1 Concurrency Specification

2 Outline 4 Issues in concurrent systems 4 Programming language support for concurrency 4 Concurrency analysis - A specification based approach 4 Concurrency and other formal methods 4 Deadlock checker 4 Concurrency and architectures

3 Concurrency 4 Coexistence 4 Sharing of resources 4 Issues – Asynchronicity – Non-determinism 4 Solution – Locks 4 Results – Deadlock and starvation

4 Concurrency in Various Disciplines 4 Databases – Transaction serializability 4 Operating systems – Multithreading 4 Electronic circuits – Flip flops 4 Real life – Gas station example

5 Concurrency in Architecture Implementations

6 PL Support for Concurrency Fork and join constructs 4 Queue construct and the signal operation – Concurrent Pascal 4 The Java synchronized keyword

7 PL Support for Concurrency Communicating sequential processes [CSP] – Producer command : consumer!m – Consumer command : producer?n – Guarded commands —› guard : list of declarations, boolean expressions or an input command Alternative guarded command –[ G1 —› C1 ƀ G2 —› C2 ƀ …. ƀ Gn —› Cn]

8 From Specification to Implementation Specification Phase Implementation Phase 4 Easy to verify safety Difficult to verify and liveness safety and liveness 4 State spaces small State spaces and manageable large and unmanageable; testing difficult 4 Cost of correcting Cost of correcting flaws is low flaws is high

9 Specification-Based Model Synchronizer construct – Set of variables defining the state of shared resources – Set of operations on these variables (with pre/post conditions) – Set of invariants Safety conditions Liveness conditions

10 Specification-Based Model Process construct – Independent thread of execution – Multiple processes coexist – Control allocation/deallocation of synchronizer controlled resources 4 Example: – Web server : synchronizer – Web browser : process

11 Gas-Station Model

12 Gas-Station Model - Program Spec

13 Gas Station Model - RSTG

14 Gas Station Model - Event Expressions [ Two customers trying to buy gas concurrently

15 Gas Station Model - Reachability Graph 4 Identifies the states that can be reached by executing enabled operations in processes and synchronizers 4 Constructed from event expressions and RSTG – Nodes represent states of RSTG – Edges represent operations from event expressions 4 A deadlock occurs if the graph contains terminal nodes

16 Tool Support for Concurrency Analysis 4 INCA (Inequality Necessary Condition Analysis) – Checks properties of an architectural specification (e.g. mutual exclusion) – Provides example executions that violate those properties – Verifies that a modification removes the faults

17 Detecting a Race Condition 4 Customer1 pays before Customer2 but Customer2 takes up the hose before Customer1 thus getting the amount of gas purchased by Customer1

18 The INCA Query

19 INCA Results 4 INCA generates a system of inequalities based on the violation of properties specified by the query – A consistent inequality implies such a situation is possible – An inconsistent inequality implies such a situation is impossible

20 Features Common with Other Formal Methods 4 RSTG 4 Pre and post conditions 4 State invariants

21 Unique Features Operation execution phases – Request phase – Enabled phase – Service phase Only one operation invocation can be in the service phase – Terminate phase H Example: Fair scheduler []<>enabled(o) -> <>service(o)

22 Unique Features Separation of control resources from state variables 4 Event expressions help “walk through” the concurrency aspect 4 Semantics of allocation and deallocation – Helpful in detecting deadlocks

23 Deadlock Checker 4 Performs checks on parallel programs written in CSP in order to prove freedom from deadlock 4 Takes in a network file (.net) that has been compiled from a CSP source file using a tool such as FDR

24 The Dining Philosophers Problem 4 5 philosophers and 5 chopsticks 4 All philosophers keep thinking 4 When a philosopher feels hungry, he picks up the chopsticks closest to him, eats rice and keeps the chopsticks back 4 Deadlock: – When all philosophers grab their left chopstick simultaneously

25 The Dining Philosophers Problem

26 Architectures and Concurrency 4 Component types: – Synchronizer – Process units 4 Connector – Synchronization connector

27 Synchronization Connector

28 Conclusions and Discussion 4 Analysis of concurrent systems early in the development process reduces complexity and cost of correcting errors 4 A formal analysis will help detect deadlocks and starvation and also in direct code generation 4 Concurrency in software architectures can be represented in terms of CSPs