Declarative REST clients and service contracts

One of the important aspects of building distributed systems is communicating between different services/APIs. In a microservice architecture, services interact with each other via RESTful APIs. Traditionally, building REST clients has been a tedious process, where developers need to write boilerplate code to handle HTTP requests and responses. However, with the advent of Spring Cloud and its declarative REST client support, this process has become much easier and more efficient.

Declarative REST clients

In Spring Cloud, a declarative REST client allows you to define a Java interface with annotations that describe the desired HTTP endpoints and their behavior. This interface acts as a contract that defines the structure of the REST API and the expected request and response formats. Spring Cloud then generates a fully functional implementation of this interface for you, eliminating the need to write repetitive code for handling HTTP requests and responses.

The benefits of using declarative REST clients are numerous. First and foremost, it simplifies the development process. Developers can focus on defining the API contract and the business logic, rather than the low-level implementation details. Additionally, by using annotations, it provides a clean and readable way to document the REST API. The generated implementation also benefits from the rich ecosystem of Spring Cloud, with support for features like load balancing, circuit breaking, and service discovery.

Service contracts

Declarative REST clients heavily rely on the concept of service contracts. A service contract defines the agreement between the client and the server on how they will communicate with each other. It specifies the structure and semantics of the requests and responses, including the expected data formats, headers, and error handling mechanisms.

By using service contracts, you can enforce consistency and compatibility between services. Changes to the API can be communicated through versioning and negotiation, allowing services to evolve independently without breaking existing clients. This decoupling of client and server enables teams to work independently and promotes service autonomy.

Service contracts can be defined using various technologies and standards like OpenAPI (formerly known as Swagger), RAML, or even simple documentation formats. In the Spring Cloud ecosystem, OpenAPI is a widely used standard for describing RESTful APIs. Using tools like SpringFox or Swagger Codegen, you can generate a service contract from your existing code or create it from scratch.


Declarative REST clients and service contracts are powerful tools in building and maintaining distributed systems. By using declarative REST clients, developers can write concise and readable code, focusing on business logic rather than low-level HTTP interactions. Service contracts enable teams to communicate effectively and ensure compatibility between services. Spring Cloud's support for declarative REST clients and its integration with OpenAPI standards make it an excellent choice for developing RESTful microservices.

So, go ahead and leverage the power of declarative REST clients and service contracts in your Spring Cloud applications, and experience the benefits of clean, efficient, and maintainable API integrations.

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