RESTful API Integration

RESTful API integration uses HTTP to exchange representations of resources between services. A Java backend typically builds HTTP requests, sends them to resource-oriented endpoints, handles status codes and response data, and deals with failures such as timeouts or invalid responses.

REST models application data as resources identified by URIs, such as `/users/42` or `/orders/1001`. Clients interact with those resources through standard HTTP methods and exchange representations, commonly JSON.

The usual method semantics are `GET` for retrieval, `POST` for creating or triggering an operation, `PUT` for replacing a resource, `PATCH` for partial modification, and `DELETE` for removal. The exact behavior should follow the API contract rather than relying only on the method name.

HTTP status codes communicate the outcome. For example, `200 OK` commonly indicates a successful request with a response body, `201 Created` indicates successful creation, `204 No Content` indicates success without a response body, and `4xx` or `5xx` statuses indicate client-side or server-side failures respectively.

In Java backend applications, REST integration commonly involves an HTTP client such as Java's `HttpClient`, Spring's `RestClient`, or another framework-supported client. The integration code should set appropriate headers, serialize request bodies, deserialize responses, validate status codes, and handle timeouts and transport failures.

Reliable integrations also consider authentication, connection and read timeouts, retries, idempotency, logging, and observability. Retries should be used carefully: automatically repeating a non-idempotent operation such as an unsafe `POST` can create duplicate effects unless the API provides an idempotency mechanism.

Key points

• Resources are identified by URIs and exchanged through HTTP representations such as JSON.
• Use HTTP methods according to their defined semantics: `GET`, `POST`, `PUT`, `PATCH`, and `DELETE`.
• Use status codes to distinguish successful responses, client errors, and server errors.
• Java integrations must handle serialization, deserialization, HTTP failures, timeouts, and authentication.
• Retries require care because repeating some operations can produce duplicate side effects.

Interview questions

What is REST, and how would you integrate a Java service with a REST API?

REST is an architectural style that commonly represents application data as resources identified by URIs and accessed through HTTP. In Java, I would create an HTTP client, construct the request with the required method, URI, headers, and body, then send it to the remote service. I would check the HTTP status, deserialize a successful response, and handle errors and timeouts appropriately. I would also follow the API contract for authentication, request formats, and error responses.

What is the difference between GET, POST, PUT, PATCH, and DELETE?

`GET` retrieves a representation of a resource and should not be used to perform state-changing operations. `POST` is commonly used to create a subordinate resource or submit an operation, while `PUT` is generally used for replacing a resource at a known target URI. `PATCH` is used for partial modifications, and `DELETE` requests removal of a resource. The API contract determines the precise behavior, but clients should respect standard HTTP semantics.

How should a Java REST client handle HTTP errors?

The client should first distinguish transport failures, such as connection errors and timeouts, from valid HTTP responses containing error status codes. For HTTP responses, it should inspect the status code and, when useful, parse the API's structured error body. The application can then map expected failures to appropriate domain behavior while preserving enough context for logging and troubleshooting. Retries should be limited to failures where repeating the operation is safe or explicitly supported by the API.

How would you make a REST integration reliable in production?

I would define explicit connection and request timeouts and make failure behavior part of the integration design rather than an afterthought. Retries would use bounded attempts and backoff, with idempotency considered before repeating requests. I would add structured logging, metrics, tracing, and correlation information so failures can be diagnosed across service boundaries. I would also validate response schemas, protect credentials, and consider rate limits and circuit-breaking where the system's failure characteristics justify them.