Executable Specifications using Message Sequence Charts

1. Executable Specifications using Message Sequence Charts Abhik Roychoudhury School of Computing National University of Singapore Internal Talk, Oct 1 2002

2. MSCs – What • Depict scenarios in system exection. • Used to specify requirements in early stages of system design. • A MSC depicts only a possible scenario of system behavior. Internal Talk, Oct 1 2002

3. MSC - Visually Mem CPU mem CPU req req ack nack addr val MSC 2 MSC 1 Internal Talk, Oct 1 2002

4. Concurrency modeling • Depicts concurrently executing processes (the vertical lines). • Processes communicate via a explicit message passing (instead of shared variables). • Realistic MSCs will also contain data attributes as part of the exchanged msg Internal Talk, Oct 1 2002

5. Message Attributes req ack Addr(a) v := lookup(a) val(v) CPU Mem Internal Talk, Oct 1 2002

6. Internal Computations req Internal Action ack Addr(a) v := lookup(a) val(v) CPU Mem Internal Talk, Oct 1 2002

7. MSC – (Very) Basics • Time flows from top to bottom in each process. • Message exchange shown by horizontal lines. • Each process in the MSC is assumed to contain message queues to store incoming messages. • The message queues contribute to the global state of the system. • If the data in messages come from a finite domain, then we identify a finite set of messages exchanged, called M. Internal Talk, Oct 1 2002

8. MSC -formally • P : set of processes • M: set of messages • A : set of internal actions • E : set of events. Mapped to • <p ? q , m>  P  P  M • <p ! q, m>  P  P  M • <p, a>  P  A • <, a partial order on events. • We will consider the “Visual Order” for these purposes … Internal Talk, Oct 1 2002

9. MSC order • Intuitively, a “happens-before” relation between events. • e < e’ if the MSC requires e to happen before e’ • If e, e’ occur in the same process and e occurs above e’ then e < e’ • If e, e’ are the send and receive of the same message then e < e’ • These orderings constitute the visual order. • Think of our partial order as the transitive closure of the visual order. Internal Talk, Oct 1 2002

10. Combining MSCs • Models a single scenario and states its possible execution in the system implementation. • How could we use MSCs to model the behavior of a concurrent system ? • One attempt is to describe the system behavior as a graph of MSCs. • Each node of this graph is a MSC. • We call such graphs as Message Sequence Graphs (MSG). Internal Talk, Oct 1 2002

11. MSG - Visually req M1 M3 M2 ack nack addr val Internal Talk, Oct 1 2002

12. Choice and Concatenation • Choice of scenarios at a certain point in system execution modeled by several outgoing branches • M1  M2 and M1  M3 • Concatenation of MSCs (by following the edges in the MSG) produces infinite execution traces. Client Server Internal Talk, Oct 1 2002

13. More on concatenation • Synchronous concatenation • All processes synchronize at the end of each MSC (a node of the MSG) • For any edge M1  M2 • All events in M1 happen before all events in M2 • Costly to implement since the natural control flow in a process is disrupted by the termination of a MSC (requiring handshake with other processes). Internal Talk, Oct 1 2002

14. Synchronous concatenation r1 s1 s1 < r1 < s2 < r2 r2 s2 Internal Talk, Oct 1 2002

15. An alternative concentation • Asynchronous concatenation • If M1  M2 is an edge in the MSG, then concatenate M1 and M2 process by process • If a process finishes its role in M1 ahead of others, it can start executing M2 • Amenable to efficient distributed implementation. Internal Talk, Oct 1 2002

16. Asynchronous concatenation r1 s1 s1 < r1 s2 < r2 s1 < s2 Is r1 < s2 ?? r2 s2 Internal Talk, Oct 1 2002

17. Unboundedness of queues q p Message queue of q can grow unboundedly. Model Checking will be applicable to only “Bounded MSGs”. Internal Talk, Oct 1 2002

18. Hierarchical MSC (HMSC) • Improves MSGs by incorporating hierarchy. • A graph, each node of which is: • a Message Sequence Chart, or • a HMSC • Clearly a MSG can appear as a node of HMSC. • Different nodes in the graph can be labeled by the same HMSC • The HMSC is like a subprogram which is invoked in various contexts. Internal Talk, Oct 1 2002

19. HMSC example Th1 CPU Th2 CPU-Mem MSG CPU-Mem MSG can be invoked in another context involving thread Th2. Th1 CPU data Internal Talk, Oct 1 2002

20. MSG - Recap req M1 M3 M2 ack nack addr val Internal Talk, Oct 1 2002

21. Need for Universal Charts M1 req Mem.busy Mem.busy Mem CPU M3 M2 ack nack addr val CPU Mem CPU Mem Internal Talk, Oct 1 2002

22. A Universal chart Mem.busy 1. Mem.busy is the activation condition (AC) 2. If the AC holds, thenthis chart must be executed. 3. But the AC need not be true in every execution. nack Mem CPU Internal Talk, Oct 1 2002

23. Semantics of universal charts • A chart only constrains events and variables appearing in the chart. • A system execution trace  = s0 s1 … satisfies a universal chart M iff •  positions I s I l= AC(M)   I |= M, that is •  I = s I s I+1 …. contains a linearization of the events in M • Events not in M may appear in  I • A system satisfies a universal chart M if all its execution traces satisfy M. Internal Talk, Oct 1 2002

24. Pre-charts • The trigger of a universal chart need simply be an activation condition. • Can be an activation message: when it is received the chart is required to be activated. • Can even be another full blown chart (a pre-chart). This chart can contain msg send/recv, conditions. • Once the pre-chart is executed, the universal chart is required to follow. Internal Talk, Oct 1 2002

25. Example of Pre-chart Mem req CPU Mem.busy = true Pre-chart Concatenation CPU Mem Univ. chart nack Internal Talk, Oct 1 2002

26. What kind of Concatenation ? • Live Sequence Charts(LSC) [Damm and Harel] • Synchronous concatenation • The pre-chart/activation condition must be completely executed before the universal chart can initiate. • Still allows for several active copies of pre(M).M e.g. pre(M) may be activated while M is still executing. Internal Talk, Oct 1 2002

27. What kind of Concatenation ? • Cyclic Transaction Processes(CTP) • Universal charts triggered by activation conditions. • Activation condition may involve variables of diff. processes. • A process can initiate its role in the chart once the condition on its variables is evaluated to true. • Other important differences with LSC formalism … Internal Talk, Oct 1 2002

28. Distributed Activation Cond. m1 Reset(r.b) m2 … p q r AC(M) = p.a  r.b r can start even when p has not started. Internal Talk, Oct 1 2002

29. Requirements or Exec. spec ? • A universal chart denotes a safety property that the actual system must satisfy. • Can be seen as a requirement with “must” flavor. • Possible to enforce it on system executions by monitoring for the occurrence of its pre-chart. • LSC spec. used for monitoring/testing/simulating an executable model early in the design cycle. • How do we obtain a high-level executable specification directly based on MSCs ? Internal Talk, Oct 1 2002

30. LSC-like executable specifications • A collection of universal charts {M1, …, Mk} • Each chart has activation condition AC( Mi ) • Mi becomes active when AC(Mi) becomes true. • Several charts may be active simultaneously. • At any point in execution, no event occurs unless it is required to occur by the . • Can use this executable model to check for inconsistencies in the LSC spec. itself ! Internal Talk, Oct 1 2002

31. Checking for inconsistencies Pre-chart m1 m1 a b b a The two universal charts are inconsistent. Internal Talk, Oct 1 2002

32. Control flow in LSC • Intra-process control flow is completely implicit in an LSC based executable specification. • Raises efficiency issues in implementation, since: • Processes need to handshake at the end of a pre-chart. • Need a global watchdog which always monitors for pre-charts of any Univ. chart being initiated. • Can we make these decisions (initiation/termination of pre-charts) local to a process ? Internal Talk, Oct 1 2002

33. Adding Per-process Control flow • Describe the control flow in each process by giving the sequence of univ. charts it participates in • Can start with a simple cyclic control flow (CTP) • A process initiates a chart when its activation condition is true. • A process leaves a chart when its role is finished. • A transition in local control state sp  sp’ in process p takes place by executing a universal chart. Internal Talk, Oct 1 2002

34. Transaction scheme • A transition sp  sp’ in process p involves a non-trivial interaction of p with other processes q,r… • Depending on the local variable values in p,q,r… the interaction may be different. • Captured by a choice of universal charts • Gp  Gq  Univ. Chart 1 • Gp’  Gq’  Univ. Chart 2 • This choice is not non-deterministic. Resolved at run-time by either of the guards being true. Internal Talk, Oct 1 2002

35. References • LSCs: Breathing Life into Message Sequence Charts Damm and Harel, Formal Methods in System Design, Vol. 19, pages 45-80, 2001. • Model Checking of Message Sequence Charts Alur and Yannakakis, Proceedings of CONCUR, 1999. • Communicating Transaction Processes, Roychoudhury and Thiagarajan, International Conf. on Applications of Concurrency in System Design (ACSD) 2003. Internal Talk, Oct 1 2002