Question

I have this scenario ,on a airline website (using Java) two separate customers send two requests at same time to book a same seat in same airline
from New York to Chicago. I am using the oracle database and isolation level is read committed.My question here is that does oracle database provide
any solution to deal with this kind of concurrent scenario? what I know is when first transaction DML statement is fired it will get a lock on affected
rows and will release when transaction completes i.e on issuing rollback or commit.But as soon as commit is done and second request will proceed as soon as
first is completed and will override the first one.So it does not help?

Yes in Java I can deal with making my db class as singleton and using synchronized keyword on method which is doing update. But want to know is there
anyway we can this kind of issue at database level itself?Probably isolation level as serializable can help. But not sure?

Answer 1

To handle concurrency in a web site a common practice it to have a column on each record that allows you to check it has not been updated since you got it. Either last update date or a sequential version number (auto incremented by a trigger).

Typically you will read the data (plus the concurrency column)

SELECT seat,etc,version_no
FROM t1
WHERE column = a_value

Then when the user eventually gets round to booking the seat the update will work unless there has been an update.

(the version number or update date will change after every update)

BEGIN
    UPDATE t1
    SET seatTaken = true
    WHERE seatid = .....
    AND version_no = p_version
    RETURNING version_no INTO p_version;
EXCEPTION WHEN NOT_FOUND THEN
    --Generate a custom exception 
    --concurrency viloation the record has been updated already
END;

the trigger to auto update the version number would look a little like this

CREATE OR REPLACE TRIGGER t1_version
AFTER INSERT OR UPDATE ON t1
FOR EACH ROW
BEGIN
    IF :new.version_no IS NULL THEN
       :new.version_no  := 0;
    ELSE
       :new.version_no  := :old.version_no  + 1;
    END IF;
END;