snapshots
- 76. 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?
- 77. …sum of states… Snapshots ! ! Journal E1 E2 E3 E4 E5 E6 E7 E8
- 78. state until [E8] Snapshots S8 ! ! Snapshot Store snapshot! ! ! Journal E1 E2 E3 E4 E5 E6 E7 E8
- 79. state until [E8] Snapshots S8 ! ! Snapshot Store ! ! Journal E1 E2 E3 E4 E5 E6 E7 E8 S8 crash!
- 80. Snapshots ! ! Snapshot Store ! ! Journal E1 E2 E3 E4 E5 E6 E7 E8 S8 crash!
- 81. “bring me up-to-date!” Snapshots ! ! Snapshot Store ! ! Journal E1 E2 E3 E4 E5 E6 E7 E8 S8 restart! replay!
- 82. “bring me up-to-date!” Snapshots ! ! Snapshot Store S8 restart! replay! S8 ! ! Journal E1 E2 E3 E4 E5 E6 E7 E8
- 83. state until [E8] Snapshots ! ! Snapshot Store S8 restart! replay! S8 ! ! Journal E1 E2 E3 E4 E5 E6 E7 E8
- 84. state until [E8] Snapshots ! ! Snapshot Store S8 S8 ! ! Journal E1 E2 E3 E4 E5 E6 E7 E8 We could delete these!
- 85. 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!
- 86. 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, success final case class SnapshotMetadata(! processorId: String, sequenceNr: Long, ! timestamp: Long)
- 87. 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, success
- 88. Snapshot Recovery class Counter extends Processor {! var total = 0! ! def receive = {! case SnapshotOffer(metadata, snap: Int) => ! total = snap! ! case Persistent(payload, sequenceNr) => // ...! }! }
- 89. Snapshots Upsides • Simple! • Faster recovery (!) • Allows to delete “older” events • “known state at point in time”
- 90. 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!
- 91. Views
- 92. Journal (DB) ! ! ! Views ! Processor ! def processorId = “a” ! ! View ! def processorId = “a” ! ! ! pooling
- 93. Journal (DB) ! ! ! Views ! Processor ! def processorId = “a” ! ! View ! def processorId = “a” ! ! ! pooling ! View ! def processorId = “a” ! ! ! pooling different ActorPath, same processorId
- 94. View class DoublingCounterProcessor extends View {! var state = 0! override val processorId = "counter"! ! def receive = {! case Persistent(payload, seqNr) =>! // “state += 2 * payload” ! ! }! }
- 95. 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 …
- 97. Extra: Channels
- 99. Extras: PersistentChannel Features: • “at least once delivery”
- 100. Try it now
- 101. Try it now() // ! typesafe.com/activator akka-sample-persistence-scala
- 102. Links • 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
- 104. Links Eric Evans: "The DDD book” Talk:“Acknowledging CAP at the Root” ! ! ! ! ! ! VaughnVernon’s Book and Talk ! !
- 105. 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:
- 106. ©Typesafe 2014 – All Rights Reserved