Avoiding concurrency errors when updating a database through AJAX

Question

What are some strategies for avoiding concurrency errors when updating a database through AJAX. That is, in cases where a given user could attempt to modify the same row in the database with simultaneous requests (say, that he updates one column with information with an AJAX call, then immediately submits a form that updates the same row), what are some good ways yo prevent the two updates from colliding? On the JavaScript side one might make the AJAX call synchronous, but I question whether that is the best way to do it. Has anyone else confronted this?

Answer 1

Well, since no one seems to have any ideas so far, I'll throw in my two cents worth.
I'm not too familiar with AJAX so my advice may be limited.
I suggest you do a google search on Optimistic concurrency database to get some ideas on how to handle it.
If your update/insert/delete query is thread safe, then even if the same user double clicks the button fast enough to immediately have another update to the same record, then the first update will complete before the second one is carried out. Therefore no harm is done since he is just updating the same record. Since a typical update/insert/delete takes only a few milliseconds (querying may take a lot more time), its not likely he can click the button that fast. (I assume your using connection pooling for speed).
The trouble comes up when two people call up data from the same record in different browsers. The first one updates/inserts/deletes the record. Now the other user is looking at old data. When he updates/inserts/deletes, there is a problem. One way to handle this is to keep a copy of all the fields from that record in each user's session scope when they first fetch it from the database (example: oldName). Then when you go to update some time later, to this:
sql= update person set name=newValue where personID=3344 and name=oldName
Note oldName is from session scope.
If name in the database has changed, the update will not occur and will return 0 records updated. If it didn't change, it will update.
Note your update function should not throw an exception if it cant update, it should return false. Return true if it worked.
Example: public boolean updateName(String name)
Similiarly, your insert should not throw an exception if the record already exists, it should return false meaning it cant insert.
Exaple: public bolean insertName(String name). Same thing with delete function.
If a record cant be updated due to someone else updating the record while the user was looking at data, return a message that it wasn't updated due to that problem. Then show the user the new value in the database ask him if he wants to overwrite the new change.
Note even if you show 100 records on the JSP, only update the ones that have changed and not all 100. This way, its not likely he has updated the same records someone else has altered. Less likely a colision compared to updating all 100.
If your updating/inserting/deleting more than 1 record (say, 3), then you should look into setting up a transaction in addition to all the above and rolling back all of them if one fails.
Reading the section on databases from http://www.javapractices.com/home/HomeAction.do may also help.