1. SCOOPLI for .NET: a library for concurrent object-oriented programming
Volkan Arslan, Piotr Nienaltowski
Chair of Software Engineering, ETH Zurich
2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003

2. 2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003
Overview
Motivation
The SCOOP model for concurrent OO programming
SCOOPLI library
Implementation for .NET
Conclusions
2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003

3. 2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003
Motivation
Extend a pure, strongly typed, object-oriented language with a simple, general and powerful concurrency and distribution model
2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003

4. 2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003
The SCOOP model
Simple Concurrent Object-Oriented Programming
High-level concurrency mechanism
Full use of inheritance and other object-oriented techniques (genericity, agents, …)
Applicable to many physical setups: multiprocessing, multithreading, distributed execution, etc.
Minimal extension of Eiffel
one new keyword separate
Based on Design by Contract™
new semantics for preconditions
2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003

5. Two-level implementation of SCOOP
SCOOP can be implemented in several environments
Microsoft .NET is our reference platform
SCOOP
platform-independent
.NET
Compact
Framework
POSIX …
2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003

6. 2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003
Asynchronous calls
Fundamental scheme of the O-O computation: feature call x.f (a)
Caller and callee handled by different processors
Asynchronous semantics
2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003

7. 2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003
Processors
Processor is an autonomous thread of control capable of supporting the sequential execution of instructions on one or more objects
Principal new concept introduced by SCOOP
Not to be confused with a physical CPU! It can be implemented as:
piece of hardware (computer, CPU),
process,
thread,
AppDomain,
etc.
2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003

8. 2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003
Access control policy
Target of a separate call must be a formal argument of the enclosing routine
store (buffer: separate BUFFER; value: INTEGER) is
-- Store value in buffer. do
buffer.put (value)
end
...
buf: separate BUFFER
create buf.make
store (buf, 10) -- instead of buf.put (10)
In order to obtain exclusive access to a separate object, use the attached entity as an argument of the corresponding call, as in store (buf, 10).
2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003

9. From preconditions to wait-conditions
Contracts in Eiffel
store (buffer: BUFFER; value: INTEGER) is
-- Store value in buffer.
require
buffer_not_full: not buffer.is_full do
buffer.put (value)
ensure
buffer_not_empty: not buffer.is_empty
end
...
store (buf, 10)
If buffer is separate, correctness condition buffer_not_full becomes wait condition
2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003

10. 2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003
Synchronization
No special mechanism is required for a client to resynchronize with its supplier after a separate call.
The client will wait if and only if it needs:
x.f
x.g (a)
y.f
...
value := x.some_query
This mechanism is called wait by necessity.
2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003

11. 2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003
SCOOPLI library
The library relies on two concepts:
separate client
separate supplier
Separate client is handled by a different processor than each of its separate suppliers.
SCOOPLI uses multiple inheritance to provide separateness:
SEPARATE_
SUPPLIER X
SEPARATE_X
2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003

12. 2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003
Use of SCOOPLI (1/2)
SCOOP
SCOOPLI
x: separate X
x: X -- class X is separate
x: SEPARATE_X
-- SEPARATE_X inherits from X and
-- SEPARATE_SUPPLIER
r (x, y)
-- x and y are separate
r (x: separate X; y: separate Y) is
require
not x.is_empty
y.count > 5
i > 0 -- i non-separate do
...
end
separate_execute ([x, y],
agent r (x, y), agent r_precondition)
r_precondition: BOOLEAN is
Result := not x.is_empty and y.count > 5
-- separate_execute defined in class SEPARATE_CLIENT
-- client class inherits from
-- class SEPARATE_CLIENT
2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003

13. Feature separate_execute of SEPARATE_CLIENT
separate_execute (requested_objects: TUPLE [SEPARATE_SUPPLIER];
action: PROCEDURE [ANY, TUPLE];
wait_condition: FUNCTION [ANY, TUPLE, BOOLEAN])
Formal arguments:
requested_objects Denotes the (tuple of) objects on which exclusive locks should be acquired before calling action.
action Denotes the routine to be called on the separate client object; action corresponds to the routine that “wraps” separate calls
wait_condition Denotes the boolean function representing the wait condition for the call
2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003

14. 2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003
Use of SCOOPLI (1/2)
SCOOP
SCOOPLI
x: separate X
x: X -- class X is separate
x: SEPARATE_X
-- SEPARATE_X inherits from X and
-- SEPARATE_SUPPLIER
r (x, y)
-- x and y are separate
r (x: separate X; y: separate Y) is
require
not x.is_empty
y.count > 5
i > 0 -- i non-separate do
...
end
separate_execute ([x, y],
agent r (x, y), agent r_precondition)
r_precondition: BOOLEAN is
Result := not x.is_empty and y.count > 5
-- separate_execute defined in class SEPARATE_CLIENT
-- client class inherits from
-- class SEPARATE_CLIENT
2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003

15. 2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003
Use of SCOOPLI (2/2)
SCOOP
SCOOPLI
x.f (a) -- a is expanded
separate_routine (x, agent x.f (a))
res := x.g (a)
-- g is function and returns
-- reference value
-- res is non-separate
res ?= separate_value (x, agent x.g (a))
i := x.h (a)
-- h is function and returns
-- expanded value
i := separate_integer_value (x, agent x.h (a))
-- use function corresponding to returned type:
-- boolean_value, integer_value, real_value,
-- double_value, char_value
if x.count > 0 then ...
if separate_integer_value (x,
agent x.g (a)) > 0 then ...
2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003

16. Feature separate_routine of SEPARATE_CLIENT
separate_routine (supplier: SEPARATE_SUPPLIER; procedure: PROCEDURE [ANY, TUPLE])
Formal arguments:
supplier Denotes the separate supplier object on which the separate call to procedure is made
procedure Denotes the routine to be called on the separate supplier object
2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003

17. 2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003
Use of SCOOPLI (2/2)
SCOOP
SCOOPLI
x.f (a) -- a is expanded
separate_routine (x, agent x.f (a))
res := x.g (a)
-- g is function and returns
-- reference value
-- res is non-separate
res ?= separate_value (x, agent x.g (a))
i := x.h (a)
-- h is function and returns
-- expanded value
i := separate_integer_value (x, agent x.h (a))
-- use function corresponding to returned type:
-- boolean_value, integer_value, real_value,
-- double_value, char_value
if x.count > 0 then ...
if separate_integer_value (x,
agent x.g (a)) > 0 then ...
2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003

18. Feature separate_value of SEPARATE_CLIENT
separate_value (supplier: SEPARATE_SUPPLIER; function: FUNCTION [ANY, TUPLE, ANY]): ANY
Formal arguments:
Supplier Denotes the separate supplier object on which the separate call to function is made.
Function Denotes the function to be evaluated.
Return value is of type ANY
2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003

19. 2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003
Use of SCOOPLI (2/2)
SCOOP
SCOOPLI
x.f (a) -- a is expanded
separate_routine (x, agent x.f (a))
res := x.g (a)
-- g is function and returns
-- reference value
-- res is non-separate
res ?= separate_value (x, agent x.g (a))
i := x.h (a)
-- h is function and returns
-- expanded value
i := separate_integer_value (x, agent x.h (a))
-- use function corresponding to returned type:
-- boolean_value, integer_value, real_value,
-- double_value, char_value
if x.count > 0 then ...
if separate_integer_value (x,
agent x.count) > 0 then ...
2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003

20. 2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003
SCOOPLI for .NET
Mapping of SCOOP concepts to .NET constructs
Processors  AppDomains
Namespace System.Runtime.Remoting
Use of multithreading
Single feature calls on separate objects, thus one thread per AppDomain
Namespace System.Threading
ThreadPool
2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003

21. Distributed execution
Processors (AppDomains) located on different machines
.NET takes care of the “dirty work”
Marshalling
Minimal cost of inter-AppDomain calls
Computer1
Computer3
AppDomain1
o1.g
AppDomain3 o1 o2 o4 o5
o4.f
Computer2
AppDomain4
o8.g
o9.f
o6.f(o3)
AppDomain2 o6 o7 o8 o3 o9
2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003

22. CCF – Concurrency Control File
Location of processors does not need to be specified at compile time
On-the-fly specification with CCF
creation
local_nodes:
system
"susi" (1): "c:\prog\appl1\appl1.exe"
"ruth" (1): "c:\prog\appl2\appl2.dll"
"schlemmer" (2): "c:\prog\appl3\appl3.dll"
end
2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003

23. 2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003
Conclusions
SCOOP model is simple yet powerful
Full concurrency
Full use of object-oriented techniques
One keyword separate
Based on Design by Contract™
Several platforms
SCOOPLI library
SCOOP-based syntax
Separate clients, separate suppliers
.NET as reference platform
Processors mapped to AppDomains
Distributed execution with .NET Remoting
2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003

24. 2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003
Future research
Extension of access control policy
multiple locking of separate objects based on the concept of pure functions
SCOOP for real-time systems
specifying timing constraints
adding priorities to the duel mechanism
Implementation
Use of agents and delegates
Porting of SCOOPLI to .NET CF using threads
2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003

25. 2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003
Questions ?
Thank you !
2nd Microsoft Rotor Workshop, Pisa, April 23-25, 2003