@RestController
public class DataController {

    private volatile boolean synchronizing;

    @GetMapping(value = "api/sync")
    public ResponseEntity synchronize() {
        if (synchronizing) {
            throw new RuntimeException("Already Synchronizing");
        }
        synchronizing = true;
        try {
            //Do very long process
        } catch (SynchronizationException e) {
            throw new RuntimeException(e);
        } finally {
            synchronizing = false;
        }
        return ResponseEntity.ok();
    }
}