Spores: A Type-Based Foundation for Closures in the Age of Concurrency and Distribution

Functional programming (FP) is regularly touted as the way forward for bringing parallel, concurrent, and distributed programming to the mainstream. The popularity of the rationale behind this viewpoint has even led to a number of object-oriented (OO) programming languages outside the Smalltalk tradition adopt- ing functional features such as lambdas and thereby function closures. However, despite this established viewpoint of FP as an enabler, reliably distributing function closures over a network, or using them in concurrent environments nonetheless remains a challenge across FP and OO languages. This paper takes a step towards more principled distributed and concurrent programming by introducing a new closure-like abstraction and type system, called spores, that can guarantee closures to be serializable, thread-safe, or even have custom user-defined properties. Crucially, our system is based on the principle of encoding type information corresponding to captured variables in the type of a spore. We prove our type system sound, implement our approach for Scala, evaluate its practicality through a small empirical study, and show the power of these guarantees through a case analysis of real-world distributed and concurrent frameworks that this safe foundation for closures facilitates.

1. SPORES ECOOP 2014, Uppsala, Sweden August 1st, 2014 Heather Miller A Type-Based Foundation for Closures in the Age of Concurrency & Distribution , Philipp Haller, Martin Odersky
2. SOME MOTIVATION…
3. TWO TRENDS DATA-CENTRIC APPLICATIONS FUNCTIONAL PROGRAMMING APPLY FUNCTIONS TO IMMUTABLE DATA. CLOSURE-HEAVY. CALLBACKS/REACTIVE.
4. OBSERVATION:
5. THESE ARECOMPLIMENTARY TRENDS OBSERVATION:
6. THESE ARECOMPLIMENTARY TRENDS OBSERVATION: FUNCTIONAL PROGRAMMING A BOON TO DATA-CENTRIC PROGRAMMING
7. WHY? The basic philosophy to transform immutable data by applying first-class functions. The observation that this functional style simplifies reasoning about data in parallel, concurrent, and distributed code. 1 2
8. THE OF DATA-PARALLEL POPULARITY FRAMEWORKS HENCE,
9. Spark MapReduce DISTRIBUTED Java 8’s monadic optionally-parallel streams Scala’s parallel collections Haskell’s Par Monad PARALLEL/ CONCURRENT Intel’s Concurrent Collections DATA-PARALLEL FRAMEWORKS
10. WHY SPORES ? ARE NECESSARY
11. Well, CLOSURES ARE OFTEN A SOURCE OF HEADACHES YOU CAN’T REALLY DISTRIBUTE THEM.
12. Well, CLOSURES ARE OFTEN A SOURCE OF HEADACHES NOT JUST IN SCALA OR JAVA. BUT CROSS-PARADIGM. YOU CAN’T REALLY DISTRIBUTE THEM.
13. PROBLEMS 1. Accidental capture of non-serializable
 variables (like this) 2. Language-specific compilation schemes
 that create implicit references to objects
 that are not serializable 3. transitive references that inadvertently hold
 on to excessively large object graphs
 creating memory leaks THELAUNDRY LIST W/CLOSURES
14. PROBLEMS 4. Capturing references to mutable objects,
 leading to race conditions in a concurrent
 setting. 5. Unknowingly accessing object members
 that are not constant such as methods, 
 which in a distributed setting can have 
 logically different meanings on different 
 machines. THELAUNDRY LIST W/CLOSURES CONT’D
15. SPARK motivating example: http://docs.scala-lang.org/sips/pending/spores.html Scala Improvement Proposal: class MyCoolRddApp { val param = 3.14 val log = new Log(...) ... def work(rdd: RDD[Int]) { rdd.map(x => x + param) .reduce(...) } }
16. SPARK motivating example: http://docs.scala-lang.org/sips/pending/spores.html Scala Improvement Proposal: class MyCoolRddApp { val param = 3.14 val log = new Log(...) ... def work(rdd: RDD[Int]) { rdd.map(x => x + param) .reduce(...) } } PROBLEM: not serializable because it captures this of type MyCoolRddApp which is itself not serializable (x => x + param)
17. AKKA/FUTURES def  receive  =  {    case  Request(data)  =>        future  {            val  result  =  transform(data)            sender  !  Response(result)        }   } Akka actor spawns future to concurrently process incoming results AKKA ACTOR SPAWNS A FUTURE TO CONCURRENTLY PROCESS INCOMING REQS NOT A STABLE VALUE! IT’S A METHOD CALL! PROBLEM: motivating example: http://docs.scala-lang.org/sips/pending/spores.html Scala Improvement Proposal:
18. AKKA/FUTURES def  receive  =  {    case  Request(data)  =>        future  {            val  result  =  transform(data)            sender  !  Response(result)        }   } Akka actor spawns future to concurrently process incoming results AKKA ACTOR SPAWNS A FUTURE TO CONCURRENTLY PROCESS INCOMING REQS NOT A STABLE VALUE! IT’S A METHOD CALL! PROBLEM: motivating example: http://docs.scala-lang.org/sips/pending/spores.html Scala Improvement Proposal:
19. Need a solution that… INTEGRATES WITH EXISTING PRACTICE: PRACTICAL FOR INCLUSION/ USE IN FULL-FEATURED SCALA. PRACTICAL FOR INTEGRATION WITH SPARK, AKKA, ETC.
20. Need a solution that… ENABLES LIBRARY AUTHORS TO CONFIDENTLY RELEASE LIBRARIES THAT EXPOSE FUNCTIONS IN USER-FACING APIS without concerns of runtime exceptions or other dubious errors falling on users.
21. ENTER: SPORES
22. ENTER: SPORES PROPOSED FOR INCLUSION IN SCALA.
23. WHAT ARE THEY? A closure-like abstraction GOAL: Well-behaved closures with controlled environments that can avoid various hazards. http://docs.scala-lang.org/sips/pending/spores.html Scala Improvement Proposal: SPORES A type system …designed for use in distributed or concurrent environments.
24. http://docs.scala-lang.org/sips/pending/spores.html Scala Improvement Proposal: SPORES THIS IS ACHIEVED BY: (a) enforcing a specific syntactic shape which dictates how the environment of a spore is declared. (b) providing additional type checking to ensure that types being captured have certain properties.
25. http://docs.scala-lang.org/sips/pending/spores.html Scala Improvement Proposal: SPORES THIS IS ACHIEVED BY: (a) enforcing a specific syntactic shape which dictates how the environment of a spore is declared. (b) providing additional type checking to ensure that types being captured have certain properties. CRUCIALLY, spores encode extra type information corresponding to the captured environment in their type.
26. Contributions spores abstraction & type system type-based constraints that can be combined with existing type systems formalization and soundness proof implementation of spores for Scala small empirical study of practicality
27. Contributions spores abstraction & type system type-based constraints that can be combined with existing type systems formalization and soundness proof implementation of spores for Scala small empirical study of practicality details in the paper… (covered in this presentation)
28. WHAT DO SPORES LOOK LIKE? Basic usage: val  s  =  spore  {      val  h  =  helper      (x:  Int)  =>  {          val  result  =  x  +  "  "  +  h.toString          println("The  result  is:  "  +  result)      }   } THE BODY OF A SPORE CONSISTS OF 2 PARTS 2 a closure a sequence of local value (val) declarations only (the “spore header”), and 1 http://docs.scala-lang.org/sips/pending/spores.html Scala Improvement Proposal
29. WHAT DO SPORES LOOK LIKE? Basic usage: val  s  =  spore  {      val  h  =  helper      (x:  Int)  =>  {          val  result  =  x  +  "  "  +  h.toString          println("The  result  is:  "  +  result)      }   } THE BODY OF A SPORE CONSISTS OF 2 PARTS 2 a closure a sequence of local value (val) declarations only (the “spore header”), and 1 http://docs.scala-lang.org/sips/pending/spores.html Scala Improvement Proposal
30. WHAT DO SPORES LOOK LIKE? Basic usage: val  s  =  spore  {      val  h  =  helper      (x:  Int)  =>  {          val  result  =  x  +  "  "  +  h.toString          println("The  result  is:  "  +  result)      }   } THE BODY OF A SPORE CONSISTS OF 2 PARTS 2 a closure a sequence of local value (val) declarations only (the “spore header”), and 1 http://docs.scala-lang.org/sips/pending/spores.html Scala Improvement Proposal
31. SPORE 1. All captured variables are declared in 
 the spore header, or using capture 2. The initializers of captured variables 
 are executed once, upon creation of 
 the spore 3. References to captured variables do 
 not change during the spore’s execution vsCLOSURES ( ) A Guarantees... http://docs.scala-lang.org/sips/pending/spores.html Scala Improvement Proposal:
32. SPORES&CLOSURES EVALUATION SEMANTICS: Remove the spore marker, and the code behaves as before SPORES & CLOSURES ARE RELATED: You can write a full function literal and pass it to something that expects a spore. (Of course, only if the function literal 
 satisfies the spore rules.) http://docs.scala-lang.org/sips/pending/spores.html Scala Improvement Proposal:
33. HOW CAN YOU USE A SPORE? Ok. So. IN APIS http://docs.scala-lang.org/sips/pending/spores.html Scala Improvement Proposal def  sendOverWire(s:  Spore[Int,  Int]):  Unit  =  ...   //  ...   sendOverWire((x:  Int)  =>  x  *  x  -­‐  2)   If you want parameters to be spores, then you can write it this way
34. WHAT DOES ALL OF THAT http://docs.scala-lang.org/sips/pending/spores.html Scala Improvement Proposal GET YOU? SINCE... 
 Captured expressions are evaluated upon spore creation. ! Spores are like function values with an immutable environment. Plus, environment is specified and checked, no accidental capturing. THAT MEANS...
35. But… SOCKET? WHAT IF I CAPTURE A
36. THIS IS WHERE TYPE SYSTEM COMES IN. THE
37. KEEP TRACK OF CAPTURED TYPES Idea: The spore macro can synthesize precise types automatically for newly created spores: Spore[Int,  ...]  {      type  Excluded  =  No[Actor]      type  Captured  =  (Int,  ActorRef)   } w/ constraints CreatingSPORES ...at compile-time spore  {  val  x:  Int  =  list.size;  val  a:  ActorRef  =  this.sender      (y:  Int)  =>  ...   }  without[Actor] SYNTHESIZED TYPE:
38. w/ constraints ComposingSPORES BASIC COMPOSITION OPERATORS (same as for regular functions) andThen compose How do we synthesize the result type of 
 s1 andThen s2? RESULT TYPE SYNTHESIZED BY andThen MACRO type member Captured: concatenation of the captured types of s1 and s2 type member Excluded: conjunction of excluded types, needs to check Captured type members to see if possible
39. w/ constraints Example:Composing val  s1:  Spore[Int,  String]  {      type  Excluded  =  No[Actor]      type  Captured  =  (Int,  ActorRef)   }  =  ...   val  s2:  Spore[String,  String]  {      type  Excluded  =  No[RDD[Int]]      type  Captured  =  Actor   }   s1  andThen  s2    //  does  not  compile   SPORES
40. w/ constraints Example: SPORES Composing val  s1:  Spore[Int,  String]  {      type  Excluded  =  No[Actor]      type  Captured  =  (Int,  ActorRef)   }  =  ...   val  s2:  Spore[String,  String]  {      type  Excluded  =  No[RDD[Int]]   }   s1  andThen  s2:  Spore[Int,  String]  {      type  Excluded  =  No[Actor]  with  No[RDD[Int]]      type  Captured  =  (Int,  ActorRef)   }   !
41. But… HOW ARE THEY SERIALIZABLE?
42. But… HOW ARE THEY SERIALIZABLE? THEY’RE NOT. YET.
43. PROPERTIES Enter: IDEA: http://docs.scala-lang.org/sips/pending/spores.html Scala Improvement Proposal case  class  Person(name:  String,  age:  Int)   ! val  fun  =  spore  {      val  p:  Person  =  ...      val  luckyNum:  Int  =  ...      ()  =>  s"${p.name}'s  lucky  number  is:  $luckyNum"   } allow expressing a type-based property that must be satisfied by all captured variables upon creation of a spore. EXAMPLE: ENSURE THE FOLLOWING SPORE IS SERIALIZABLE.
44. PROPERTIES Enter: IDEA: http://docs.scala-lang.org/sips/pending/spores.html Scala Improvement Proposal case  class  Person(name:  String,  age:  Int)   ! val  fun  =  spore  {      val  p:  Person  =  ...      val  luckyNum:  Int  =  ...      ()  =>  s"${p.name}'s  lucky  number  is:  $luckyNum"   } allow expressing a type-based property that must be satisfied by all captured variables upon creation of a spore. EXAMPLE: ENSURE THE FOLLOWING SPORE IS SERIALIZABLE. To serialize a spore, it's necessary that for all captured variables of type T, there is an implicit pickler of type Pickler[T] in scope.
45. PROPERTIES Enter: IDEA: http://docs.scala-lang.org/sips/pending/spores.html Scala Improvement Proposal case  class  Person(name:  String,  age:  Int)   ! val  fun  =  spore  {      val  p:  Person  =  ...      val  luckyNum:  Int  =  ...      ()  =>  s"${p.name}'s  lucky  number  is:  $luckyNum"   } allow expressing a type-based property that must be satisfied by all captured variables upon creation of a spore. EXAMPLE: ENSURE THE FOLLOWING SPORE IS SERIALIZABLE. To serialize a spore, it's necessary that for all captured variables of type T, there is an implicit pickler of type Pickler[T] in scope. Pickler[T] CAN BE A SUCH A PROPERTY
46. PROPERTIES Enter: WHAT ARE THEY? http://docs.scala-lang.org/sips/pending/spores.html Scala Improvement Proposal Properties are defined using a type Property[T] Use a property by importing it. When importing a property of type Property[Prop] all spores in scope are guaranteed to satisfy the property or not compile (= all captured types have property Prop)
47. PROPERTIES Enter: WHAT ARE THEY? http://docs.scala-lang.org/sips/pending/spores.html Scala Improvement Proposal Properties are defined using a type Property[T] Use a property by importing it. When importing a property of type Property[Prop] all spores in scope are guaranteed to satisfy the property or not compile (= all captured types have property Prop) so, in the case of pickling, just: import  picklable and the framework ensures that a Pickler[T] exists for every captured type T. (has to have type Property[Pickler])
48. NOT LIMITED TO PICKLING CAN HAVE ARBITRARY PROPERTIES. http://docs.scala-lang.org/sips/pending/spores.html Scala Improvement Proposal …plugs in nicely with other pluggable type systems. Integrate with a deep immutability checker like OIGJ (Zibin et al. 2010) EXAMPLE: DEEP IMMUTABILITY
49. NOT LIMITED TO PICKLING IDEA: http://docs.scala-lang.org/sips/pending/spores.html Scala Improvement Proposal Automatically generate type class instances for all types that satisfy a transitive predicate, using macros. implicit  def  isImmutable[T:  TypeTag]:  Immutable[T] which returns a type class instance for all types of the shape C[O, Immut] that's deeply immutable (analyzing the TypeTag).
50. NOT LIMITED TO PICKLING IDEA: http://docs.scala-lang.org/sips/pending/spores.html Scala Improvement Proposal Automatically generate type class instances for all types that satisfy a transitive predicate, using macros. implicit  def  isImmutable[T:  TypeTag]:  Immutable[T] which returns a type class instance for all types of the shape C[O, Immut] that's deeply immutable (analyzing the TypeTag). To enforce transitive immutability for a spore, it's then sufficient to define an implicit of type Property[Immutable].
51. FORMALIZATION CENTRAL IDEA: Spore types are refinements of function types. | ‹’‘”– pn ‹ t SURSHUW\ LPSRUW | t …‘’‘•‡ t VSRUH FRPSRVLWLRQ v ::= (x : T) ⇒ t DEVWUDFWLRQ | {l = v} UHFRUG YDOXH | •’‘”‡ { x : T = v ; pn; (x : T) ⇒ t } VSRUH YDOXH T ::= T ⇒ T IXQFWLRQ W\SH | {l : T} UHFRUG W\SH | S S ::= T ⇒ T { –›’‡ C = T ; pn } VSRUH W\SH | T ⇒ T { –›’‡ C ; pn } DEVWUDFW VSRUH W\SH P ∈ pn → T SURSHUW\ PDS T ∈ P(T) W\SH IDPLO\ Γ ::= x : T W\SH HQYLURQPHQW ∆ ::= pn SURSHUW\ HQYLURQPHQW HU 3 +DOOHU DQG 0 2GHUVN\ ::= ... WHUPV | •’‘”‡ { x : T = t ; T; pn; (x : T) ⇒ t } VSRUH ::= ... YDOXHV | •’‘”‡ { x : T = v ; T; pn; (x : T) ⇒ t } VSRUH YDOXH ::= T ⇒ T { –›’‡ C = T ; –›’‡ E = T ; pn } VSRUH W\SH | T ⇒ T { –›’‡ C ; –›’‡ E = T ; pn } DEVWUDFW VSRUH W\SH )LJ  &RUH ODQJXDJH V\QWD[ H[WHQVLRQV ϿϱЁζ T2 <: T1 R1 <: R2 n′ ⊆ pn M1 = M2 ∨ M2 = –›’‡ C ∀T′ ∈ U′. ∃T ∈ U. T′ <: They include more information than typical functions: properties, excluded, and captured
52. FORMALIZATION This additional information is used when type- checking (assume S1 and S2 are the types of two spores): properties: S1 <: S2 implies S1 must satisfy at least the properties of S2 excluded: S1 <: S2 implies S1 must exclude at least the types that S2 excludes captured: S1 <: S2 implies S2 either has abstract type Captured or S1 and S2 agree on the captured types (Intuition)
53. FORMALIZATION This additional information is used when type- checking (assume S1 and S2 are the types of two spores): properties: S1 <: S2 implies S1 must satisfy at least the properties of S2 excluded: S1 <: S2 implies S1 must exclude at least the types that S2 excludes captured: S1 <: S2 implies S2 either has abstract type Captured or S1 and S2 agree on the captured types (Intuition)
54. FORMALIZATION Sound composition of constraints: avoid calculating constraints that are not guaranteed to hold at runtime SPORE COMPOSITION PRESERVES TYPE CONSTRAINTS …In the paper: ! Soundness proof based on a small-step operational semantics and progress+preservation. ! Correspondence to the Scala implementation
55. HOW ARESPORES PRACTICAL ?
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59. THANK YOU.