Pengantar Logika : Kerancuan Berpikir dan Jenisnya

Analisis dan Desain Perangkat Lunak : Module, Cohesion, dan Coupling

What Is a Module ?

A lexically contiguous sequence of program statements, bounded by boundary elements, with an aggregate identifier

“Lexically contiguous”

Adjoining in the code

“Boundary elements”

{ ... } 

begin ... end

“Aggregate identifier”

A name for the entire module

Design of Computer

A highly incompetent computer architect decides to build an ALU, shifter, and 16 registers with AND, OR, and NOT gates, rather than NAND or NOR gates

The architect designs three silicon chips

Redesign with one gate type per chip. Resulting “masterpiece”

The two designs are functionally equivalent

• The second design is
• Hard to understand
• Hard to locate faults
• Difficult to extend or enhance
• Cannot be reused in another product

Modules must be like the first design 

• Maximal relationships within modules, and
• Minimal relationships between modules

Composite/Structured Design

A method for breaking up a product into modules to achieve 

• Maximal interaction within a module, and 
• Minimal interaction between modules 

Module cohesion 

• Degree of interaction within a module 

Module coupling 

• Degree of interaction between modules

Function, Logic, and Context of a Module

In C/SD, the name of a module is its function 

Example: 

• A module computes the square root of double precision integers using Newton’s algorithm. The module is named compute_square_root

The underscores denote that the classical paradigm is used here

Cohesion

The degree of interaction within a module. Seven categories or levels of cohesion (non-linear scale)

Coincidental Cohesion

A module has coincidental cohesion if it performs multiple, completely unrelated actions 

Example: 

• print_next_line,
• reverse_string_of_characters_comprising_second_parameter,
• add_7_to_fifth_parameter,
• convert_fourth_parameter_to_floating_point 

Such modules arise from rules like 

“Every module will consist of between 35 and 50 statements”

Why Is Coincidental Cohesion So Bad ?

• It degrades maintainability 
• A module with coincidental cohesion is not reusable 
• The problem is easy to fix 
• Break the module into separate modules, each performing one task

Logical Cohesion

A module has logical cohesion when it performs a series of related actions, one of which is selected by the calling module

Example 1: 

function_code = 7; 

new_operation (op code, dummy_1, dummy_2, dummy_3); 

// dummy_1, dummy_2, and dummy_3 are dummy variables, 

// not used if function code is equal to 7 

Example 2: 

An object performing all input and output

Example 3: 

One version of OS/VS2 contained a module with logical cohesion performing 13 different actions. The interface contains 21 pieces of data

Why Is Logical Cohesion So Bad?

• The interface is difficult to understand 
• Code for more than one action may be intertwined 
• Difficult to reuse

A new tape unit is installed 

• What is the effect on the laser printer?

Temporal Cohesion

A module has temporal cohesion when it performs a series of actions related in time

Example: 

open_old_master_file,
new_master_file,
transaction_file,
print_file;

initialize_sales_district_table,
read_first_transaction_record,
read_first_old_master_record (a.k.a. perform_initialization)

Why Is Temporal Cohesion So Bad?

• The actions of this module are weakly related to one another, but strongly related to actions in other modules. Consider sales_district_table 
• Not reusable

Procedural Cohesion

A module has procedural cohesion if it performs a series of actions related by the procedure to be followed by the product 

Example: 

read_part_number_and_update_repair_record_on_ master_file

Why Is Procedural Cohesion So Bad ?

The actions are still weakly connected, so the module is not reusable

Communicational Cohesion

A module has communicational cohesion if it performs a series of actions related by the procedure to be followed by the product, but in addition all the actions operate on the same data 

Example 1: 

update_record_in_database_and_write_it_to_audit_trail 

Example 2: 

calculate_new_coordinates_and_send_them_to_terminal

Why Is Communicational Cohesion So Bad?

Still lack of reusability

Functional Cohesion

A module with functional cohesion performs exactly one action

Example 1: 

get_temperature_of_furnace 

Example 2: 

compute_orbital_of_electron 

Example 3: 

write_to_diskette 

Example 4: 

calculate_sales_commission

Why Is Functional Cohesion So Good?

• More reusable 
• Corrective maintenance is easier 
• Fault isolation 
• Fewer regression faults 
• Easier to extend a product

Informational Cohesion

A module has informational cohesion if it performs a number of actions, each with its own entry point, with independent code for each action, all performed on the same data structure

Why Is Informational Cohesion So Good?

Cohesion Example

Coupling

The degree of interaction between two modules. Five categories or levels of coupling (non-linear scale)

Content Coupling

Two modules are content coupled if one directly references contents of the other

Example 1: 

Module p modifies a statement of module q

Example 2: 

Module p refers to local data of module q in terms of some numerical displacement within q 

Example 3: 

Module p branches into a local label of module q

Why Is Content Coupling So Bad?

Almost any change to module q, even recompiling q with a new compiler or assembler, requires a change to module p

Common Coupling

Two modules are common coupled if they have write access to global data.

Example 1 

Modules cca and ccb can access and change the value of global_variable

Example 2: 

Modules cca and ccb both have access to the same database, and can both read and write the same record

Example 3: 

• FORTRAN common 
• COBOL common (nonstandard) 
• COBOL-80 global

Why Is Common Coupling So Bad ?

• It contradicts the spirit of structured programming. The resulting code is virtually unreadable 

What causes this loop to terminate?

• Modules can have side-effects 
• This affects their readability 
• Example: edit_this_transaction (record_7) 
• The entire module must be read to find out what it does 
• A change during maintenance to the declaration of a global variable in one module necessitates corresponding changes in other modules 
• Common-coupled modules are difficult to reuse
• Common coupling between a module p and the rest of the product can change without changing p in any way 
• Clandestine common coupling 
• Example: The Linux kernel 
• A module is exposed to more data than necessary 
• This can lead to computer crime

Control Coupling

Two modules are control coupled if one passes an element of control to the other

Example 1: 

An operation code is passed to a module with logical cohesion

Example 2:

A control switch passed as an argument



Example

Module p calls module q 

Message: 

I have failed — data 

Message: 

I have failed, so write error message ABC123 — control

Why Is Control Coupling So Bad ?

• The modules are not independent 
• Module q (the called module) must know the internal structure and logic of module p 
• This affects reusability 
• Associated with modules of logical cohesion

Stamp Coupling

Some languages allow only simple variables as parameters 

• part_number 
• satellite_altitude 
• degree_of_multiprogramming 

Many languages also support the passing of data structures 

• part_record 
• satellite_coordinates 
• segment_table

Two modules are stamp coupled if a data structure is passed as a parameter, but the called module operates on some but not all of the individual components of the data structure

Why Is Stamp Coupling So Bad ?

• It is not clear, without reading the entire module, which fields of a record are accessed or changed. 
• Example calculate_withholding (employee_record) 
• Difficult to understand 
• Unlikely to be reusable 
• More data than necessary is passed 
• Uncontrolled data access can lead to computer crime
• However, there is nothing wrong with passing a data structure as a parameter, provided that all the components of the data structure are accessed and/or changed.
• Examples: 
• invert_matrix (original_matrix, inverted_matrix);
• print_inventory_record (warehouse_record);

Data Coupling

Two modules are data coupled if all parameters are homogeneous data items (simple parameters, or data structures all of whose elements are used by called module)

Examples: 

• display_time_of_arrival (flight_number); 
• compute_product (first_number, second_number); 
• get_job_with_highest_priority (job_queue);

Why Is Data Coupling So Good ?

• The difficulties of content, common, control, and stamp coupling are not present 
• Maintenance is easier

Coupling Example

The Importance of Coupling

• As a result of tight coupling 
• A change to module p can require a corresponding change to module q 
• If the corresponding change is not made, this leads to faults 
• Good design has high cohesion and low coupling 
• What else characterizes good design? (see over)

Sumber

Object-Oriented and Classical Software Engineering, Eighth Edition, WCB/McGraw-Hill, 2011 by Stephen R. Schach

Slide ADPL : Modules and Objects