The Open/Closed Principle

Image created with Midjourney. Image prompt:
Image created with Midjourney. Image prompt: A 2D, minimalist figure is depicted adding blocks (representing new features) to a solid structure (representing an existing module) without changing the core structure. Each block has a unique symbol, signifying different functionalities
Entities should be open for extension and closed for modification.

In the realm of software design and development, principles and guidelines help maintain code quality and improve the software's maintainability. One such principle is the Open/Closed Principle, an integral part of the SOLID principles of object-oriented design.

Understanding the Open/Closed Principle

The Open/Closed Principle states that entities like classes, modules, or functions should be "open for extension but closed for modification"1. This means that a software entity should be designed to allow its behavior to be extended to accommodate new requirements, while its existing functionality should remain unchanged.

Imagine a module that converts a Markdown document into HTML. If a new syntax is added to the Markdown specification, like support for mathematical equations, the module should be open for extension to implement this new syntax. However, the existing implementations, such as paragraphs and bullets, should be closed for modification. These aspects already work as expected, and altering them could lead to unexpected results1.

The Open/Closed Principle in Digital Software Products

Let's delve into three examples illustrating how the Open/Closed Principle applies to digital software products:

Example 1: E-commerce Platforms

E-commerce platforms like Shopify or WooCommerce often add new features to improve the user experience. However, these features are added in such a way that the core functionality remains unchanged. This is an example of the Open/Closed Principle in action.

Example 2: Content Management Systems

Content Management Systems (CMS) like WordPress allow for the addition of plugins that extend the core functionality. However, these plugins do not modify the CMS's existing behavior, hence adhering to the Open/Closed Principle.

Example 3: Social Media Platforms

Social media platforms like Facebook and Twitter frequently introduce new features to enhance user engagement. Yet, they ensure that these additions do not disrupt the core features that users rely on. This is another practical implementation of the Open/Closed Principle.


The Open/Closed Principle is a crucial aspect of software design that ensures the stability, robustness, and maintainability of a software product. By adhering to this principle, digital product developers can add new functionalities without disrupting existing ones, leading to an improved product and a better user experience1.