CS 5381 Juan C. González Authors: Paola Inverardi and Alexander L. Wolf

1. Formal Specification and Analysis of Software Architectures Using the Chemical Abstract Machine Model CS 5381 Juan C. González Authors: Paola Inverardi and Alexander L. Wolf

2. Outline • Introduction • A model of Architectural Description • The Chemical Abstract Machine Model • Specifying Architectures using CHAM’S • Conclusion

3. Introduction • Importance of studying the architectures of large and complex systems • Improved education • Increased reuse • Reduced development cost • Enhanced quality

4. Formal specification of software architecture • Motivations of using a formal framework • Semantic precision • Uniform description • Common basis for formal comparison • Susceptibility to formal analysis

5. Challenges of choosing a framework • A general and flexible formalism in which it is possible to describe very different kinds of architectures • The description has to be understandable to the widely varying consumers of the specification

6. Operational semantic formalism as the more appropriate choice • In the belief that can be more easily understood by a broader range of practitioners than other, abstract mathematical formalism • Operational semantics reflects the familiar idea of specifying the computational behavior of a more abstract and precisely defined system.

7. Ideal approach • To use an operational semantic formalism that is based on a more flexible, relatively neutral computational model.

8. Chemical Abstract Machine (CHAM) model • Proposed by Berry and Boudol in the domain of theoretical computer science • The abstract machine is fashioned after chemical and chemical reactions

9. Metaphor of the CHAM • The states of the machine are chemical solutions • Molecules can only interact according to a set of reaction rules • The CHAM formalism has already been used to describe several different and important computational paradigms.

10. A Model of architectural descriptionExample: Multiphase compiler

11. The model identifies three basic kind of architectural elements • Processing elements • Data elements • Connecting elements

12. The Chemical Abstract Machine Model • It is built upon the chemical metaphor first proposed by Banatre and Le Metayer to illustrate their Gamma formalism or parallel computing.

13. Gamma computation • Set of transformations or reactions that consume elements of the multiset and produce new ones according to the rules that constitute the program.

14. The CHAM formalism • Extends the Gamma language by specifying a syntax for molecules and by providing a classification scheme for reaction rules. • It also introduces the membrane construct.

15. Basics • A CHAM is specified by defining molecules m,m’, … and solutions S,S’,… of molecules. • Also contains transformation rules T, T’,… that define a transformation relation S  S’ • General laws • Specific rules

16. More… • Any solution can be considered as a single molecule with respect to other solutions by means of an encapsulation construct called a membrane. • A reversible operator called an airlock is used to selectively extract molecules form a solution within a membrane.

17. Molecules, Solutions and Membranes • Solutions S, S’, … are finite multisets of molecules, each denoted as a comma – separated list of molecules • m1, m2, …, mn • Solutions can be built form other solutions by combining them through the multiset union operator. • S=m1, …, mn and S’=m’1,…,m’k then • S U S’ = m1,…,mn,m’1,…,m’k

18. More… • A solution enclosed in {| . |} denotes a membrane. • The reversible airlock operator applied to solution S = m1, …, mn to extract mi from S is denoted • S’ = mi ◄ {|m1, …, mi-1, mi+1, …, mn|} • The model provides a context abstraction, denoted as C[.], derived from λ-calculus.

19. General Laws • CHAM’s obey four general laws: • The reaction law • The chemical law

20. More… • The membrane law • The airlock law

21. Rules • Heating rules • Is heating if it decomposes a molecule into its constituents. • A solution is said to be hot if no heating rule is applicable. • Cooling rules • Is a cooling if it composes a compound molecule. • A solution is said to be frozen if no cooling rule is applicable. • Reaction rules • Is reaction if it actually changes the nature of the solutions.

22. Example • Sequential Multiphase Compiler

23. Conclusion • The CHAM is a reasonably simple model, yet very expressive. • The CHAM model might be one useful tool in the software architect’s chest of useful tools.