Runtime Session Types in C# Per Andersson Christian Genne - PowerPoint PPT Presentation

Runtime Session Types in C# Per Andersson Christian Genne http://www.pean.se/sessiontypes/

Runtime Checking Idea: Generate and check types when parties connect +Problems can be detected in a determinstic way with a single test run + Small (hopefully no) changes to C# - Problems can't be detected at compile time

Session Class  User interface implemented in the class Session  Sessions are started with Accept or Open  Accept and Open takes a delegate that is called twice  1:st call infers session type  2:nd call executes the session  User must encapsulate side effects in Session.Do

public class Buyer { public void Run() { Session.Open (sellerHost, sellerPort, delegate( Session s ) { Log( s, "I want to buy " + productid ); s.Send (productid); int price = s.Recv <int>(); s.SendIf (price < maxPrice, delegate() { Log(s, "Fair price, i'm buying! Here is my address."); s.Send (address); DateTime delivdate = s.Recv <DateTime>(); s.Do (delegate() { notifyUser(delivTime); }); }, delegate() { Log(s, price + "? That is to expensive for me."); }); }); } }

public class Seller { public void Run() { Session.Accept (IPAddress.Loopback, port, delegate( Session s ) { String prodId = s.Recv <String>(); int price = 50; Log(s, "So you want to buy " + prodId + ". " + price + " sounds like a nice deal?"); string prodDetails = prodId + " details"; s.Send (price); s.RecvIf (delegate() { Log(s, "Good choice. I'll transfer your request to the shipper"); s.OpenNew ( shipperHost, shipperPort, delegate( Session s2 ) { s2.Send (prodDetails); s2.SendS (s); }); s.Do (delegate() { createOrder(prodid); }); }, delegate() { Log(s, "Yes we are expensive!"); }); }); } void createOrder(String prodid, Address address) { ... } }

public class Shipper { public void Run() { Session.Accept (IPAddress.Loopback, port, delegate( Session s ) { string prodDetails = s.Recv <string>(); Log(s, "So you want me to ship a " + prodDetails + ". Let me talk to the buyer!" ); s.RecvS (delegate( Session s2 ) { Address address = s2.Recv <Address>(); s2.Do (delegate() { Log(s2, "Ok, i've got the address. " + address.mStreet + " " + address.mNr); Log(s2, "I'm creating an order"); }); DateTime delivdate = null; s2.Do (delegate() { delivdate = createShipment(prodDetails, address); }); s2.Send (delivDate); }); }); } DateTime createShipment(String prodid, Address address) { ... } }

Problem 1 – Side Effects  The type inference step must not execute side effects like DB transactions  A) Create a custom type system for C# with better control on side effects + Safe - Time consuming to implement - May break with new C# version  B) Move responsibility to user by using Session.Do - Unsafe + Easy to implement + Just a new method, no C# language changes

Problem 2 – Higher Order Sessions  How does Seller know its type when Buyer connects? It depends on Shippers type!  A) Type infer Seller-Shipper when type infering Seller-Buyer + Type errors occur at Buyer-Seller connection, or not at all - Shipper IP must be static  B) Delay higer order type checking - Type errors becomes runtime errors + No restriction on Shipper IP

User Restrictions  All communication through a Session s  Encapsulate side effects in s.Do  s.Do( delegate() { WriteDB( value ); } );  If-statements must be replaced by s.SendIf and s.RecvIf  Other conditional statements are not allowed  Only server sessions are allowed to send sessions

Conclusions  Session types can be implemented reasonbly well in pure C#  Syntax would be nicer with C# 3.0 support for lambda expressions  Easy implementation but requires more from the user compared to static type checking  Any conditional statement can easily be added to the Session class

Questions?