1 Polyphonic C# Nick Benton Luca Cardelli Cédric Fournet Microsoft Research

2 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

3 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

4.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)

5 Polyphonic C# An extension of the C# language with new concurrency constructs Based on the join calculus A foundational process calculus like the -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 its also simple – if Mort can do any kind of concurrency, he can do this

6 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.

7 A simple buffer class Buffer { String get() & async put(String s) { return s; }

8 A simple buffer class Buffer { String get() & async put(String s) { return s; } An ordinary (synchronous) method with no arguments, returning a string

9 A simple buffer class Buffer { String get() & async put(String s) { return s; } An ordinary (synchronous) method with no arguments, returning a string An asynchronous method (hence returning no result), with a string argument

10 A simple buffer class Buffer { String get() & async put(String s) { return s; } An ordinary (synchronous) method with no arguments, returning a string An asynchronous method (hence returning no result), with a string argument Joined together in a chord

11 A simple buffer class Buffer { String get() & async put(String s) { return s; } Calls to put() return immediately (but are internally queued if theres no waiting get() ). Calls to get() block until/unless theres a matching put() When theres a match the body runs, returning the argument of the put() to the caller of get(). Exactly which pairs of calls are matched up is unspecified.

12 A simple buffer class Buffer { String get() & async put(String s) { return s; } Does example this involve spawning any threads? No. Though the calls will usually come from different pre- existing threads. So is it thread-safe? You dont seem to have locked anything… Yes. The chord compiles into code which uses locks. (And that doesnt mean everything is synchronized on the object.) Which method gets the returned result? The synchronous one. And there can be at most one of those in a chord.

13 Reader/Writer …using threads and mutexes in Modula 3 An introduction to programming with threadsAn introduction to programming with threads. Andrew D. Birrell, January 1989.

14 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) … Reader/Writer in five chords

15 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); }

16 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

17 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

18 Active Objects public abstract class ActiveObject : MarshalByRefObject { protected bool done; abstract protected void processmessage(); public ActiveObject () { done = false; mainloop(); } async mainloop() { while (!done) { processmessage(); } }

19 …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 } … }

20 Extending C# with chords 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. 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)

21 JoCaml allows multiple synchronous methods to be joined, as in the following rendezvous 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, its still easy to code up an asymmetric rendezvous in Polyphonic C#.) Why only one synchronous method in a chord? int f(int x) & int g(int y) { return y to f; return x to g; }

22 The problem with inheritance Weve half overridden f Too easy to create deadlock or async leakage class C { virtual void f() & virtual async g() {…} virtual void f() & virtual async h() {…} } class D : C { override async g() { …} } void m(C x) { x.g(); x.f();} … m(new D());

23 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.

24 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

25 Implementation Translate Polyphonic C# -> C# Built on Proebsting & Hansons lcsc 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,…

26 Samples animated dining philosophers web service combinators (Cardelli & Davies) adaptive scheduler (cf. Larus & Parkes), accessing web services (Terraserver), active objects and remoting (stock trader)

27 Current and future work Direct syntactic support for timeouts Limited pattern-matching on message contents Adding joinable transactions with explicit compensations Behavioural types?

28 Conclusions A clean, simple, new model for asynchronous concurrency in C# Declarative, local synchronization Model good for both local and distributed settings Efficiently compiled to queues and automata Easier to express and enforce concurrency invariants Compatible with existing constructs, though they constrain our design somewhat Minimalist design – pieces to build whatever complex synchronization behaviours you need Solid foundations Works well in practice

29 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() {} }

30 Predictable Demo: Dining Philosophers eating waiting to eat waiting to eat thinking

31 Code extract class Room { public Room (int size) { hasspaces(size); } public void enter() & private async hasspaces(int n) { if (n > 1)hasspaces(n-1); elseisfull(); } public void leave() & private async hasspaces(int n) { hasspaces(n+1); } public void leave() & private async isfull() { hasspaces(1); } }

32 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) { if (n==1) Idle(); else S(n-1); } Could even allow when patterns as general statements, though this seems in dubious taste…

33 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

34 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; }

35 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 }

36 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); }

37 Pattern Matching async Sell(string item, Client seller) & async Buy (string item, Client buyer) {... // match them up } Very useful, but hard to compile efficiently

38 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); }... }

39 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); }... }