D EPARTMENT OF C OMPUTER S CIENCES Structured Concurrent Programming Jayadev Misra Department of Computer Science University of Texas at Austin Email: misra@cs.utexas.edu web: http://www.cs.utexas.edu/users/psp Collaborators: William Cook, David Kitchin U NIVERSITY OF T EXAS AT A USTIN 0
D EPARTMENT OF C OMPUTER S CIENCES Example: Airline � Contact two airlines simultaneously for price quotes. � Buy ticket from either airline if its quote is at most $300. � Buy the cheapest ticket if both quotes are above $300. � Buy any ticket if the other airline does not provide a timely quote. � Notify client if neither airline provides a timely quote. U NIVERSITY OF T EXAS AT A USTIN 1
D EPARTMENT OF C OMPUTER S CIENCES Wide-area Computing Acquire data from remote services. Calculate with these data. Invoke yet other remote services with the results. Additionally Invoke alternate services for failure tolerance. Repeatedly poll a service. Ask a service to notify the user when it acquires the appropriate data. Download an application and invoke it locally. Have a service call another service on behalf of the user. U NIVERSITY OF T EXAS AT A USTIN 2
D EPARTMENT OF C OMPUTER S CIENCES The Nature of Distributed Applications Three major components in distributed applications: Persistent storage management databases by the airline and the hotels. Specification of sequential computational logic does ticket price exceed $300? Methods for orchestrating the computations We look at only the third problem. U NIVERSITY OF T EXAS AT A USTIN 3
D EPARTMENT OF C OMPUTER S CIENCES � Orchestration language. Overview of Orc � A Program execution – Invoke services by calling sites – Manage time-outs, priorities, and failures � Simple – calls sites, – publishes values. – Language has only 3 combinators. – Semantics described by labeled transition system and traces. – Combinators are (monotonic and) continuous. U NIVERSITY OF T EXAS AT A USTIN 4
D EPARTMENT OF C OMPUTER S CIENCES Structure of Orc Expression � Simple: just a site call, CNN ( d ) � composition of two Orc expressions: Publishes the value returned by the site. f and g in parallel f j g x from f do g f > x > g x from g do f f where x : 2 g do Symmetric composition for all Piping for some Asymmetric composition U NIVERSITY OF T EXAS AT A USTIN 5
f j g D EPARTMENT OF C OMPUTER S CIENCES CNN j B B C : calls both CNN and B B C simultaneously. Symmetric composition: 0 , 1 or 2 values) � Evaluate f and g independently. Publishes values returned by both sites. ( � Publish all values from both. � No direct communication or interaction between f and g . They may communicate only through sites. U NIVERSITY OF T EXAS AT A USTIN 6
f > x > g D EPARTMENT OF C OMPUTER S CIENCES f do g . Publish only the values from g . Pipe: � CNN > x > E mail ( addr ess; x ) CNN . Bind result (if any) to x . Call E mail ( addr ess; x ) . For all values published by E mail . � ( C N N j B B C ) > x > E mail ( addr ess; x ) Call E mail twice. Publishes up to two values from E mail . Publish the value, if any, returned by May call U NIVERSITY OF T EXAS AT A USTIN 7
D EPARTMENT OF C OMPUTER S CIENCES f � g for f > x > g if x unused in g . Notation f > x > g j h > y > u ( f > x > g ) j ( h > y > u ) Write Precedence: U NIVERSITY OF T EXAS AT A USTIN 8
D EPARTMENT OF C OMPUTER S CIENCES Schematic of piping f f > x > g g0 g1 g2 Figure 1: Schematic of U NIVERSITY OF T EXAS AT A USTIN 9
( f where x : 2 g ) D EPARTMENT OF C OMPUTER S CIENCES g do f . Publish only the values from f . Asymmetric parallel composition: E mail ( addr ess; x ) where x : 2 ( CNN j B B C ) x to the first value from CNN j B B C . For some value published by � Evaluate f and g in parallel. x are suspended; other site calls proceed. ( M j N ( x )) where x : 2 g Binds � When g returns a value, assign it to x and terminate g . Site calls that need � Values published by f are the values of ( f where x : 2 g ) . Resume suspended calls. U NIVERSITY OF T EXAS AT A USTIN 10
D EPARTMENT OF C OMPUTER S CIENCES Some Fundamental Sites 0 : never responds. l et ( x; y ; � � � ) : returns a tuple of its argument values. if ( b ) : boolean b , b is true; remains silent if b is false. S ig nal returns a signal immediately. Same as if ( true ) . returns a signal if R timer ( t ) : integer t , t � 0 , returns a signal t time units later. U NIVERSITY OF T EXAS AT A USTIN 11
D EPARTMENT OF C OMPUTER S CIENCES Centralized Execution Model � An expression is evaluated on a single machine (client). � Client communicates with sites by messages. � All fundamental sites are local to the client. R timer respond immediately. � Concurrent and distributed executions are derived from an expression. All except U NIVERSITY OF T EXAS AT A USTIN 12
D EPARTMENT OF C OMPUTER S CIENCES MailOn e ( a ) � Expression Definition E mail ( a; m ) where m : 2 ( CNN j BBC ) MailL o op ( a; t ) � MailOn e ( a ) � R timer ( t ) � MailL o op ( a; t ) � Expression is called like a procedure. MailL o op does not publish a value. � Site calls are strict; expression calls non-strict. May publish many values. U NIVERSITY OF T EXAS AT A USTIN 13
D EPARTMENT OF C OMPUTER S CIENCES Metronome Metr onome � S ig nal j ( R timer (1) � Metr onome ) Publish a signal at every time unit. S R n signals. S R BM (0) � 0 BM ( n ) � S ig nal j ( R timer (1) � BM ( n � 1)) Publish U NIVERSITY OF T EXAS AT A USTIN 14
D EPARTMENT OF C OMPUTER S CIENCES Example of Expression call � Site Quer y returns a value (different ones at different times). � Site A ept ( x ) returns x if x is acceptable; � Produce all acceptable values by calling Quer y at unit intervals it is silent otherwise. Metr onome � Quer y > x > A ept ( x ) forever. U NIVERSITY OF T EXAS AT A USTIN 15
D EPARTMENT OF C OMPUTER S CIENCES M ’s response if it arrives before t , and 0 otherwise. Time-out l et ( z ) Publish z : 2 M j R timer ( t ) � l et (0) where U NIVERSITY OF T EXAS AT A USTIN 16
D EPARTMENT OF C OMPUTER S CIENCES M and N in parallel. Fork-join parallelism l et ( u; v ) Call u : 2 M v : 2 N Return their values as a tuple after both respond. where ( l et ( u; v ) u : 2 M ) v : 2 N This stands for: where where U NIVERSITY OF T EXAS AT A USTIN 17
D EPARTMENT OF C OMPUTER S CIENCES Recursive definition with time-out tal l y ([ ℄) � l et (0) Call a list of sites. tal l y ( M : M S ) � u + v Count the number of responses received within 10 time units. u : 2 ( M � l et (1)) j ( R timer (10) � l et (0)) v : 2 tal l y ( M S ) where U NIVERSITY OF T EXAS AT A USTIN 18
M � f j N � g D EPARTMENT OF C OMPUTER S CIENCES f and g start only after both M and N complete. Barrier Synchronization in l et ( u; v ) u : 2 M v : 2 N ) � ( f j g ) ( where U NIVERSITY OF T EXAS AT A USTIN 19
D EPARTMENT OF C OMPUTER S CIENCES � :P + � :Q Arbitration In CCS/ Pi-Calculus: if ( b ) � P j if ( : b ) � Q b : 2 ( Al pha � l et ( true )) j ( B eta � l et ( false )) In Orc: where Orc does not permit non-deterministic internal choice. U NIVERSITY OF T EXAS AT A USTIN 20
D EPARTMENT OF C OMPUTER S CIENCES Priority � Publish N ’s response asap, but no earlier than 1 unit from now. D el ay � ( R timer (1) � l et ( u )) where u : 2 N � Call M , N together. M responds within one unit, take its response. l et ( x ) where x : 2 ( M j D el ay ) If Else, pick the first response. U NIVERSITY OF T EXAS AT A USTIN 21
f D EPARTMENT OF C OMPUTER S CIENCES f can not be directly interrupted. Interrupt � Interrupt :set : to interrupt f Evaluation of Introduce two sites: � Interrupt :g et : responds after Interrupt :set has been called. f , evaluate l et ( z ) where z : 2 ( f j Interrupt :g et ) Instead of U NIVERSITY OF T EXAS AT A USTIN 22
D EPARTMENT OF C OMPUTER S CIENCES M and N return booleans. Compute their parallel or. ift ( b ) � if ( b ) � l et ( true ) : returns true if b is true ; silent otherwise. Parallel or ift ( x ) j ift ( y ) j or ( x; y ) Sites x : 2 M ; y : 2 N where l et ( z ) z : 2 ift ( x ) j ift ( y ) j or ( x; y ) To return just one value: x : 2 M y : 2 N where U NIVERSITY OF T EXAS AT A USTIN 23
D EPARTMENT OF C OMPUTER S CIENCES A and B . Airline quotes: Application of Parallel or as soon as it is available, Contact airlines thr eshold ( x ) returns x if x < ; silent otherwise. Min ( x; y ) returns the minimum of x and y . Return any quote if it is below otherwise return the minimum quote. l et ( z ) z : 2 thr eshold ( x ) j thr eshold ( y ) j Min ( x; y ) x : 2 A y : 2 B where U NIVERSITY OF T EXAS AT A USTIN 24
D EPARTMENT OF C OMPUTER S CIENCES Sequential Computing � ( S ; T ) is ( S � T ) � b then S T if ( b ) � S j if ( : b ) � T if else is � B ( x ) do x := S ( x ) l oop ( x ) � B ( x ) > b > ( if ( b ) � S ( x ) > y > l oop ( y ) j if ( : b ) � l et ( x )) while U NIVERSITY OF T EXAS AT A USTIN 25
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