How to implement safety against repeated calls in a REST controller so that it is resistant to re-execution?

To make a REST method idempotent, you can use several approaches:

1. Idempotent operations: design methods so that repeated calls with the same parameters do not alter the system's state further.

2. Using unique identifiers (idempotency keys): the client generates a unique key for each operation and sends it in the request. The server stores processed keys and ignores repeated requests with the same key.

3. Locks and transactions: use locking mechanisms or transactions to ensure that the operation is executed only once.

Example using a unique key:

@PostMapping("/process")
public ResponseEntity<?> process(@RequestHeader("Idempotency-Key") String key, @RequestBody Data data) {
    if (processedRequests.containsKey(key)) {
        return ResponseEntity.ok(processedRequests.get(key)); // Return the result of the previous call
    }
    Result result = service.processData(data);
    processedRequests.put(key, result);
    return ResponseEntity.ok(result);
}

Here, processedRequests is, for example, a thread-safe cache or database where results are stored by keys.