Swagger: What is Swagger / OpenAPI?

What is Swagger?

Swagger is an ecosystem of open-source tools centered around the OpenAPI specification (formerly the Swagger Specification), an industry standard for describing REST APIs in a formal, machine-readable way. The OpenAPI specification defines a format (YAML or JSON) that exhaustively describes an API's endpoints, their parameters, request and response bodies, HTTP status codes, and authentication mechanisms.

The most well-known tool in the Swagger ecosystem is Swagger UI, an interactive web interface that transforms an OpenAPI specification into explorable visual documentation. Developers can not only read the documentation but also execute requests directly from the interface, making it an indispensable testing and exploration tool.

At Kern-IT, we use Swagger/OpenAPI on all our projects that expose REST APIs. For our Django projects, the drf-spectacular library automatically generates the OpenAPI specification from Django REST Framework views and serializers, and Swagger UI makes it accessible via a dedicated endpoint. This living documentation, always synchronized with the code, is a major asset for collaboration with frontend teams and clients.

Why Swagger Matters

API documentation is often the neglected child of software development. Swagger solves this problem by making documentation automatic, interactive, and impossible to ignore.

• Always up-to-date documentation: by generating documentation from source code, Swagger eliminates the risk of outdated documentation. If the API changes, the documentation changes automatically.
• Built-in test interface: Swagger UI allows testing each endpoint directly from the browser, with the ability to fill in parameters, authentication headers, and request bodies.
• API contract: the OpenAPI specification serves as a formal contract between backend and frontend. Teams can work in parallel based on the specification, without waiting for the API to be implemented.
• Code generation: from an OpenAPI specification, tools like Swagger Codegen or openapi-generator can automatically generate API clients in dozens of languages.
• Standardization: OpenAPI is an industry standard backed by the Linux Foundation. Tools, integrations, and skills are transferable from one project to another.

How It Works

The Swagger process can follow two approaches. The "code first" approach consists of writing the API code first, then automatically generating the OpenAPI specification from the code. This is the preferred approach at Kern-IT with Django REST Framework: serializers, views, and routers are annotated, and drf-spectacular generates the complete specification.

The "design first" approach consists of writing the OpenAPI specification before implementing the API. The Swagger Editor tool allows writing the specification in YAML with real-time preview. This approach is useful for projects where the API must be validated by multiple stakeholders before implementation.

The OpenAPI specification describes each endpoint with its path, HTTP method (GET, POST, PUT, DELETE), parameters (query, path, header), request body schema, possible response schemas (by status code), and required security mechanisms. Data schemas are defined according to the JSON Schema standard, with support for types, validation, and cross-references between schemas.

Ecosystem Tools

The Swagger/OpenAPI ecosystem comprises several complementary tools. Swagger UI is the interactive web interface for exploring documentation. Swagger Editor is an online editor for writing OpenAPI specifications. Swagger Codegen generates clients and servers in multiple languages from the specification.

In the Django ecosystem, drf-spectacular is the standard for OpenAPI 3.0 generation. It supports serializers, filters, pagination, authentication, and most common DRF patterns. For non-Django projects, FastAPI natively generates OpenAPI documentation thanks to Python type annotations.

Concrete Example

A Kern-IT project develops a REST API for a mobile order management application. The backend is built with Django REST Framework with drf-spectacular configured. Each endpoint is automatically documented: the order list (GET /api/orders/), order creation (POST /api/orders/), detail view (GET /api/orders/{id}/), and so on.

The frontend team accesses the URL /api/docs/ which displays Swagger UI. The mobile developer can test each endpoint with real data, understand the expected parameters and response formats. When the backend adds a new field to the serializer, the documentation is updated instantly, without manual intervention.

The client also receives access to this documentation and can follow the API development progress in real time. The OpenAPI specification is also exported as JSON and used to automatically generate the TypeScript client used by the mobile application.

Best Practices

1. Document from the start: integrate Swagger/OpenAPI from the first endpoint. Retrospective documentation is painful and often incomplete.
2. Enrich auto-generated documentation: add detailed descriptions, request and response examples to make the documentation genuinely useful.
3. Version your API: include the version in the path (/api/v1/) or headers, and document changes between versions.
4. Test via the documentation: use Swagger UI as a testing tool during development. If an endpoint is hard to test through the UI, it is often a sign the API design needs improvement.
5. Share the documentation: make Swagger UI accessible to frontend teams, clients, and technical partners. Good API documentation is a business asset.

Conclusion

Swagger and the OpenAPI specification have transformed how REST APIs are documented and consumed. By combining auto-generated documentation, an interactive test interface, and an industry standard, they bridge the gap between the backend and its consumers. For any company developing APIs, investing in Swagger/OpenAPI is a choice that improves developer productivity, integration quality, and API user satisfaction.