1 SOFTWARE DESIGN QUALITY COHESION and COUPLING (Part II)

2 Quality Characteristics for Modules independent modules –avoid aggregation of multiple tasks per module –single-minded function ===> high COHESION –minimal interaction with other modules –simple interface ===> low COUPLING [Steven, Myers, Constantine]

3 Module COUPLING quality of interaction between modules based on module interfaces and types and number of connections coupling should be low types of coupling (high to low):

4 Content Coupling characteristics: –direct reference into another module –modification of called module at run-time example(s): –at calling time: jump to an address entry point –change local data of another module problems: –modules rely on each others internal structure –no clear interface - side effects –high degree of interdependency

5 Common Coupling characteristics: –sharing of a common, external data area –data area is unstructured example(s): –FORTRAN “COMMON” –C external data types problems: –M modules and N data elements: M x (M - 1) x N possible connections –high degree of (potential) interdependency

6 External Coupling characteristics: –sharing of a common, external data area –data area is structured example(s): –global data structures (variables) –communication to external tools and devices problems: –high potential for side effects –missing access control –modules are “bound” to the global structure

7 Control Coupling characteristics: –one module controls the execution path of another module via control flow (flag) example(s): –control parameter selects from a set of functions –error-checking decides on further execution problems: –the controlling module must have a minimum of knowledge about the controlled module –cohesion of the controlled module? (logical, procedural)

8 read & check registration reg_i 1 read registration 1.1 check registration 1.2 error-handler registration 1.3 reg_i reg_flag error_reason Control Coupling - control inversion

9 Stamp (Data) Coupling characteristics: –parameter passing of a complete data structure –only portions of the data structure are used example(s): –global, imported data structure declarations –copying real life limitations into your system problems: –change of data structure => change of module –module has to handle irrelevant information –module’s cohesion? (communicational, sequential,...)

10 Data Coupling characteristics: –parameter passing of (atomic) data elements –no unnecessary information –no control parameters –no external, shared data structures –independent and reusable –execute one single-minded function (good cohesion) problems: –cannot always be achieved

11 A good system design exhibits HIGH COHESION and LOW COUPLING

12 Cohesion & Coupling many times, high cohesion and low coupling complement and cause each other sometimes, high cohesion and low coupling contradict each other ===> the designer decides on the trade-offs

13 Cohesion & Coupling heuristics (sanitized experience) relative measures non-linear scale

14 Beyond Cohesion & Coupling Module size – objective: “understood without flipping the pages” – guideline: module size < statements on a page Fan-out (span of control): – objective : ”boss” modules should control a reasonable number of workers; – guideline: number of modules controlled by a “boss” < 7 Fan-in – objective: maximize fan-in, create generalized modules – guideline: modules must have strong or moderate cohesion (3) (4)

15 End of Section 4b