Question

I am developing a web-service that will be used by several clients. I am using EclipseLink as JPA provider. When calling a method in the web-service from a client, then I have observed that EclipseLink uses a thread in a thread pool to call the method which means the method can be called simultaneously at the same time from different threads in that thread pool.

In some of the methods I do some critical writes to some tables in the database and I need do implement some kind of locking mechanism. Is it a good practice to synchronize the methods when writing to database or do I need to lock the entire table in MySQL?

Some of the writes looks like this in pseudo code:

   1. insert_into_TableA(getLastInsertedIdInTableA + 1);
   2. insert_into_TableB(getLastInsertedIdInTableB + 1);
   3. int id_A = getLastInsertedIdInTableA + 1;
   4. int id_B = getLastInsertedIdInTableB + 1;
   5. insert_into_TableAB(id_A, id_B);

Database structure below:

TableA          TableAB         TableB
pk tableA_id    pk tableA_id    pk tableB_id
                pk tableB_id

My second question: I am not using autoincrement in my tables, so is it a good practice, when inserting new rows, to get the ID of the last inserted row and then just increment it by 1 to insert the new row? Or can I use some good methods in EclipseLink to do this for me?

Answer 1

What you need is a “transaction.”

Usually, the container will make sure that a new transaction is started when a new request comes in and commits the transaction after the code to process the request completes (unless an exception is thrown).

The problem is getLastInsertedId – this won’t work. Different transactions might see the same value here. The solution is to let the database figure out a new, free ID for you.

Use an “identity column” instead. In MySQL, that would be NOT NULL AUTO_INCREMENT.