Synchronize concurrent batch updates with JPA

Question

This example is modified and simplified, but resembles a real problem I have with legacy code that I am trying to fix.

I have a MSSQL CAR table (where id is the primary key)

id	make	status
1	BMW	WASHED
2	BMW	DIRTY
3	BMW	DIRTY
4	Ford	DIRTY
...	...	...

and a /washNextCars endpoint.

@PatchMapping("/washNextCars")
public ResponseEntity<List<Car>> washNextCars(
      @RequestBody WashCarBody body,
      @RequestParam String status,
      @RequestParam String make,
      @RequestParam Integer limit) {
  ...
}

The following request changes the state of the next 10 cars from DIRTY to WASHED

PATCH /washNextCars?status=DIRTY&make=BMW&limit=10
{
    "status": "WASHED"
}

This is the rough business logic:

  @Transactional
  public List<Car> washNextCars(WashCarDTO dto) {

    // SELECT TOP ? * FROM CAR WHERE make=? AND status='DIRTY' ORDER BY id ASC 
    List<Car> nextCars = carRepository.findNextCars(limit, make, status);

    // do some complex validation on every car if it can be washed, therefore can't do everything in one single UPDATE query

    cars.forEach(car -> {
      car.setStatus("WASHED");
    });
    carRepository.saveAll(nextCars);
  }

This endpoint works fine when called in sequence, but when called in parallel, it tries to perform the update to the same set of cars at the same time, not the next batch of cars.

Questions:

• How can I synchronize the calls until the first one is done?
  • Note this has to be achieved on the database level, because the API is deployed as multiple replicas
• I found the following options:
  • @Transactional(isolation = READ_UNCOMMITED)
    • This would require to update the cars to WASHED before any other statement in the business logic is executed, is that correct?
  • @Lock(PESSIMISTIC_WRITE_LOCK) - Did not provide the desired effect
• What is the right way to achieve this?

Answer 1

Try with UPDLOCK:

SELECT ... FROM tab1 WITH (UPDLOCK) WHERE ...

Something similar to

SELECT FOR UPDATE

in Oracle.