RAML

RAML, the RESTful API Modeling Language, is a YAML-based specification for describing REST APIs, launched in 2013 by a group of vendors led by MuleSoft. It set out to make API design a first-class, human-readable activity, leaning hard on reuse through resource types, traits, and includes so that teams could describe large APIs without repeating themselves. For a stretch of the early API-definition era, RAML was one of the three formats every practitioner debated.

• YAML-first and readable - Definitions are written in clean YAML meant to be authored and reviewed by people, not just generated.
• Reuse primitives - Resource types and traits let you define a pattern once and apply it across many endpoints.
• Modular includes - !include pulls in external files, so schemas, examples, and fragments live as reusable, versioned assets.
• Design-first workflow - RAML pushed teams to model the contract before writing code, feeding mocks, docs, and validation.

RAML was one corner of the design-format contest I covered constantly in 2014, alongside Swagger/OpenAPI and API Blueprint, and it shaped how the industry thought about reuse and design-first practice. In the years since, the market consolidated overwhelmingly around OpenAPI, and most new work is authored there, but RAML definitions still exist across many established API programs and its ideas about traits and resource types continue to echo in how teams think about consistency. If you are inheriting an older API estate, RAML is a format you will still encounter and occasionally need to convert forward.