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principles:principle_of_separate_understandability

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Principle Of Separate Understandability (PSU)

Variants and Alternative Names

Context

Principle Statement

Each module shall be understandable on its own – without knowing anything about other modules.

Description

PSU means that:

  • By looking at the public methods of a class it should be clear why they are there. That means there should be no method that is only there because a specific other module needs it.
  • By looking at the implementation of a module it should be clear how it works and why it was done that way. That means there should be no code that is solely there in order to make another module work.
  • By looking at a private method it should be clear what it does. That means there should be no (private) method that is only meaningful in the context of another method.

Rationale

When a module is separately understandable, it is easier to maintain, as no other modules have to be considered during maintenance. It is furthermore more testable, as a unit test can easily test only this particular module without requiring integration with other modules.

Another point of view is that a violation of PSU either means that a part of the functionality does not belong to that module or the module has the wrong abstraction. So this is a sign of a design that needs improvement.

Strategies

When a module does not comply with PSU, this means that either a part of the functionality of the module does not belong here or the module has the wrong abstraction. So strategies for making a solution more compliant with PSU are:

  • Move the conflicting functionality to another module where it fits better (see IE).
  • Build up a new module for the conflicting functionality (see HC).
  • Find the right abstraction for the module that allows the functionality to stay here (see MP).

Origin

This principle is newly proposed in this wiki. Nevertheless it is believed that it is not “new” in the sense that its a new insight. Its rather something that is commonly known but hasn't been expressed as a principle, yet.

Evidence

Relations to Other Principles

Generalizations

Specializations

Contrary Principles

Complementary Principles

  • Information Hiding/Encapsulation (IH/E): PSU is about constructing a module such that its inner workings (and its usage also) can be understood without knowledge about other modules. IH/E on the other hand is about constructing a module in a way that hides the inner workings so it can be used without knowing them.
  • Low Coupling (LC): One kind of couplings are logical couplings. These are especially hard to detect but should be avoided. PSU describes one aspect of these logical couplings, whereas LC relates this kind of coupling to others.
  • Model Principle (MP): The model contains the only information that should be necessary to understand the module.

Principle Collections

OOD Principle Language
General Principles
ML KISS MIMC DRY GP RoE
Modularization Principles
MP HC ECV
Module Communication Principles
TdA/IE LC DIP
Interface Design Principles
EUHM PLS UP
Internal Module Design Principles
IH/E IAP LSP PSU

Example

Example 1: Parsing Data

Suppose a program parses data stored in an spreadsheet file. There are three classes:

  • SpreadsheetReader: This reads the spreadsheet and creates DomainObject objects.
  • DomainObject: This is the data which was contained in the spreadsheet and is now processed by the program in some way.
  • SpreadsheetWriter: This class takes a DomainObject and writes it back to the spreadsheet.

In such a scenario it might be convenient to simplify SpreadsheetWriter by adding information about the spreadsheet to DomainObject. This might be some cell coordinates for example. SpreadsheetReader can store them into the newly created DomainObject and SpreadsheetWriter uses the data to store the DomainObject to the correct position in the spreadsheet.

This is a simple solution (see KISS) but it violates PSU. DomainObject is not understandable on its own. It holds data (namely the cell position in the spreadsheet) that is only meaningful in the context of the other two modules. During maintenance this data could accidentally be altered (resulting in a corrupted output file). Maintenance effort is also increased simply by distracting the maintainers who might wonder what this data is and if it is relevant for their task.

A better solution (wrt. PSU) would be to give SpreadSheetWriter the ability to determine the correct position in the spreadsheet itself. This is more complicated and may involve searching the spreadsheet for the correct position. But DomainObject is easier to understand and less prone to errors.

Description Status

Further Reading

principles/principle_of_separate_understandability.1360665435.txt.gz · Last modified: 2013-05-20 12:46 (external edit)