2. START

3. Translation of process algebras to Java Paul Bilokon Samuel Lau Andrew Roberts

4. Introduction Modelling Finite State Processes (FSP) Threads and monitors Mission statement

5. Modelling Simplicity: abstraction from irrelevant details Determination of relevant factors Predict long term behaviour “What if” scenarios

6. Finite State Processes (FSP) “A process calculus is a small language that allows us to give precise descriptions of the essential properties of concurrent and communicating programs.” FSP is a process calculus. FSP is finite, so we can do proofs, safety checks, etc. Example: SUPREMA_GROUP = (enter -> present -> leave -> SUPREMA_GROUP). AUDIENCE = (yawn -> sleep -> wakeUp -> AUDIENCE). ||FUN = (SUPREMA_GROUP || AUDIENCE). Wishful thinking: yawn  sleep  enter  wakeUp  present  leave…

7. Mission statement Investigate the relationship between FSP and Java Restrict the subset of FSP under consideration Consider examples, develop the theory Discover interesting patterns and methods Formalize it, if possible, using mathematical notation Evaluate the results. Refine the model, if necessary Could we use these results to build an automated FSP to Java translator? Would this be a useful tool?

8. Translation The action prefix and simple processes Guarded actions Choice Variables Simple parallel composition Parallel composition of dependent processes Other FSP constructs

9. Quick FSP Recap P1 = (a1 -> a2 -> … -> an -> STOP). P = (when(B) a -> STOP). DRINKS_MACHINE = (red -> coffee -> DRINKS_MACHINE |blue -> tea -> DRINKS_MACHINE). Actions Guarded Actions Choice FSP Processes  Java classes FSP Actions  Java Methods Guards  While-wait loop Choice  External factor allows choice

10. Variables public class public class Store { private int private int i; public () { public Store() { this this.i = 0; } synchronized public int put(int i) { this this.i = i; } } //END class range T = 0..5 STORE = STORE[0], STORE[i:T]=(put[i:T]->STORE[i]). FSP Constructor  Java constructor STORE has a state variable, represented in Java as a field variable ‘put’ has an action variable, represented as a parameter for the put method

11. Simple parallel composition public class public class Composition { protected protected A1 _a1; protected protected A2 _a2; … protected protected An _an; public public Composition() { newnew new Thread(_a1 = new A1()); newnew new Thread(_a2 = new A2()); … newnew new Thread(_an = new An()); } ||COMPOSITION = (a1 || a2 || … || an), No shared actions! Instantiate and start threads Non-simple composition uses similar composite class

12. Composition: caller/callee pattern Two shared actions: designate one the caller and the other callee Caller is part of a thread Calls callee method, which is part of a monitor Only works with 2 shared actions! public classimplements public class Caller implements Runnable { public public run() { … Callee.a(); … } public class public class Callee { synchronized public void synchronized public void a() { //do something }

13. Composition: semaphores public classimplements public class Bill implements Runnable { public void public void run() { play(); release(A); acquire(B); eat(); } } //END class BILL = (play -> meet -> eat -> STOP). BEN = (work -> meet -> sleep -> STOP). public classimplements public class Ben implements Runnable { public void public void run() { work(); acquire(A); release(B); sleep(); } } //END class Uses semaphores – not intuitive Java Works for many shared actions – can become complex!

14. General composition of processes P1 = (… -> a -> … -> STOP). P2 = (… -> a -> … -> STOP). … P(n – 1) = (… -> a -> … -> STOP). Pn = (… -> a -> … -> STOP). Method 1. Extend the semaphore method. For n shared methods, (2n-2) semaphores required. Initialize to an ‘acquired’ state. Method 2. Synchronization object. Uses Java’s inbuilt synchronization. This object is a monitor and counts in the shared actions. Once they have all ‘reported in’ it will let all the threads continue.

15. Case study: Roller Coaster Apology Monitors and threads revisited Caller/callee pattern Problem: parameters or return values? Problem: action order Well-formed FSP

16. Caller/callee pattern Only one type of process interaction – monitor/thread. In general, this is the most common form of process communication. ‘Directionality’ is an important criterion for preferring this design pattern. E.g. the thread Passengers tells the monitor Controller that a newPassenger has arrived.

17. Problem: parameters vs return COASTERCAR has action getPassenger[i:1..MCar] with a ‘free’ variable (cf. Prolog). CONTROLLER has action getPassenger[carSize]. The variable carSize is bound. A method of Coastercar (thread) to be called from Controller (monitor)? Use return values instead: synchronized public int getPassenger() in Controller.

18. Problem: action order PLATFORMACCESS = (arrive -> depart -> PLATFORMACCESS). PLATFORMACCESS = ({arrive -> depart} -> PLATFORMACCESS). publicclass public class PlatformAccess { public boolean false public boolean arrive_done = false; synchronized public void synchronized public void arrive() { while while (arrive_done) wait(); true arrive_done = true; } synchronized public void synchronized public void depart() { while while (! arrive_done) wait(); false arrive_done = false; }

19. Well-formed FSP Is this translation ‘natural’? Does Java ‘match’ FSP? Is it easy for an automated FSP2Java program to spot this? Could ask the user to re-write the FSP to comply with a well-formed FSP standard. For example…

20. Well-formed FSP II PLATFORMACCESS = (arrive -> depart -> PLATFORMACCESS). PLATFORMACCESS = PLATFORMACCESS[0], PLATFORMACCESS[i:0..1] = (when(i=0) arrive->PLATFORMACCESS[1] |when(i=1) arrive->PLATFORMACCESS[0] ).

21. Conclusions What is a good translation? A subset of FSP Limitation Automatic Translation Further work  FSP modification  Validation of translation  Rest of FSP

22. What is a good translation? The factors are: Complexity Readability We have found out that the Caller/Callee pattern is the most intuitive. However, this pattern cannot be used for more than 2 shared actions. We would like to read and understand the Java code easily.

23. Limitation Caller/Callee pattern: Monitor? Thread? Data flow between processes can be difficult to translate. One solution is to rewrite the FSP and merge the actions. A FSP can send more than 1 item of data but Java cannot.

24. Automatic translation Given more time, we may write a program that automatically translates FSP to Java. Closer relation between FSP and Java code Humans have benefit of context, which will effect implementation Is this feasible?

25. FSP modification We may rewrite the FSP in some circumstances. Aim: Make the translation more effective and concise.

26. Validation of translation We are able to translate a large subset of FSP. But how can we prove that the Java code actually corresponds to the FSP given? Very difficult: Logic reasoning?

27. I Want More! Prof. Magee’s home page: http://www.doc.ic.ac.uk/~jnm/ http://www.doc.ic.ac.uk/~jnm/ Our project home page: http://www.doc.ic.ac.uk/~pb401/ Suprema/ http://www.doc.ic.ac.uk/~pb401/ Suprema/ Web articles and project report. Test yourself – try Q&A! You are dazzled, fascinated, intrigued… where do you want to go today?

28. That’s all, folks! http://www.doc.ic.ac.uk/~pb401/Suprema