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Polyphonic C# Nick Benton Luca Cardelli Cédric Fournet Microsoft Research 1 Asynchrony is where its at Distribution = concurrency + latency = asynchrony = more concurrency Message-passing, event-based programming, dataflow models For programming languages, coordination (orchestration) languages & frameworks, workflow 2 Language support for concurrency Make invariants and intentions more apparent (part of the interface) Good software engineering Allows the compiler much more freedom to choose different implementations Also helps other tools 3 .NET today Java-style “monitors” OS shared memory primitives Clunky delegate-based asynchronous calling model Hard to understand, use and get right Different models at different scales Support for asynchrony all on the caller side – little help building code to handle messages (must be thread-safe, reactive, and deadlock-free) 4 Polyphonic C# An extension of the C# language with new concurrency constructs Based on the join calculus A foundational process calculus like the p-calculus but better suited to asynchronous, distributed systems A single model which works both for local concurrency (multiple threads on a single machine) distributed concurrency (asynchronous messaging over LAN or WAN) It is different But it’s also simple – if Mort can do any kind of concurrency, he can do this 5 In one slide: Objects have both synchronous and asynchronous methods. Values are passed by ordinary method calls: If the method is synchronous, the caller blocks until the method returns some result (as usual). If the method is async, the call completes at once and returns void. A class defines a collection of chords (synchronization patterns), which define what happens once a particular set of methods have been invoked. One method may appear in several chords. When pending method calls match a pattern, its body runs. If there is no match, the invocations are queued up. If there are several matches, an unspecified pattern is selected. If a pattern containing only async methods fires, the body runs in a new thread. 6 A simple buffer class Buffer { String get() & async put(String s) { return s; } } 7 A simple buffer class Buffer { String get() & async put(String s) { return s; } } ãAn ordinary (synchronous) method with no arguments, returning a string 8 A simple buffer class Buffer { String get() & async put(String s) { return s; } } ãAn ordinary (synchronous) method with no arguments, returning a string ãAn asynchronous method (hence returning no result), with a string argument 9 A simple buffer class Buffer { String get() & async put(String s) { return s; } } ãAn ordinary (synchronous) method with no arguments, returning a string ãAn asynchronous method (hence returning no result), with a string argument ãJoined together in a chord 10 A simple buffer class Buffer { String get() & async put(String s) { return s; } } ãCalls to put() return immediately (but are internally queued if there’s no waiting get()). ãCalls to get() block until/unless there’s a matching put() ãWhen there’s a match the body runs, returning the argument of the put() to the caller of get(). ãExactly which pairs of calls are matched up is unspecified. 11 A simple buffer class Buffer { String get() & async put(String s) { return s; } ãDoes example this involve spawning any threads? } ãNo. Though the calls will usually come from different pre- existing threads. ãSo is it thread-safe? You don’t seem to have locked anything… ãYes. The chord compiles into code which uses locks. (And that doesn’t mean everything is synchronized on the object.) ãWhich method gets the returned result? ãThe synchronous one. And there can be at most one of those 12 in a chord. Reader/Writer …using threads and mutexes in Modula 3 An introduction to programming with threads. Andrew D. Birrell, January 1989. 13 Reader/Writer in five chords public class ReaderWriter { public void Exclusive() & async Idle() {} public void ReleaseExclusive() { Idle(); } public void Shared() & async Idle() { S(1); } public void Shared() & async S(int n) { S(n+1); } public void ReleaseShared() & async S(int n) { if (n == 1) Idle(); else S(n-1); } public ReaderWriter() { Idle(); } } A single private message represents the state: none Idle() S(1) S(2) S(3) … 14 Asynchronous requests and responses Service exposes an async method which takes parameters and somewhere to put the result: a buffer, or a channel, or a delegate public delegate async IntCB(int v); public class Service { public async request(String arg, IntCB callback) { int result; // do something interesting… callback(result); } 15 } Asynchronous requests and responses - Join class Join2 { void wait(out int i, out int j) & async first(int r1) & async second(int r2) { i = r1; j = r2; return; } } // client code: int i,j; Join2 x = new Join2(); service1.request(arg1, new IntCB(x.first)); service2.request(arg2, new IntCB(x.second)); // do something useful // now wait until both results have come back x.wait(out i,out j); // do something with i and j 16 Asynchronous requests and responses - Select class Select { int wait() & async reply(int r) { return r; } } // client code: int i; Select x = new Select(); service1.request(arg1, new IntCB(x.reply)); service2.request(arg2, new IntCB(x.reply)); // do something useful // now wait until one result has come back i = x.wait(); // do something with i 17 Active Objects public abstract class ActiveObject : MarshalByRefObject { protected bool done; abstract protected void processmessage(); public ActiveObject () { done = false; mainloop(); } async mainloop() { while (!done) { processmessage(); } } } 18 …continued class Stock : ActiveObject { override protected void processmessage() & public async bid(BidOffer thebid) { // process bid messages } override protected void processmessage() & public async register(Client who) { // process registration requests } … } 19 Extending C# with chords Classes can declare methods using generalized chord-declarations instead of method-declarations. chord-declaration ::= method-header [ & method-header ]* body method-header ::= attributes modifiers [return-type | async] name (parms) Interesting well-formedness conditions: 1. At most one header can have a return type (i.e. be synchronous). 2. The inheritance restriction. 3. “ref” and “out” parameters cannot appear in async headers. 20 Why only one synchronous method in a chord? JoCaml allows multiple synchronous methods to be joined, as in the following rendezvous int f(int x) & int g(int y) { return y to f; return x to g; } But in which thread does the body run? In C#, thread identity is “very” observable, since threads are the holders of particular re- entrant locks. So we rule this out in the interests of keeping & commutative. (Of course, it’s still easy to code up an asymmetric rendezvous in Polyphonic C#.) 21 The problem with inheritance class C { virtual void f() & virtual async g() {…} virtual void f() & virtual async h() {…} } class D : C { override async g() { …} } We’ve “half” overridden f Too easy to create deadlock or async leakage void m(C x) { x.g(); x.f();} … m(new D()); 22 The inheritance restriction Two methods are co-declared if they appear together in a chord declaration. Whenever a method is overridden, every co-declared method must also be overridden. Hence, the compiler rejects patterns such as public virtual void f() & private async g() {…} In general, inheritance and concurrency do not mix well. Our restriction is simple; it could be made less restrictive. 23 Types etc. async is a subtype of void Allow covariant return types on those two: An async method may override a void one A void delegate may be created from an async method An async method may implement a void method in an interface async methods are given the [OneWay] attribute, so remote calls are non-blocking 24 Demo 25 Coming Soon: C 26 C Integrates Polyphonic C# with “the language formerly known as Xen” Erik Meijer, Wolfram Shulte and others in WebData Claudio Russo, Gavin Bierman Data integration: unifying objects with relational and XML data Features: Stream types Tuples Unions XML literals Generalized member access 27 Simple streams public static int* FromTo(int s, int e) { for (i = s; i = e; i++) yield i; } int* OneToTen = FromTo(1,10); foreach(int j in OneToTen) { Console.WriteLine(j); }; 28 Stream typing Possibly T* = 0 null = 1 T? T+ T S : T S* : T* Don't care T! T** ≈ T* Never null 29 Tuple typing (simplified) covariance S : T […,S,…] : […,T,…] forget labels […, T m,…] : […,T,…] […, T,…] : (…|T|…)* forget ordering 30 Typing simple queries type Sales = [ int Sale, int Employee, string Customer, string Item, string Supplier, decimal Price ] Sales* Sale Employee Customer Item Supplier Price 1 1 Simpson Sofa Harrison $235.67 … … … … … … [int Sale, string Item, decimal Price]* s = select Sale, Item, Price from Sales; 31 Unions XSD-style class Address { content sequence{ model choice { string Street; int POBox; }; string City; int ZipCode; string Country; }; }; Address! a = …; string? street = a.Street; // maybe null choice{string; int;}* m = a.*; 32 XML literals (with holes) class Email {…} static Email OOF (Date d, TimeSpan s, string to){ return Email Header To {to} /To From Wolfram /From Subject OOF /Subject /Header Body P I am OOF from {d} until {d+s}. /P /Body /Email ; } 33 Conclusions A clean, simple, new model for asynchronous concurrency in C# Model good for both local and distributed settings Efficiently compiled to queues and automata Works well in practice Plus relational, XML, object integration streams, tuples, unions, XML, select, … http://research.microsoft.com/~nick/polyphony/ 34 Fairer reader/writer lock class ReaderWriterFair { ReaderWriter() { idle(); } private int n = 0; // protected by s() or t() public void Shared() & async idle() { n=1; s(); } public void Shared() & async s() { n++; s(); } public void ReleaseShared() & async s() { if (--n == 0) idle(); else s(); } public void Exclusive() & async idle() {} public void ReleaseExclusive() { idle(); } public void ReleaseShared() & async t() { if (--n == 0) idleExclusive(); else t(); } public void Exclusive() & async s() { t(); wait(); } void wait() & async idleExclusive() {} } 35 Predictable Demo: Dining Philosophers eating waiting to eat waiting to eat thinking eating 36 Code extract class Room { public Room (int size) { hasspaces(size); } public void enter() & private async hasspaces(int n) { if (n 1) hasspaces(n-1); else isfull(); } public void leave() & private async hasspaces(int n) { hasspaces(n+1); } public void leave() & private async isfull() { hasspaces(1); } } 37 A Better Syntax? class ReaderWriter { private async Idle(); // declare asyncs private async S(int n); public void Exclusive() when Idle() {} public void ReleaseExclusive() { Idle(); } public void Shared() // syncs can have sequence of when Idle() {S(1);} // “when” patterns involving | when S(int n) {S(n+1);} // asyncs public void ReleaseShared() when S(int n) { Could even allow when if (n==1) Idle(); else S(n-1); patterns as general statements, } though this seems in dubious } taste… 38 Santa Claus problem (Trono, Ben-Ari) Santa sleeps until awakened by either all 9 reindeer or by 3 of the 10 elves. If woken by reindeer he harnesses them all up, they deliver presents together, he unharnesses them, they go off on holiday and he goes back to sleep. If woken by a group of elves, he shows them into his office, consults with them on toy R&D then shows them all out and goes back to sleep. Surprisingly tricky to avoid bugs such as Santa going off without the reindeer, queue-jumping elves Trono posed problem and gave incorrect solution using semaphores Ben-Ari gave a non-trivial solution using Ada primitives and ugly, inefficient and unsatisfactory solution in Java 39 public class nway { public async produce(int n) & public void consume() { if (n==1) { alldone(); } else { produce(n-1); } } public void waitforalldone() & async alldone() { return; } } 40 class santa { static nway harness = new nway(); static nway unharness = new nway(); static nway roomin = new nway(); static nway roomout = new nway(); static void santalife() { while (true) { waittobewoken(); // get back here when dealt with elves or reindeer } } static void waittobewoken() & static async elvesready() { roomin.produce(3); roomin.waitforalldone(); elveswaiting(0); // all elves in the room, consult roomout.produce(3); roomout.waitforalldone(); // all elves shown out, go back to bed } static void waittobewoken() & static async reindeerready() { // similar to elvesready chord } 41 static async elflife(int elfid) { while (true) { // work elfqueue(); // wait to join group of 3 roomin.consume(); // wait to be shown in // consult with santa roomout.consume(); // wait to be shown out again } } static void elfqueue() & static async elveswaiting(int e) { if (e==2) { elvesready(); // last elf in a group so wake santa } else { elveswaiting(e+1); } } 42 Pattern Matching async Sell(string item, Client seller) & async Buy (string item, Client buyer) { ... // match them up } Very useful, but hard to compile efficiently 43 Ordered processing, currently class SequenceProcessor : ActiveObject { private SortedList pending = new SortedList(); private int next = 0; public async Message(int stamp, string contents) & override protected void ProcessMessage() { if (stamp == next) { DealWith(contents); while (pending.ContainsKey(++next)) { DealWith((string)pending[next]); pending.Remove(next); } } else { pending.Add(stamp,contents); } } ... 44 } with matching class SequenceProcessor : ActiveObject { public async Message(int stamp, string contents) & override protected void ProcessMessage() & async waitingfor(int stamp) { DealWith(contents); waitingfor(stamp++); } SequenceProcessor() { waitingfor(0); } ... } 45 Implementation Translate Polyphonic C# - C# Introduce queues for pending calls (holding blocked threads for sync methods, arguments for asyncs) Generated code (using brief lock to protect queue state) looks for matches and then either Enqueues args (async no match) Enqueues thread and blocks (sync no match) Dequeues other args and continues (sync match) Wakes up blocked thread (async match with sync) Spawns new thread (async match all async) Efficient – bitmasks to look for matches, no PulseAlls,… 46 Samples animated dining philosophers web service combinators (Cardelli & Davies) adaptive scheduler (cf. Larus & Parkes), accessing web services (Terraserver), active objects and remoting (stock trader) 47 Current and future work Direct syntactic support for timeouts Limited pattern-matching on message contents Adding joinable transactions with explicit compensations Behavioural types? 48
