如何保存表顺序并防止并发问题

Question

I have the following problem I have to save the order of some elements in a table. This is done by a order number. So the first element has ordering 1 and the tenth the ordering 10 etc. Also I have a brand value saved, which means the ordering is only inside all rows of one brand. My configuration is Spring 3.1 with Hibernate 4.1.6 and Spring Data, I also use the JPA interface, underlying is a Mysql Database.

I have right now a save method which gets the maximum value by a namedquery. Then increments the value by one. And now sets the value with the object setter. Then saves it.

    if(tag.getOrdering()==0) {
        int newOrderingNumber = tagRepository.getMaximumOrderingNumber();
        newOrderingNumber++;
        if(LOGGER.isDebugEnabled()) {
            LOGGER.debug("Next Ordering Number for Tag is " + newOrderingNumber);
        }
        tag.setOrdering(newOrderingNumber);
    }

Here I have a problem what would happen if two threads are doing this at the same time. They would both save the same value. But one would have to increment twice. My question is how could I prevent this. First I tried to do something with the isolation level of the spring transaction, but JPA doenst know transaction levels and the ways I found how I could extend the jpadialect like here ( http://amitstechblog.wordpress.com/2011/05/31/supporting-custom-isolation-levels-with-jpa/ ) But that didnt work, my Connection was always readonly the readOnly=false flag was not set correctly and I didnt find out what was wrong. Also I think that wouldn`t solve my problem.

Then I tried to work with a @Version Property, but this doesnt work because I do not work on one object instance. The two threads are working on two different rows, so the versioning doesnt solve the problem.

So now I see four different approaches.

First: I could use a synchronize lock object inside my code to let one thread wait for the other to free the lock on the object

Second: I could acquire a full table lock, which I dont know how to do with hibernate and it also seems oversized to block the complete table.

Third: When I wouldnt have the brand column inside which the ordering could be the same, I could add a Unique constraint which would throw an Exception which I can use to increment it once again, til it works.

Fourth: Right now I found an additional way, which could work. I dont save an ordering number. Instead I add a column which contains the id of the next element in order. This would mean that there is a Join in every request with the order, but I know that this table has not many rows, so the performance doesnt really matter. Also the updating when the order changes would be much easier.

So what would be the right way to solve this problem. I also have to solve the problem when I have to update all objects when the ordering is changed.

Answer 1

使用mysql触发器应该可以使您进行直接检查其他SO问题 。

Answer 2

Have you tried @OrderColumn ? It is business of JPA provider how to deal with concurrency in case of ordered list (I am thinking by some constraints). Here example of ordered tree structure.

Also you may try to use JPA locks . See for PESSIMISTIC .

Answer 3

An additional solution for my problem would be to make the order column and the brand column both UNIQUE ( ADD CONSTRAINT UNIQUE(order,brand) ) . Then there could be one order number only once inside a specific brand. Then I would have to react to a ConstraintViolationException, and redo the insert with new values. But I thank its not very nice, because you dont know exactly if the constraintViolationException is really due to the Unique constraint or some other Constraint Violation