分离的实体已传递以保留

Question

CacheId aci = new CacheId();
aci.setId(cacheObject.getId());
aci.setSequence(cacheObject.getSequence());
Cache persisted = em.find(Cache.class, aci);
if (persisted == null){
    System.out.println("========== PERSISTING");
    em.persist(cacheObject);
}else{
    System.out.println("========== MERGING");
    em.merge(cacheObject);
}

Mean while im running my project another instance deletes the entry with current CacheId. Now what i need to do is set the new value to the database with same cache id. But when i do a em.find() this does not return null. (but in the database entry is already deleted. So this will end up calling merge. And merge failed with trying to update 1 but updated 0...

But if i stop checking the search and call persist always then i get "Detached entity passed to persist ". How can i work around this issue ?

Answer 1

You have a problem of concurrent access and the answer is locking . If you use optimistic locking in your application, just catch the Exception and retry (a limited number or times).

You can also use the pessimistic locking pattern by using :

Cache persisted = em.find(Cache.class, aci, LockModeType.PESSIMISTIC_WRITE);

(or LockModeType.PESSIMISTIC_READ ). The lock is obtained immediately from database, that is you will know immediately if the data has been removed. And once the lock is obtained, no other transaction will be allowed to removed it.

The only remaining race condition would be if two transaction try to create the same data.

But BEWARE : the application should commit as soon as possible to release the lock to avoid potential deadlocks or unnecessary blocking of other transactions.