# Design pattern

> A named, reusable solution to a problem that keeps showing up in software design.

- Level: Advanced
- Topics: Software
- URL: https://tidbit.legios.com.ar/design-pattern/
- In Spanish: [Patrón de diseño](https://tidbit.legios.com.ar/es/design-pattern/)

A **design pattern** is a proven way to structure code for a recurring problem, described in general terms so you can adapt it to your own program. It is not a library you install or code you paste: it is a shape, with a name, a typical situation where it helps, and its trade-offs. The idea was borrowed from architecture and popularized in 1994 by the book Design Patterns, whose four authors are known as the **Gang of Four**. It catalogued 23 patterns for object-oriented programming.

A few examples. The **observer pattern** lets one object notify many others when something changes, without knowing who they are: it is behind every "subscribe" and event listener, from a spreadsheet updating its charts to a UI reacting to a click. The strategy pattern swaps one algorithm for another behind the same interface, like choosing between shipping calculators. The adapter wraps a component with an incompatible interface so it fits, the way a travel plug adapter does. The factory centralizes how objects are created, and the singleton ensures there is only one instance of something.

The biggest value is shared vocabulary. Saying "put an adapter in front of the old payments API" communicates a whole design in five words during a code review. Patterns also travel beyond classes: retries with backoff, circuit breakers, publish and subscribe, and the many patterns of microservices and cloud architecture follow the same idea. The risk is the opposite: forcing patterns where a plain function would do adds layers nobody needs. A pattern that reliably causes trouble is called an **anti-pattern**.

## An analogy

Recipes in a cookbook for techniques rather than dishes: how to make a roux, how to emulsify a sauce. Every cook adapts them to their own dish, and saying "start with a roux" tells another cook exactly what you mean.

## Keep in mind

- A design pattern is a named, general solution to a recurring design problem.
- The 1994 Gang of Four book catalogued 23 classic object-oriented patterns.
- Their main value is a shared vocabulary for discussing designs.
- Overusing patterns adds needless complexity; some habits are anti-patterns.

## Go deeper

- [Software design pattern on Wikipedia](https://en.wikipedia.org/wiki/Software_design_pattern)
- [Design patterns catalog (Refactoring.Guru)](https://refactoring.guru/design-patterns)

## Related terms

- [Object-oriented programming](https://tidbit.legios.com.ar/object-oriented-programming/): Organizing code as objects that bundle data with the actions that use it.
- [Spreadsheet vs database](https://tidbit.legios.com.ar/spreadsheet-vs-database/): Why a company that runs on Excel eventually needs a real database.
- [UX and UI](https://tidbit.legios.com.ar/ux-and-ui/): UI is what you see and touch on a screen; UX is how the whole experience feels.
- [Algorithm](https://tidbit.legios.com.ar/algorithm/): A precise, step-by-step method for solving a problem or doing a task.
- [API](https://tidbit.legios.com.ar/api/): The menu of things one program lets another program ask for.
- [Code review](https://tidbit.legios.com.ar/code-review/): Having another person read a code change before it becomes part of the product.
- [Microservices](https://tidbit.legios.com.ar/microservices/): Building one application as many small services that each do one job.
- [Cloud computing](https://tidbit.legios.com.ar/cloud-computing/): Renting computers over the internet instead of buying your own.