RAML vs OpenAPI: When to Use Each in MuleSoft API Design

APIs are everywhere. Whether it’s a mobile app fetching user details, an e-commerce site checking inventory, or banks verifying transactions — everything depends on APIs. But before an API is built, it must be designed, and that’s where API specifications come in.

Today, two major specification formats dominate the industry:

• RAML (RESTful API Modeling Language)
• OpenAPI Specification (OAS) — previously Swagger

Most developers hear these terms when they enter the API world, but the real question is:

• Why do we have two standards doing the same thing?
• Why did MuleSoft create RAML when Swagger already existed?
• When should I use RAML? When should I use OAS?

1. Evolution of API Specifications

Before standardized specifications, API design was chaotic. Teams documented endpoints in Word files, spreadsheets, or wiki pages. There was no single source of truth, which led to:

• Confusing or inconsistent endpoints
• Missing documentation
• Integration failures between teams

To solve this, Swagger (now OAS) was introduced. It allowed APIs to be described in a machine-readable format that developers and tools could understand. This made API development faster, more consistent, and easier to maintain.

As enterprises grew, they faced new challenges: multiple APIs across large teams, a need for design consistency, and a desire for reusability. Existing solutions like OAS focused mainly on documentation and were not enough for design-heavy internal workflows. This gap led to the creation of RAML.

2. Why RAML Exists: Beyond OAS for Enterprise APIs

OAS came first and quickly became a global standard. Its primary focus was on documenting APIs and enabling interoperability across platforms and tools. While this worked well for many scenarios, large enterprises building API-led integrations needed more than documentation. They needed structured, reusable, design-first specifications that could enforce consistency across numerous APIs.

MuleSoft created RAML to address these enterprise requirements. RAML allows teams to:

• Promote reusable API components using traits, resourceTypes, and libraries
• Provide a human-friendly syntax that architects and developers can easily understand
• Ensure consistency across internal APIs
• Integrate deeply with MuleSoft tools like Design Center, Studio, and Exchange

Importantly, RAML was not designed to replace OAS globally. Instead, it serves enterprises heavily invested in MuleSoft, where internal API modeling, reuse, and design-first development are critical for maintaining large, scalable API ecosystems.

3. RAML Outside MuleSoft

RAML is rarely adopted. Occasionally, teams may use it to define API endpoints and generate mock servers or SDKs, but this is uncommon. Most organizations prefer OAS for broader, cross-platform API standards.

4. What is RAML?

RAML (RESTful API Modeling Language) is a YAML-based specification from MuleSoft, designed for design-first API development. It helps teams create consistent, reusable internal APIs and integrates seamlessly with MuleSoft tools like Design Center, Studio, and Exchange.

Key Characteristics

• Human-friendly and easy to read
• Built for designing APIs before implementation
• Reusability through traits, resourceTypes, libraries
• Native support inside MuleSoft tools
• Strong design-first approach
• Best suited when the organization uses MuleSoft heavily

Example RAML Snippet

#%RAML 1.0
title: Customer API
version: v1
baseUri: /api/customers

types:
  Customer:
    type: object
    properties:
      id: integer
      name: string
      email: string
      phone: string

/customers:
  get:
    description: Retrieve all customers
    responses:
      200:
        body:
          application/json:
            type: Customer[]
            example:
              - id: 1
                name: John Doe
                email: john@example.com
                phone: "9999999999"

  post:
    description: Create a new customer
    body:
      application/json:
        type: Customer
        example:
          name: Jane Doe
          email: jane@example.com
          phone: "8888888888"
    responses:
      201:
        body:
          application/json:
            example:
              message: "Customer created successfully"

/customers/{id}:
  uriParameters:
    id:
      type: integer

  get:
    description: Retrieve a customer by ID
    responses:
      200:
        body:
          application/json:
            type: Customer
            example:
              id: 1
              name: John Doe
              email: john@example.com
              phone: "9999999999"

  put:
    description: Update an existing customer
    body:
      application/json:
        type: Customer
    responses:
      200:
        body:
          application/json:
            example:
              message: "Customer updated successfully"

  delete:
    description: Delete a customer
    responses:
      200:
        body:
          application/json:
            example:
              message: "Customer deleted successfully"

5. What is OAS?

OAS (OpenAPI Specification), formerly known as Swagger, is a globally accepted standard for describing REST APIs. It supports YAML and JSON, focuses on documentation and interoperability, and is widely used for APIs shared externally or across multiple platforms.

OAS is ideal when APIs need to be accessible to different teams, platforms, or partners and is supported by most tools like Postman, SwaggerHub, and API gateways.

Key Characteristics

• The industry standard across most companies
• Tooling support everywhere (Postman, SwaggerHub, Azure, AWS, etc.)
• JSON & YAML formats
• Machine-readable + strong documentation capabilities
• Supports API-first or documentation-first approaches
• Used when APIs need to be shared with external partners or across platforms

Example OAS Snippet

openapi: 3.0.0
info:
  title: Customer API
  version: 1.0.0

paths:
  /customers:
    get:
      summary: Get all customers
      responses:
        '200':
          description: List of customers
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: '#/components/schemas/Customer'
              examples:
                sample:
                  value:
                    - id: 1
                      name: John Doe
                      email: john@example.com
                      phone: "9999999999"

    post:
      summary: Create customer
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/Customer'
            examples:
              sample:
                value:
                  name: Jane Doe
                  email: jane@example.com
                  phone: "8888888888"
      responses:
        '201':
          description: Customer created
          content:
            application/json:
              examples:
                msg:
                  value:
                    message: Customer created successfully

  /customers/{id}:
    parameters:
      - name: id
        in: path
        required: true
        schema:
          type: integer

    get:
      summary: Get customer by ID
      responses:
        '200':
          description: Customer details
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Customer'
              examples:
                sample:
                  value:
                    id: 1
                    name: John Doe
                    email: john@example.com
                    phone: "9999999999"

    put:
      summary: Update customer
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/Customer'
      responses:
        '200':
          description: Updated
          content:
            application/json:
              examples:
                sample:
                  value:
                    message: Customer updated successfully

    delete:
      summary: Delete customer
      responses:
        '200':
          description: Deleted
          content:
            application/json:
              examples:
                sample:
                  value:
                    message: Customer deleted successfully

components:
  schemas:
    Customer:
      type: object
      properties:
        id:
          type: integer
        name:
          type: string
        email:
          type: string
        phone:
          type: string

6. RAML vs OAS Key Differences

7. When to Use RAML and When to Use OAS?

Use RAML when:

• Your entire organization uses MuleSoft
• You want strong reusability (traits, resourceTypes)
• You need a pure modeling language
• Internal APIs that stay within MuleSoft ecosystem

Example: A bank connecting customer and transaction systems uses RAML for consistent, reusable API designs.

Use OAS when:

• You share APIs with external partners
• Multiple platforms consume the API
• You need to publish APIs in SwaggerHub, Azure, AWS, Apigee, Kong, etc.
• You follow industry-standard API documentation requirements

Example: An e-commerce platform exposes inventory and order APIs to third-party vendors using OAS for easy integration.

8. OAS with MuleSoft — Step-by-Step Guide

Step 1: Login to Anypoint Platform

1. Go to: https://anypoint.mulesoft.com
2. Enter your credentials
3. Choose your organization & environment

Step 2: Open Design Center

1. Click Design Center
2. Click Create New
3. Choose API Specification

Step 3: Choose OpenAPI Specification (OAS)

1. Select OpenAPI Specification (OAS 3.0)
2. Click Create Specification

Design Center opens a new editor with:

• Left panel: Files
• Middle: OAS editor
• Right: API console preview

Step 4: Import an Existing OAS File (Optional)

If you already have a Swagger/OAS file:

1. Click + Add File
2. Select Import
3. Upload your .yaml or .json OAS file

Design Center automatically validates & shows API console.

Step 5: Use Mocking Service

1. Click the top-right Mocking Service button
2. Enable Mocking
3. Copy the Mock URL
4. Test API in Postman / browser

This helps developers develop before backend implementation is ready.

Step 6: Publish to Anypoint Exchange

1. Click Publish (top right)
2. Select your organization
3. Provide version, description
4. Click Publish to Exchange

The API is now reusable by your entire organization.

Step 7: Create API Implementation in MuleSoft

1. Go to Anypoint Studio
2. Click File → New → Mule Project
3. Select API Specification from Exchange
4. Choose your published OAS spec
5. Studio automatically generates:

You only need to add business logic inside each generated flow.

Step 8: Deploy API to CloudHub

1. Right-click your project
2. Click Anypoint Platform → Deploy to CloudHub
3. Select environment
4. Provide app name
5. Deploy

Your OAS-based API is now live with MuleSoft runtime.

9. Summary

RAML and OAS serve different purposes in API development. RAML is ideal for internal, design-first APIs within MuleSoft, offering reusability and consistency. OAS is the industry standard for APIs shared externally or across multiple platforms, ensuring interoperability and broad tool support.

10. Final Conclusion

Choosing between RAML and OAS depends on your business needs and API strategy.

• Use RAML for internal APIs where design consistency and reuse matter most.
• Use OAS when APIs are exposed externally, shared across teams, or require industry-standard documentation.

By leveraging MuleSoft’s Design Center, Studio, Exchange, and Mocking Service, organizations can quickly design, test, and deploy APIs, making OAS-based API development simple, fast, and enterprise-ready.