Validation rules of passing steps for use case based on three-valued logic

Abstract

Validation rules are implemented as methods of program classes in the simplest case. Another approach involves using  "Specification" template, in which rules are formed as Boolean algebra predicates. The paper proposes to link validation rules with steps of use cases, through which functional requirements for software system are described. Validation can result in one of three situations at the execution step of use case: (1) the step was completed correctly, (2) an error occurred at step execution leading to an alternative path, (3) uncertainty that requires the actant decision to continue execution of the current path or to move towards an alternative path. Therefore, three-value logic is proposed to use for validation, while the values "true", "false" and "uncertainty" correspond to the above situations. The system status at validation moment is described by facts – atomic logical values reflecting status of program elements. A statement is attributed to each such value explaining the meaning in a fashion understandable to the actant in natural language. Combination of value and statement is conclusion. Thus, the purpose of validation is to obtain a final conclusion with respect to the predicate formulated for the step of the use case. Statement of the operation result is formed when performing a two-place logical operation on operands. The statement for the value that defined the operation logical result is included into the result. The statement after applying one-place operation on interium conclusion remains unchanged. "NOT", "AND", "OR" are proposed to additionally be included into the set of operations traditionally used in program development, one-place operations of  possibility and necessity known in three-value logic. One-place operations are moved to leaves in parse tree of validation predicate according to known formulas. Recommendation while implementing validation are to provide the final conclusion to the actant without mentioning operations’ names and without duplicate statements. The proposed approach to validation is implemented by the authors as programming library.