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Runtime Checking
+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
Runtime Session Types in C# Per Andersson Christian Genne - - PowerPoint PPT Presentation
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
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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
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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."); }); }); } }
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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) { ... } }
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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) { ... } }
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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
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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
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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
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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
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