ObjectOriented Software Construction Bertrand Meyer Chair of Software

Object-Oriented Software Construction Bertrand Meyer Chair of Software Engineering OOSC Lecture 20 - Concurrency

Lecture 20: Object-oriented concurrency Piotr Nienaltowski Chair of Software Engineering OOSC Lecture 20 - Concurrency

Overview § A bit of history § O-O and concurrency § SCOOP model § Examples Chair of Software Engineering OOSC Lecture 20 - Concurrency

Sequential programming § Used to be messy § It § § § is still hard but: Structured programming Data abstraction & object technology Design by Contract Genericity Architectural techniques § Switch from operational reasoning to logical deduction (e. g. invariants) Chair of Software Engineering OOSC Lecture 20 - Concurrency

Concurrent programming § Used to be messy § And it still is! § Java/C# threading models § No real breakthrough § Only understandable through operational reasoning Chair of Software Engineering OOSC Lecture 20 - Concurrency

Impedance mismatch § OO: high-level abstraction mechanisms § Concurrency: very low-level mechanisms § semaphores, locks, critical sections, suspend, … Chair of Software Engineering OOSC Lecture 20 - Concurrency

O-O and concurrency § “I don’t understand why objects are non concurrent in the first place” (Robin Milner) § Many attempts, e. g. active objects § Problems: inheritance anomaly § No mechanism widely accepted § In practice, low-level mechanisms on top of O-O language Chair of Software Engineering OOSC Lecture 20 - Concurrency

Motivations § Extend object technology with general and powerful concurrency support § Provide the industry with simple techniques for parallel, distributed, Internet, real-time programming § Let programmers sleep better! Chair of Software Engineering OOSC Lecture 20 - Concurrency

The SCOOP model § Simple Concurrent Object-Oriented Programming § High-level concurrency mechanism § Fully uses inheritance and other O-O techniques § Applicable to many physical setups: multiprocessing, multithreading, distributed execution, Web services. . . § Minimal extension of Eiffel § Based on Design by Contract Chair of Software Engineering OOSC Lecture 20 - Concurrency

Object-oriented computation Actions Objects Processor To perform a computation is § to apply certain actions § to certain objects § using certain processors. Chair of Software Engineering OOSC Lecture 20 - Concurrency

What makes an application concurrent? Processor: autonomous thread of control supporting sequential execution of instructions on one or more objects Actions Can be implemented as: § Process § Thread Processor § current implementation based on. NET threading § App. Domain (. NET) § Web service Chair of Software Engineering OOSC Lecture 20 - Concurrency Objects

Handling rule All calls on an object are executed by its processor’s handler Chair of Software Engineering OOSC Lecture 20 - Concurrency

Feature call: synchronous x: CLASS_X. . . x. f (a) Object 1 Object 2 f (a: A) require a /= Void ensure not a. empty previous_instruction x. f (a) next_instruction (CLASS_T ) (CLASS_X) Processor Chair of Software Engineering OOSC Lecture 20 - Concurrency

Separate feature call: asynchronous x: separate CLASS_X. . . x. f (a) Object 1 previous_instruction x. f (a) next_instruction Object 2 f (a: A) require a /= Void ensure not a. empty (CLASS_T) Processor 1 Chair of Software Engineering (CLASS_X) Processor 2 OOSC Lecture 20 - Concurrency

Separateness rule Calls to non-separate objects are synchronous Calls to separate objects are asynchronous Chair of Software Engineering OOSC Lecture 20 - Concurrency

Reasoning about objects {Prer and INV} bodyr {POSTr and INV} {Prer’} x. r (a) {POSTr’} Only n proofs for n exported routines! Chair of Software Engineering OOSC Lecture 20 - Concurrency

In a concurrent context {Prer and INV} bodyr {POSTr and INV} {Prer’} x. r (a) {POSTr’} Client 1, r 1 Client 3, r 3 Client 2, r 2 Only n proofs for n exported routines? Chair of Software Engineering OOSC Lecture 20 - Concurrency

Mutual exclusion rule At most one feature may execute on any one object at any one time Chair of Software Engineering OOSC Lecture 20 - Concurrency

Access control policy Target of separate call must be formal argument of enclosing routine: store (buffer: separate BUFFER [INTEGER]; value: INTEGER) is -- Store value into buffer. do buffer. put (value) end Call with separate argument gives exclusive access: store (my_buffer, 10) Chair of Software Engineering OOSC Lecture 20 - Concurrency

Contracts in Eiffel store (buffer: BUFFER [INTEGER]; value: INTEGER) is -- Store value into buffer. require not buffer. is_full value > 0 do buffer. put (value) ensure not buffer. is_empty end. . . store (my_buffer, 10) Precondition § If b is separate, precondition becomes wait condition (instead of correctness condition) Chair of Software Engineering OOSC Lecture 20 - Concurrency

From preconditions to wait-conditions store (buffer: separate BUFFER [INTEGER]; value: INTEGER) is -- Store value into buffer. require not buffer. is_full value > 0 do buffer. put (value) ensure not buffer. is_empty end. . . store (my_buffer, 10) Chair of Software Engineering On separate target, precondition becomes wait condition OOSC Lecture 20 - Concurrency

Wait by necessity § No special mechanism for client to resynchronize with supplier after separate call. § The client will wait only when it needs to: x. f x. g (a) y. f … value : = x. some_query Wait here! Chair of Software Engineering OOSC Lecture 20 - Concurrency

Duels Problem: Impatient client (challenger) wants to snatch object from another client (holder) § Can’t just interrupt holder, service challenger, and resume holder: would produce inconsistent object. § But: can cause exception, which will be handled safely. Chair of Software Engineering OOSC Lecture 20 - Concurrency

Duels Challenger → normal_service immediate_service ↓ Holder retain Challenger waits Exception in challenger yield Challenger waits Exception in holder; serve challenger Chair of Software Engineering OOSC Lecture 20 - Concurrency

Mapping processors to physical resources Concurrency Control File (CCF) create system "palin" (4): "c: progappl 1. exe" "cleese" (2): "c: progappl 2. dll" "Current" (5): "c: progappl 3. dll" end external Database_handler: "jones" port 9000 ATM_handler: "chapman" port 8001 end default port: 8001; instance: 10 end Chair of Software Engineering OOSC Lecture 20 - Concurrency

Implementation of SCOOP § Can be implemented on various platforms § Microsoft. NET is our reference platform SCOOP platform-independent . NET / ROTOR Chair of Software Engineering NET Compact Framework. POSIX Threads OOSC Lecture 20 - Concurrency …

SCOOPLI: a library for SCOOP § Implemented in Eiffel on. NET § Aim: try out solutions without bothering with compiler issues § Can serve as a basis for compiler implementations Chair of Software Engineering OOSC Lecture 20 - Concurrency

Example: Bounded buffers separate class BOUNDED_BUFFER [G] inherit BOUNDED_QUEUE [G] end Chair of Software Engineering OOSC Lecture 20 - Concurrency

Example: Bounded buffers class BUFFER_ACCESS [G] feature put (b: BOUNDED_BUFFER [G]; x: G) is -- Insert x into b. require not b. full do b. put (x) ensure not b. empty end Chair of Software Engineering OOSC Lecture 20 - Concurrency

Example: Bounded buffers remove (b: BOUNDED_BUFFER [G]) is -- Remove an element from b require not b. empty do b. remove ensure not b. full end item (b: BOUNDED_BUFFER [G]): G is -- Oldest element not yet consumed require not b. empty do Result : = b. item end -- class BUFFER_ACCESS [G] Chair of Software Engineering OOSC Lecture 20 - Concurrency

Example: Bounded buffers Usage of bounded buffers my_buffer_access: BUFFER_ACCESS [INTEGER] my_bounded_buffer: BOUNDED_BUFFER [INTEGER] create my_buffer_access create my_bounded_buffer my_buffer_access. put (my_bounded_buffer, 25) my_buffer_access. put (my_bounded_buffer, 50) my_result : = my_buffer_acces. item (my_bounded_buffer) Chair of Software Engineering OOSC Lecture 20 - Concurrency

Example: Elevator In order to achieve maximal concurrency, all objects are separate Chair of Software Engineering OOSC Lecture 20 - Concurrency

Class BUTTON Chair of Software Engineering OOSC Lecture 20 - Concurrency

Class BUTTON separate class BUTTON feature target: INTEGER end -- class BUTTON Chair of Software Engineering OOSC Lecture 20 - Concurrency

Class CABIN_BUTTON separate class CABIN_BUTTON inherit BUTTON feature cabin: ELEVATOR request is -- Send to associated elevator a request to stop on level target. do actual_request (cabin) end Chair of Software Engineering OOSC Lecture 20 - Concurrency

Class CABIN_BUTTON actual_request (e: ELEVATOR) is -- Get hold of e and send a request to stop on level target. do e. accept (target) end -- class CABIN_BUTTON Chair of Software Engineering OOSC Lecture 20 - Concurrency

Class ELEVATOR separate class ELEVATOR feature {BUTTON, DISPATCHER} accept (floor: INTEGER) is -- Record and process a request to go to floor. do record (floor) if not moving then process_request end Chair of Software Engineering OOSC Lecture 20 - Concurrency

Class ELEVATOR feature {MOTOR} record_stop (floor: INTEGER) is -- Record information that elevator has stopped on the floor. do moving : = False position : = floor process_request end Chair of Software Engineering OOSC Lecture 20 - Concurrency

Class ELEVATOR feature {NONE} process_request is -- Handle next pending request, if any. local floor: INTEGER do if not pending. is_empty then floor : = pending. item actual_process (puller, floor) pending. remove end Chair of Software Engineering OOSC Lecture 20 - Concurrency

Class ELEVATOR actual_process (m: MOTOR; floor: INTEGER) is -- Handle next pending request, if any. do moving : = true m. move (floor) end feature {NONE} puller: MOTOR pending: QUEUE [INTEGER] -- Queue of pending requests. end -- class ELEVATOR Chair of Software Engineering OOSC Lecture 20 - Concurrency

Class MOTOR separate class MOTOR feature {ELEVATOR} move (floor: INTEGER) is -- Go to floor; once there report. do -- Direct the physical device to move to floor gui_main_window. move_elevator (cabin_number, floor) signal_stopped (cabin) end signal_stopped (e: ELEVATOR) is -- Report that elevator e stopped on level position. do e. record_stop (position) end Chair of Software Engineering OOSC Lecture 20 - Concurrency

Class MOTOR feature {NONE} cabin: ELEVATOR position: INTEGER -- Current floor level. gui_main_window: GUI_MAIN_WINDOW end -- class MOTOR Chair of Software Engineering OOSC Lecture 20 - Concurrency

Additional information § Nienaltowski P. , Arslan V. , Meyer B. Concurrent object-oriented programming on. NET, IEE Proceedings Software, Special Issue on ROTOR, October 2003. § Meyer B. Object-Oriented Software Construction, chapter 31, 2 nd edition, Prentice Hall, 1997. § Project website http: //se. inf. ethz. ch/research/scoop Chair of Software Engineering OOSC Lecture 20 - Concurrency