The goal of Appaserver's developers is to achieve the development standard established by Glenford J. Myers ^*. Myers ranked the idea of module cohesion — the degree of interaction within a model. Myers also ranked the idea of module coupling — the degree of interaction between modules.

Cohesion

The quality of a module's function determines the level of cohesion. From worst to best, here are the levels of cohesion:

1. Coincidental Cohesion is a module that performs two or more unrelated functions. Evidence of this occurs when a function name cannot be defined.
2. Logical Cohesion is a module that has available a series of functions, but only one of the functions is executed. This is the result of the case statement. Myers contends that this degrades readability, resulting in having to inspect the function to determine what is going to happen.
3. Temporal Cohesion is a module that performs a series of functions because it's time to. This usually occurs at the beginning of the algorithm, like calling the function perform_initialization().
4. Procedural Cohesion is a module that performs multiple functions, but only loosely related. An example would be the function read_part_number_and_update_employee_record(), where the employee record has no need for the part number.
5. Communicational Cohesion is a module that performs multiple functions, but closely related. An example would be the function read_part_number_and_update_sales_record(), where the sales record would need the part number.
6. Informational Cohesion is a module that performs multiple functions, and each function has its own entry point. However, all of the functions work on shared variables. The C++ class structure works at this level.
7. Functional Cohesion is a module that performs exactly one function. Or in the case of a module having multiple functions, each function has its own entry point, but works only on local variables. The goal is to be able to reuse the function in any context.

Coupling

Modules interact with each other via function calls, passing information back and forth. The degree of information passing sets the level of coupling. From worst to best, here are the levels of coupling:

1. Content Coupling is when a function modifies a variable local to another function.
2. Common Coupling is when a function modifies a global variable.
3. Control Coupling is when a function tells another function where to go. Difficulty occurs because the modules are no longer independent of each other.
4. Stamp Coupling is when a function receives an entire structure as a parameter. This is how the FILE Abstract Data Type behaves. Whereas stamp coupling saves typing, flexibility is lost.
5. Data Coupling is when each data element is presented in the parameter listing individually. This allows for greatest flexibility and readability.

The ideal function should have only input parameters and return a single value. However, for CPU efficiency, sometimes it is necessary to return multiple values. If multiple return values are necessary, pass the address of the variables to the function starting at the first parameter position.

The developers of Appaserver attempt to achieve Functional Cohesion and Data Coupling with the construction of Abstract Data Types (ADT). Each object is represented with a data structure, and all of the corresponding operations are contained in the same source file. Moreover, the syntax for each operation is
ObjectName_SomeVerbPhrase_ReturnDatatype( parameter list );

For example:

/* firm.h */
/* ------ */
#ifndef FIRM_H
#define FIRM_H
typedef struct
{
	char *firm_name;
	/* Firm's other attributes */
	int firm_zip_code;
} FIRM;

/* Operations */
/* ---------- */
FIRM *firm_new_firm(		char *firm_name,
				int firm_zip_code );
double firm_get_distance_miles_double(
				int firm_zip_code,
				int residential_zip_code );
#endif

/* firm.c */
/* ------ */
#include "stdio.h"
#include "firm.h"

FIRM *firm_new_firm( char *firm_name, int firm_zip_code )
{
	FIRM *firm = (FIRM *)calloc( 1, sizeof( FIRM ) );

	if ( !firm )
	{	
		fprintf(stderr,
		"ERROR in %s/%(): cannot allocate %s bytes of memory.\n",
			__FILE__,
			__FUNCTION__,
			sizeof( FIRM ) );
		exit( 1 );
	}
	firm->firm_name = firm_name;
	firm->firm_zip_code = firm_zip_code;
	return firm;
}

double firm_get_distance_miles_double(	int firm_zip_code,
					int residential_zip_code )
{
	double miles_double;

	miles_double = /* code to compute distance between two zip codes */
	return miles_double;
}

/* main.c */
/* ------ */
#include "stdio.h"
#include "firm.h"

int main( int argc, char **argv )
{
	FIRM *firm = firm_new_firm( "Acme Retailer", 33014 );

	printf( "Got distance = %lf\n",
		firm_get_distance_miles_double(
			firm->firm_zip_code,
			95825 ) );
	return 0;
}

*Stephen R. Schach; "Software Engineering"; 1990; pp. 219-233.
*W. P. Stevens, G. J. Myers, and L. L. Constantine; "Structured Design, IBM Systems Journal, Vol. 13"; 1974; pp. 115-139.