t i f a c r t A C o m p * t * l e * n t e e * A s t P i W s E n e O o C l l C D O * o * e c C s u u m E e e E R n * o e t v t y s d d a E * a e u l t a Lightweight Session Programming in Scala Alceste Scalas Nobuko Yoshida Theory and Applications of Behavioural Types Dagstuhl, February 1 st , 2017
Introduction lchannels Formal properties Demo Conclusions App server Client Frontend Auth GetSession ( Id ) GetSession ( Id ) Scala, Akka Typed & CPS protocols Alt Alt Session Id is valid notoriously Success ( S ) (R. Kuhn, “Project G˚ albma: Actors vs Types”, slide 42) main Active ( S ) on- ommunic- ges. Alt Alt Session Id does not exist, or is expired Failure () resort- GetAuthentication () Actors interact via untyped mailboxes. Authentication ( A ) ell New ( A ) Non-trivial protocols must be checked at run-time of Authenticate ( Credentials ) the Alt Alt from Invalid credentials ✗ Failure () ced- fron- fron- Alt Alt Valid credentials client CreateSession ( User ) NewSession ( S ) which ✓ Success ( S ) the ron- er, The Loop/Alt i ∈ { 1 , . . . , n } Loop/Alt i ∈ { 1 , . . . , n } Client-server session an Command i ( T i ) Response i ( T ′ i ) authen- . . . . . . a who
Introduction lchannels Formal properties Demo Conclusions App server Client Frontend Auth GetSession ( Id ) GetSession ( Id ) Scala, Akka Typed & CPS protocols Alt Alt Session Id is valid notoriously Success ( S ) (R. Kuhn, “Project G˚ albma: Actors vs Types”, slide 42) main Active ( S ) on- ommunic- ges. Alt Alt Session Id does not exist, or is expired Failure () resort- GetAuthentication () Actors interact via untyped mailboxes. Authentication ( A ) ell New ( A ) Non-trivial protocols must be checked at run-time of Authenticate ( Credentials ) the Alt Alt from Invalid credentials ✗ Failure () ced- fron- Akka Typed proposes: fron- Alt Alt Valid credentials client CreateSession ( User ) NewSession ( S ) which ✓ Success ( S ) the 1. typed references ActorRef[A] ron- er, 2. Continuation-Passing Style protocols The Loop/Alt i ∈ { 1 , . . . , n } Loop/Alt i ∈ { 1 , . . . , n } Client-server session an Command i ( T i ) Response i ( T ′ i ) authen- . . . . . . a who
Introduction lchannels Formal properties Demo Conclusions CPS protocols in Akka Typed (cont’d) (R. Kuhn, “Project G˚ albma: Actors vs Types”, slide 42) case class GetSession(id: Int, replyTo: ActorRef[GetSessionResult]) sealed abstract class GetSessionResult Client Frontend Auth App server case class Active(service: ActorRef[Command]) extends GetSessionResult GetSession ( Id ) case class New(authc: ActorRef[Authenticate]) GetSession ( Id ) extends GetSessionResult Alt Alt // ... case classes for authentication, etc ... Session Id is valid notoriously Success ( S ) main Active ( S ) on- ommunic- ges. Alt Alt Session Id does not exist, or is expired Failure () resort- GetAuthentication () Authentication ( A ) ell New ( A ) of Authenticate ( Credentials )
Introduction lchannels Formal properties Demo Conclusions CPS protocols in Akka Typed (cont’d) (R. Kuhn, “Project G˚ albma: Actors vs Types”, slide 42) case class GetSession(id: Int, replyTo: ActorRef[GetSessionResult]) sealed abstract class GetSessionResult Client Frontend Auth App server case class Active(service: ActorRef[Command]) extends GetSessionResult GetSession ( Id ) case class New(authc: ActorRef[Authenticate]) GetSession ( Id ) extends GetSessionResult Alt Alt // ... case classes for authentication, etc ... Session Id is valid notoriously Success ( S ) main Active ( S ) on- To send a message, produce its continuation ommunic- (pseudo-code follows) ges. Alt Alt Session Id does not exist, or is expired Failure () resort- GetAuthentication () def client(frontend: ActorRef[GetSession]) = { Authentication ( A ) val cont = spawn[GetSessionResult] { ell New ( A ) case New(a) => doAuthentication(a) of case Active(s) => doSessionLoop(s) Authenticate ( Credentials ) } frontend ! GetSession(42, cont) }
Introduction lchannels Formal properties Demo Conclusions CPS protocols: opportunities and limitations case class GetSession(id: Int, replyTo: ActorRef[GetSessionResult]) sealed abstract class GetSessionResult case class Active(service: ActorRef[Command]) extends GetSessionResult case class New(authc: ActorRef[Authenticate]) extends GetSessionResult case class Authenticate(username: String, password: String, replyTo: ActorRef[AuthenticateResult]) sealed abstract class AuthenticateResult case class Success(service: ActorRef[Command]) extends AuthenticateResult case class Failure() extends AuthenticateResult sealed abstract class Command // ... case classes for the client-server session loop ... CPS protocols are Scala types yielding structured interaction in Akka But they are also: ▸ low-level , cumbersome to write (and read) ▸ not related with any high-level protocol formalism ▸ ambiguous about reusability of (typed) actor references
Introduction lchannels Formal properties Demo Conclusions Our approach Desiderata : ▸ find a formal link between CPS protocols and session types ▸ represent sessions in a language without session primitives ▸ lightweight : no language extensions, minimal dependencies
Introduction lchannels Formal properties Demo Conclusions Our approach Desiderata : ▸ find a formal link between CPS protocols and session types ▸ represent sessions in a language without session primitives ▸ lightweight : no language extensions, minimal dependencies Inspiration (from concurrency theory): ▸ encoding of session types into linear types for π -calculus (Dardha, Giachino & Sangiorgi, PPDP’12)
Introduction lchannels Formal properties Demo Conclusions Our approach Desiderata : ▸ find a formal link between CPS protocols and session types ▸ represent sessions in a language without session primitives ▸ lightweight : no language extensions, minimal dependencies Inspiration (from concurrency theory): ▸ encoding of session types into linear types for π -calculus (Dardha, Giachino & Sangiorgi, PPDP’12) Result: Lightweight Session Programming in Scala
Introduction lchannels Formal properties Demo Conclusions lchannels : interface abstract class In[+A] { def receive(implicit d: Duration): A } abstract class Out[-A] { def send(msg: A): Unit } API reminds standard Promise s/ Future s ▸ similar runtime linearity checks and error handling Note input/output co/contra-variance
Introduction lchannels Formal properties Demo Conclusions lchannels : interface abstract class In[+A] { def receive(implicit d: Duration): A def ?[B](f: A => B)(implicit d: Duration): B = { f(receive) } } abstract class Out[-A] { def send(msg: A): Unit def !(msg: A) = send(msg) } API reminds standard Promise s/ Future s ▸ similar runtime linearity checks and error handling Note input/output co/contra-variance
Introduction lchannels Formal properties Demo Conclusions lchannels : interface abstract class In[+A] { def future: Future[A] def receive(implicit d: Duration): A = { Await.result[A](future, d) } def ?[B](f: A => B)(implicit d: Duration): B = { f(receive) } } abstract class Out[-A] { def promise[B <: A]: Promise[B] // Impl. must be constant def send(msg: A): Unit = promise.success(msg) def !(msg: A) = send(msg) } API reminds standard Promise s/ Future s ▸ similar runtime linearity checks and error handling Note input/output co/contra-variance
Introduction lchannels Formal properties Demo Conclusions lchannels : interface abstract class In[+A] { def future: Future[A] def receive(implicit d: Duration): A = { Await.result[A](future, d) } def ?[B](f: A => B)(implicit d: Duration): B = { f(receive) } } abstract class Out[-A] { def promise[B <: A]: Promise[B] // Impl. must be constant def send(msg: A): Unit = promise.success(msg) def !(msg: A) = send(msg) def create[B](): (In[B], Out[B]) // Used to continue a session } API reminds standard Promise s/ Future s ▸ similar runtime linearity checks and error handling Note input/output co/contra-variance
Introduction lchannels Formal properties Demo Conclusions Session programming = In[ ⋅ ] / Out[ ⋅ ] + CPS protocols How do we instantiate the In[ ⋅ ] / Out[ ⋅ ] type parameters ? Client Server Session types S S Scala types In[?] or Out[?] Out[?] or In[?]
Introduction lchannels Formal properties Demo Conclusions Session programming = In[ ⋅ ] / Out[ ⋅ ] + CPS protocols How do we instantiate the In[ ⋅ ] / Out[ ⋅ ] type parameters ? Client Server Session types S S CPS protocol classes Scala types A1 , A2 ,..., An In[A] or Out[A] Out[A] or In[A]
Introduction lchannels Formal properties Demo Conclusions Session programming = In[ ⋅ ] / Out[ ⋅ ] + CPS protocols How do we instantiate the In[ ⋅ ] / Out[ ⋅ ] type parameters ? Client Server Session types S S ? ( U ) or ! ( U ) U ! ( U ) or ? ( U ) Linear I/O types CPS protocol classes Scala types A1 , A2 ,..., An In[A] or Out[A] Out[A] or In[A]
Introduction lchannels Formal properties Demo Conclusions Programming with lchannels (I) S = µ X . ( ! Greet ( String ) . ( ? Hello ( String ) . X & ? Bye ( String ) . end ) ⊕ ! Quit . end )
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