What methods can be used to implement idempotency in REST API operations?

Idempotency in REST API means that calling the same operation with the same parameters multiple times will not change the system's state after the first call.

Ways to implement idempotency:

• Using unique identifiers (idempotency keys): the client generates a unique key for the operation, the server saves the result, and upon a repeat request with the same key, it returns the same response without performing the operation again.

• HTTP methods with idempotent behavior: for example, PUT and DELETE are idempotent by standard, as repeated execution leads to the same state.

• Handling repeated requests at the business logic level: for example, checking if the operation with such parameters has already been performed, and ignoring repeats.

Example with idempotency key in Java (simplified):

@PostMapping("/payments")
public ResponseEntity<?> createPayment(@RequestHeader("Idempotency-Key") String key, @RequestBody PaymentRequest request) {
    if (paymentService.isProcessed(key)) {
        return ResponseEntity.ok(paymentService.getResult(key));
    }
    PaymentResult result = paymentService.processPayment(request);
    paymentService.saveResult(key, result);
    return ResponseEntity.ok(result);
}