Order of processing REST API calls - spring

Question

I have a strage(for me) question to ask. I have created synchronized Service which is called by Controller:
#Controller
public class WebAppApiController {
private final WebAppService webApService;
#Autowired
WebAppApiController(WebAppService webApService){
this.webApService= webApService;
}
#Transactional
#PreAuthorize("hasAuthority('ROLE_API')")
#PostMapping(value = "/api/webapp/{projectId}")
public ResponseEntity<Status> getWebApp(#PathVariable(value = "projectId") Long id, #RequestBody WebAppRequestModel req) {
return webApService.processWebAppRequest(id, req);
}
}
Service layer is just checking if there is no duplicate in request and store it in database. Because client which is using this endpoint is making MANY requests continously it happened that before one request was validated agnist duplicate other the same was put in database - that is why I am trying to do synchronized block.
#Service
public class WebAppService {
private final static String UUID_PATTERN_TO = "[a-zA-Z0-9]{8}-[a-zA-Z0-9]{4}-[a-zA-Z0-9]{4}-[a-zA-Z0-9]{4}-[a-zA-Z0-9]{12}";
private final WebAppRepository waRepository;
#Autowired
public WebAppService(WebAppRepository waRepository){
this.waRepository= waRepository;
}
#Transactional(rollbackOn = Exception.class)
public ResponseEntity<Status> processScanWebAppRequest(Long id, WebAppScanModel webAppScanModel){
try{
synchronized (this){
Optional<WebApp> webApp=verifyForDuplicates(webAppScanModel);
if(!webApp.isPresent()){
WebApp webApp=new WebApp(webAppScanModel.getUrl())
webApp=waRepository.save(webApp);
processpropertiesOfWebApp(webApp);
return new ResonseEntity<>(HttpStatus.CREATED);
}
return new ResonseEntity<>(HttpStatus.CONFLICT);
}
} catch (NonUniqueResultException ex){
return new ResponseEntity<>(HttpStatus.PRECONDITION_FAILED);
} catch (IncorrectResultSizeDataAccessException ex){
return new ResponseEntity<>(HttpStatus.PRECONDITION_FAILED);
}
}
}
Optional<WebApp> verifyForDuplicates(WebAppScanModel webAppScanModel){
return waRepository.getWebAppByRegex(webAppScanModel.getUrl().replaceAll(UUID_PATTERN_TO,UUID_PATTERN_TO)+"$");
}
And JPA method:
#Query(value="select * from webapp wa where wa.url ~ :url", nativeQuery = true)
Optional<WebApp> getWebAppByRegex(#Param("url") String url);
processpropertiesOfWebApp method is doing further processing for given webapp which at this point should be unique.
Intended behaviour is:
when client post request contains multiple urls like:
https://testdomain.com/user/7e1c44e4-821b-4d05-bdc3-ebd43dfeae5f
https://testdomain.com/user/d398316e-fd60-45a3-b036-6d55049b44d8
https://testdomain.com/user/c604b551-101f-44c4-9eeb-d9adca2b2fe9
Only first one will be stored within database but at this moment this is not what is happening. Select from my database:
select inserted,url from webapp where url ~ 'https://testdomain.com/users/[a-zA-Z0-9]{8}-[a-zA-Z0-9]{4}-[a-zA-Z0-9]{4}-[a-zA-Z0-9]{4}-[a-zA-Z0-9]{12}$';
2019-11-07 08:53:05 | https://testdomain.com/users/d398316e-fd60-45a3-b036-6d55049b44d8
2019-11-07 08:53:05 | https://testdomain.com/users/d398316e-fd60-45a3-b036-6d55049b44d8
2019-11-07 08:53:05 | https://testdomain.com/users/d398316e-fd60-45a3-b036-6d55049b44d8
(3 rows)
I will try to add unique constraint on url column but I can't imagine this will solve the problem while when UUID changes new url will be unique
Could anyone give me a hint what I am doing wrong?
Question is related with the one I asked before but not found proper solution, so I simplified my method but still no success