MemberSeptember 19, 2018 at 10:52 pm
Try this -
Account  accts = [SELECT Id FROM Account LIMIT 2 FOR UPDATE];
While the records are locked by a client, the locking client can modify their field values in the database in the same transaction. Other clients have to wait until the transaction completes and the records are no longer locked before being able to update the same records. Other clients can still query the same records while they’re locked.
If you attempt to lock a record currently locked by another client, your process waits for the lock to be released before acquiring a new lock. If the lock isn’t released within 10 seconds, you will get a QueryException. Similarly, if you attempt to update a record currently locked by another client and the lock isn’t released within 10 seconds, you will get a DmlException.
If a client attempts to modify a locked record, the update operation might succeed if the lock gets released within a short amount of time after the update call was made. In this case, it is possible that the updates will overwrite those made by the locking client if the second client obtained an old copy of the record. To prevent this from happening, the second client must lock the record first. The locking process returns a fresh copy of the record from the database through the SELECT statement. The second client can use this copy to make new updates.
When you perform a DML operation on one record, related records are locked in addition to the record in question.
Hope this helps.