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Formal Methods. CIS 376 Bruce R. Maxim UM-Dearborn. Levels of Rigor for Formal Methods. Informal manual reviews, inspections Low modeling using logical and discrete mathematics Medium formal specification language with type checking High

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Formal methods l.jpg

Formal Methods

CIS 376

Bruce R. Maxim

UM-Dearborn


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Levels of Rigor for Formal Methods

  • Informal

    • manual reviews, inspections

  • Low

    • modeling using logical and discrete mathematics

  • Medium

    • formal specification language with type checking

  • High

    • fully formal specification language with rigorous semantics and proof checking


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Necessary Mathematics

  • Set builder notation

  • Set operations

  • Logic operators

  • Sequence properties

    • order, domain, range

  • Sequence operators

    • concatenation, head, tail, front, last


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Weaknesses of Less Formal Approaches - part 1

  • Contradictions

    • statements do not agree with one another

  • Ambiguities

    • statements have more than one interpretation

  • Vagueness

    • specifications in large documents are often not written precisely enough


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Weaknesses of Less Formal Approaches - part 2

  • Incompleteness

    • e.g. failing to list limitations and error handling required of a function

  • Mixed levels of abstraction

    • occurs when very abstract statements are intermixed randomly with statements written at lower levels of detail


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Why Consider Formal Methods?

  • Systems are increasingly dependent on software components

    • fault protection and safety are no longer allocated solely to hardware

    • software must be able to detect and isolate failures and then execute recovery scenarios

    • software systems fail in way different than hardware systems

  • Complexity of systems with embedded software has increase rapidly


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Reliability and Traditional Methods

  • Maintaining reliability in software intensive systems is very difficult

  • Quality ceilings are encountered using traditional measures (which means that reliability asymptotically approaches some level considerably less than 100%)

  • New approaches seem to be needed to achieve increases in quality measures


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Limitations to Formal Methods

  • Use formal methods as supplements to quality assurance methods not a replacement for them

  • Formal methods can increase confidence in a product’s reliability if they are applied skillfully

  • Useful for consistency checks, but formal methods cannot guarantee the completeness of a specifications

  • Formal methods must be fully integrated with domain knowledge to achieve positive results


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Ten Commandments of Formal Methods - part 1

  • Choose the appropriate notation

  • Do not over-formalize

  • Estimate costs

  • Have a formal methods guru on call

  • Do not abandon traditional development methods

  • Document sufficiently


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Ten Commandments of Formal Methods - part 2

  • Do not compromise quality standards

  • Do not be dogmatic in assuming formal specifications are flawless

  • Use of formal methods does not eliminate the need to test products

  • Reuse is still important


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Choosing a Life Cycle Phase

  • Formal methods can be applied to all phase of the life cycle

  • The benefit-to-cost ratio seems highest for the specification and design phases

  • The makes sense because the earlier defect is removed the cheaper it will be to correct


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Formal Methods and Specification Phase

  • This phase is the least automated and is not tightly coupled to specific languages or notations

  • Specification work products are less effectively analyzed that products from later phases

  • Using formal methods in this phase does not interfere much with other existing processes and can dramatically improve analysis capability


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Benefits of Formal Specification - part 1

  • Higher level of rigor leads to better problem understanding

  • Defects are uncovered that would be missed using traditional specification methods

  • Allows earlier defect identification

  • Formal specification language semantics allow checks for self-consistency

  • Enables the use of formal proofs to establish fundamental system properties and invariants


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Benefits of Formal Specification - part 2

  • Repeatable analysis allows reasoning to be checking by colleagues

  • Encourages and abstract view of the system, focusing on what a system should do rather than how to accomplish it

  • An abstract view of the system helps separate specification from design

  • Enhances existing processes by adding rigor


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Formal Specification Methods

  • Formal specifications

  • Formal Proofs

  • Model Checking

  • Abstraction


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Formal Specification

  • The translation of non-mathematical description (diagrams, table, natural language) into a formal specification language

  • It represents a concise description of high-level behavior and properties of a system

  • Well-defined language semantics support formal deduction about the specification


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Formal Proofs

  • Provide a complete and convincing and convincing argument for validity of some system property description

  • Proofs are constructed as a series of small steps, each of which is justified using a small set of rules

  • Eliminates the ambiguity and subjectivity inherent when drawing informal conclusions

  • Proofs can be done manually, but they usually constructed with some automated assistance


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Model Checking

  • Checking is operational rather than analytic

  • The finite state machine model of a system is expressed in a suitable language

  • A model checker determines if the finite state model satisfies the requirements expressed as formulas in a given logic

  • The basic method is to derive a computational tree from the finite state machine model and explore all plausible execution paths


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Abstraction

  • The process of simplifying or ignoring irrelevant details

  • Allows you to focus on and generalized the most important central properties and characteristics

  • Helps to avoid premature commitment to design and implementation choices


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Using Formal Methods

  • Define the data invariant, state, and operations for each system function

  • Specification is represented in some set theoretic type notation from some formal language

  • Specification correctness can be verified using mathematical proofs


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Review of Terms

  • data invariant

    • a condition true throughout execution of function that contains a collection of data

  • state

    • defined by the stored data accessed and altered by a particular function

  • operations

    • system actions that take place when data are read or written to the state

    • precondition and post condition is associated with each operation


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Formal Specification Properties

  • Unambiguous

    • formal syntax used by formal methods has only one interpretation (unlike natural language statements)

  • Consistency

    • ensuring through mathematical proof that initial facts can be mapped (using inference rules)into later statements within the specification

  • Completeness

    • difficult to achieve in a large system even using formal methods


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Writing Formal Specifications

  • Begin by defining state in terms of abstract items to be manipulated by the function (similar to variable declaration in a programming language)

  • Define the data invariant by writing the data relations that will not change during the execution of the function using mathematical notation

  • Write the precondition and post-condition for the function using mathematical notation to show the system state before and after function execution


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Formal Specification Language Components

  • Syntax

    • defines the specific notation used to represent a specification

  • Semantics

    • help to define the objects used to define the system

  • Set of relations

    • define the rules that indicate which objects properly satisfy the specification


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Algebraic Specification

  • Particularly appropriate from sub-system interface specification

  • Involves specifying operations for abstract data types or objects in terms of their interrelationships

  • Contains a syntax part which defines its signature and a semantic part defined by axioms

  • Algebraic specifications may be developed by defining the semantics of each inspection operation for each constructor operation

  • Display operations are hard to define algebraically and need to be defined informally instead


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Specification Operations

  • Constructor operations

    • create entities of the type specified

  • Inspection operations

    • evaluate entities of the type being specified

  • Behavior specification is created by defining the inspector operations for each constructor operation


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Algebraic Specification Example

5 language primitives (Guttag & Liskov)

  • Functional composition

  • Equality relation

  • Constants

  • "true and false"

  • infinite set of free variables


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Queue Specification - part 1

Syntactic specification

  • structure queue

  • newQ = queue

  • addQ(queue, item) = item

  • delQ(queue) = queue

  • frontQ(queue) = item

  • isNew(queue) = boolean


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Queue Specification - part 2

Semantic specification

  • declare

    q : queue

    i : item

  • delQ(addQ(q, i)) =

    if isNewQ() then newQ()

    else addQ(delQ(q), i)


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Queue Specification - part 3

Semantic specification (continued)

  • isNewQ(addQ(q, i)) = false

  • isNewQ(newQ()) = true

  • frontQ(addQ(q, i)) =

    if isNewQ(q) then i

    else FrontQ(q)


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Queue Specification - part 4

Restriction specification

  • delQ(newQ()) = error

  • frontQ(newQ()) = error


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Error Specification

  • It is important to define the behavior of an operation under both normal and abnormal conditions

  • This might be done using one of these approaches

    • Use a special flag constant like “error” or “undefined” that conforms to the operation return type

    • Define the return type to be a tuple with an element that indicates success or failure of the operation

    • Include a special failure or exception section in the specification (this may need to be defined informally)


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Primitive Constructors

  • Sometimes is it useful to introduce additional constructors to simplify a specification

  • This allows other constructors to be specified using these more primitive constructors

  • For example, adding a node constructor to simplify the specification of other list operators


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Structured Specification

  • Whenever possible specifications should be reused in the construction of other specifications

  • Similar to work in object-oriented design, a generic specification is instantiated to yield a particular specification

  • For example a generic specification may call for a sorted list and this might later be instantiated with a particular list node type and representation


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Incremental Specification

  • This involves developing a simple specification and then using this in more complex specifications

  • The development of a library reusable specification building blocks would be useful

  • For example, the specification of a Cartesian coordinate might be useful in the specification of screen objects in a GUI


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Specification Enrichment

  • Starting with a reusable specification definition, new operations are added to create a more complex type

  • Similar to the process of inheritance in object-oriented programming

  • Can be carried on for several levels

  • Enrichment creates a new type which is not the same as import/export which more like macro expansion (can only use an definition literally)


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Multi-value Operations

  • Some operations return more than one entity (for example pop may return an element and a modified stack in some specifications)

  • One solution is to define the operation using multiple operations

  • A more natural approach would be to extend our notation to allow operations to return tuples (e.g. structs or records) rather than just a single values


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Establishing Formal Methods

  • The use of formal methods is not an all or nothing proposition

  • The level of rigor employed can be tailored to fit

    • budgets

    • schedules

    • technical environments

  • Formal methods can be modified and integrated into existing processes


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Process Prerequisites

  • To make use of formal method a development process must be mature

    • has discrete phases

    • work products are defined for each phase

    • analysis procedures should be in place for work products

    • scheduled review of work products is present

  • The preferred type of analysis will be strongly influenced by the level of rigor needed by the project objectives


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Process Modifications with Formal Specification Methods

  • If the requirements analysis procedures are well-defined then few changes to the process will be required to integrate formal methods

  • Initial modeling activity employees finite-state machines, object diagrams, etc.

  • Model would be formalized by converting it to a formal language representation


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