A better way to syncronize objects in servlets

Question

Hi,
I am working on a webapp that has a servlet that invoque a instance of a object that accesses our sybase server.
What is the best secure way to syncronize to avoid problems at the database?
I have searched all of these but I am still confused:
Syncronize the servlet
public class LoginHandler extends HttpServlet implements SingleThreadModel
Syncronize the object
private DBManager dba = DBManager.getInstance() ;
private String GetUserCode(String Username, String Password)
syncronized(this) {
DBResults rsResult = dba.ExecQuery(SQLQuery);
Syncronize the object in the class
public class DBManager implements Serializable
private static DBManager _instancie = new DBManager();
public static synchronized DBManager getInstance()
if (_instance == null) _instance = new DBManager();
return _instance;
Syncronize the db connection
try
synchronized (ds)
con = ds.getConnection();
st = con.createStatement();
rs = st.executeQuery(sqlQuery);
if (rs.next()) rsValue = rs.getString(1);
catch (Exception e)
Syncronize method
public static syncronized String getDBValue(String SQL)
Thanks very much,
Lorenzo

Answer 1

I would not synchronize the servlet with SingleThreadModel
I would not have all this code in the servlet at all. (Servlets handle HTTP requests, after all.) Would you want to avoid problems at the database if you didn't have a Web interface? I think so. In that case, take care of the problem in the persistence layer, as close to the database as possible.
You don't want to do anything that will increase your sense of security and kill your performance.
I think there's a difference between synchronization and isolation. Read up on the java.sql.Connection.setTransactionIsolation(int level) method. I believe that's pertinent.
I think you should investigate pessimistic vs optimistic locking, isolation, dirty reads, phantom reads and writes. I'd enlist the help of the database and its admin as much as possible.