SOLID Principles

This is an acronym, which refers to:

• S: The Single Responsibility Principle
• O: The Open/Closed Principle
• L: The Liskov Substitution Principle
• I: The Interface Segregation Principle
• D: The Dependency Inversion Principle

These are key principles in Object-Oriented Programming. Design principles such as these should be able to aid developers build more maintainable systems.

The Single Responsibility Principle

Every module or class should have a single responsibility only.

The first of the SOLID principles. This principle suggests that modules or classes should do one thing and one thing only. In more practical terms, this means that a single, small change to a feature of a program should require a change in one component only. For example, changing how a password is validated for complexity should require a change in only one part of the program.

Theoretically, this should make the code more robust, and easier to change. Knowing that a component which is being changed has a single responsibility only means that testing that change should be easier. Using the earlier example, changing the password complexity component should only be able to affect the features which relate to password complexity. It can be much more difficult to reason about the impact of a change to a component which has many responsibilities.

The Open/Closed Principle

Entities should be open for extension and closed for modification.

The second of the SOLID principles. This principle states that entities (which could be classes, modules, functions and so on) should be able to have their behaviour extended, but that their existing behaviour should not be able to be modified.

As a hypothetical example, imagine a module which is able to turn a Markdown document into HTML. Now imagine there is a new syntax added to the Markdown specification, which adds support for mathematical equations. The module should be open to extension to implement the new mathematics syntax. However, existing syntax implementations (like paragraphs, bullets, etc) should be closed for modification. They already work, we don't want people to change them.

This principle has particular relevance for object-oriented programming, where we may design objects to be easily extended, but would avoid designing objects which can have their existing behaviour changed in unexpected ways.

The Liskov Substitution Principle

It should be possible to replace a type with a subtype, without breaking the system.

The third of the SOLID principles. This principle states that if a component relies on a type, then it should be able to use subtypes of that type, without the system failing or having to know the details of what that subtype is.

As an example, imagine we have a method which reads an XML document from a structure which represents a file. If the method uses a base type 'file', then anything which derives from 'file' should be able to be used in the function. If 'file' supports seeking in reverse, and the XML parser uses that function, but the derived type 'network file' fails when reverse seeking is attempted, then the 'network file' would be violating the principle.

This principle has particular relevance for object-oriented programming, where type hierarchies must be modeled carefully to avoid confusing users of a system.

The Interface Segregation Principle

No client should be forced to depend on methods it does not use.

The fourth of the SOLID principles. This principle states that consumers of a component should not depend on functions of that component which it doesn't actually use.

As an example, imagine we have a method which reads an XML document from a structure which represents a file. It only needs to read bytes, move forwards or move backwards in the file. If this method needs to be updated because an unrelated feature of the file structure changes (such as an update to the permissions model used to represent file security), then the principle has been invalidated. It would be better for the file to implement a 'seekable-stream' interface, and for the XML reader to use that.

This principle has particular relevance for object-oriented programming, where interfaces, hierarchies and abstract types are used to minimise the coupling between different components. Duck typing is a methodology which enforces this principle by eliminating explicit interfaces.

The Dependency Inversion Principle

High-level modules should not be dependent on low-level implementations.

The fifth of the SOLID principles. This principle states that higher level orchestrating components should not have to know the details of their dependencies.

As an example, imagine we have a program which read metadata from a website. We would assume that the main component would have to know about a component to download the webpage content, then a component which can read the metadata. If we were to take dependency inversion into account, the main component would depend only on an abstract component which can fetch byte data, and then an abstract component which would be able to read metadata from a byte stream. The main component would not know about TCP/IP, HTTP, HTML, etc.

This principle is complex, as it can seem to 'invert' the expected dependencies of a system (hence the name). In practice, it also means that a separate orchestrating component must ensure the correct implementations of abstract types are used (e.g. in the previous example, something must still provide the metadata reader component a HTTP file downloader and HTML meta tag reader). This then touches on patterns such as Inversion of Control and Dependency Injection.

TODO

this is work in progress!