Actor Model ؆հ John Lin

• Ұݸ༻ိ႔ཧฒߦӡࢉతఔࣜ໛ܕ • 1973 ೥༝ Carl Hewitt, Peter Bishop & Richard Steiger ఏग़ɻ • ༝ၷେ෦෼૊੒ Actor & message • ෆधཁ೚Կಛผత async featureɼՄҎࡏၚ౷త blocking ؀ڥத መ࡞ɻ What is Actor Model

• ఔࣜӡࢉతᄸݩɻ༗ࣗݾᘐཱతهԱᱪ࿨ӡࢉೳྗɻ • ༗ᘐಛత໊ࢠʢ஍ᅿʣ • ՄҎ၏Լ໘ز݅ࣄɻ • ઀Ꮕ㘤ଉ • 㗞ੜ৽త actorɻ • ૹ㘤ଉڅଖଞ actorɻ • վᏓࣗݾతهԱᱪʢվᏓߦҝʣ Actor

• Actor ೭ؒޓ૬ߏ௨తػ੍ɻ • Actor ՄҎૹmessageڅଖଞత actorɻ • ୠੋ୞ೳᏅࣗݾత messageɻ • Obj-C message send ༗ 87% ૾ Message

• Erisson language • ༻ࡏి৴ަ׵ػதɻ • ॏࢹฒߦӡࢉɻߴՄ༻ੑɻ Erlang

Erlang Actor

Erlang Actor

• SE-0306 Actors መ࡞ྃ Actorɻୠੋ࿨ Actor model ฒෆ׬શ૬ಉɻ • جຊ্बੋ㚎ݐྃ mutex త classɻॴ༗త var ౎ඃ mutex อޢɻ actor BankAccount { let accountNumber: Int var balance: Double init(accountNumber: Int, initialDeposit: Double) { self.accountNumber = accountNumber self.balance = initialDeposit } } Swift Actor

protocol Actor : AnyObject, Sendable { } • ໨લᔒ༗ඞཁత methodɻ೭ޙత proposal ။Ճɻ • ՄҎ์ࡏ፤ိఆٛಛԽత protocol protocol DataProcessible: Actor { // only actor types can conform to this var data: Data { get } // actor-isolated to self } extension DataProcessible { func compressData() -> Data { // actor-isolated to self } } actor MyProcessor : DataProcessible { var data: Data // okay, actor-isolated to self func doSomething() { let newData = compressData() // calling actor-isolated method on self } } Actor type

• ᔒ༗ message passingɼ୞༗ typed async method callɻ • ୞༗ࡏ method ཫ࠽ೳमվ varɻ extension BankAccount { func transfer(amount: Double, to other: BankAccount) async throws { assert(amount > 0) if amount > balance { throw BankError.insufficientFunds } balance = balance - amount await other.deposit(amount: amount) } } Swift Actor message passing

• ߟྀԼ໘తྫࢠɼݺڣ syn ՄҎਖ਼ৗӡ࡞䆩ʁ actor TCP { func syn(to other:TCP) async { await other.synAck(self) } func synAck(to other:TCP) async { await other.ack(self) } func ack(to other:TCP) async { } } Reentrant

• ᙛ actor ։࢝ await త࣌ީɼՄҎࣥߦଖଞ(҃ಉҰݸʣ methodɻ ෆ။ඃ block ɻ • ໵ՄҎࡏ actor ཫ໘ݺڣࣗݾతଖଞ methodɼෆ။ඃ blockɻ Reentrant

• ࡏ༗ reentrant త৘گԼੋ။ಈతɻ await త࣌ީՄҎ၏ଖଞతࣄɻ actor TCP { func syn(to actor2:TCP) async { await actor2.synAck(self) } func synAck(to actor1:TCP) async { await actor1.ack(self) } func ack(to actor2:TCP) async { } } Reentrant

actor Person { let friend: Friend // actor-isolated opinion var opinion: Judgment = .noIdea func thinkOfGoodIdea() async -> Decision { opinion = .goodIdea // <1> await friend.tell(opinion, heldBy: self) // <2> return opinion // 🤨 // <3> } func thinkOfBadIdea() async -> Decision { opinion = .badIdea // <4> await friend.tell(opinion, heldBy: self) // <5> return opinion // 🤨 // <6> } } Reentrant ଄੒త໰୊

Reentrant ଄੒త໰୊ let goodThink = detach { await person.thinkOfGoodIdea() } // runs async let badThink = detach { await person.thinkOfBadIdea() } // runs async let shouldBeGood = await goodThink.get() let shouldBeBad = await badThink.get() await shouldBeGood // could be .goodIdea or .badIdea ☠ await shouldBeBad

actor Person { let friend: Friend // actor-isolated opinion var opinion: Judgment = .noIdea func thinkOfGoodIdea() async -> Decision { opinion = .goodIdea // <1> await friend.tell(opinion, heldBy: self) // <2> return opinion // 🤨 // <3> } func thinkOfBadIdea() async -> Decision { opinion = .badIdea // <4> await friend.tell(opinion, heldBy: self) // <5> return opinion // 🤨 // <6> } } opinion = .goodIdea // <1> // suspend: await friend.tell(...) // <2> opinion = .badIdea // | <4> (!) // suspend: await friend.tell(...) // | <5> // resume: await friend.tell(...) // <2> return opinion // <3> // resume: await friend.tell(...) // <5> return opinion // <6>

• ᙛ actor ։࢝ await త࣌ީɼՄҎࣥߦଖଞ(҃ಉҰݸʣ methodɻ ෆ။ඃ block ɻ • ໵ՄҎࡏ actor ཫ໘ݺڣࣗݾతଖଞ methodɼෆ။ඃ blockɻ • ࠷޷ੋࡏ࠷ޙ࠶Ұ࣍मվvariableɻ • ༗㠥֎త proposal ཁ၏ non-reentrant actorɻ Reentrant

Swift Actor vs Erlang Actor Local Value Message passing Mailbox Reply Reentrant Swift Yes Static, Compiler invoke methods No, compiler invoke methods Automatic, Typed Yes. Can be called from self Erlang Yes Dynamic Send anything Yes Manual, Need to know sender No. Receive blocks actor.

• Reference: • https://www.brianstorti.com/the-actor-model/ • Wiki • Joe Armstrong's thesis • Swift Evolution • https://learnyousomeerlang.com/the-hitchhikers-guide-to- concurrency#dont-panic Q&A

Actor vs Golang Channel