Refactoring Database Access in Marauroa

At the moment database access in marauroa is designed for a single thread application. While this does work most of the time in turn based games (yes, Stendhal is turned based internally), it prevents doing non time critical stuff in another thread. In Stendhal we have the problem that any update or delete operation to the gameEvents table that takes more than a couple of seconds kills the server.

Requirements

Hard Requirements

• multi thread support
• parallel transactions in different threads

Would be nice

• a package of small classes instead of huge JDBCDatabase / StendhalPlayerDatabase
• an small abstraction layer to support different databases beside mysql (small as in "not hibernate")
• no extra dependencies on new libraries
• easier way to include variables into sql statements

Design

Concept FAQ

Where did IDatabase / JDBCDatabase go?

It was replaced by smaller classes specialized on one task each. They can be found in the package marauroa.server.game.db ending with "DAO".

What are those new ...DAO classes? How do they work?

They are "data access object". The basic idea is and has been from the start that the database related code is not in the original classes but at some central point outside the game logic. This used to be JDBCDatabase but one single class for all database operations is very unhandy. So JDBCDatabase has been splitted into a number of small classes focused on one area each: AccountDAO, CharacterDAO, GameEventDAO...

These classes replace the old IDatabase / JDBCDatabase and do the database stuff. All of their methods have two signatures: One with gets an DBTransaction object as first parameter and one without. This is for your convenience: In most cases it those functions are not part of a larger context so you do not have to care about transactions at all because the DAO-classes do the transaction handling on their own. There are, however, a small number of cases in which you want to do multiple calls to DAOs in one single transactions. In this case you get a DBTransaction from the TransactionPool and provide it as first parameter to DAO-methods. After you are done you must either commit or rollback your changes with the appropriate methods in the class TransactionPool.

I have extended the JDBCDatabase class. How does this work with DAOs?

DAO classes should never be instantiated directly. Instead you should (and marauroa does) use the DAORegistry. This allows you to write a subclass of a DAO provided by marauroa and register it instead. If you are familiar with Spring, this is a similar concept. But without all the bulk of xml configuration files, parameter injection and interfaces with only one single implementation.

Imagine you want to subclass the CharacterDAO with your class SomeGameCharaterDAO:

    public class SomeGameCharacterDAO extends CharacterDAO {
    ...

You simply register it as

    DAORegistry.get().register(CharacterDAO.class, new SomeGameCharaterDAO());

Note: In the register call the first parameter is the parent clase you want to replace.

What are those database adapters for?

The are a thin layer of database abstraction. This allows us to not only support MySQL but also database systems with don't require an external server.

Why is there a method getLastInsertId?

It is a replacement for "select LAST_INSERT_ID() as inserted_id" which is a MySQL "feature". By using getLastInsertId() instead of that SQL statement, your code will work with other database systems as well (unless you use other system specific functions).

Porting FAQ

Does this mean I have to adjust my game?

That depends. If you did not do any low level database stuff, there should be no changes required. As a rule of thumb: If your code compiles with the new version of Marauroa, you are fine.

What happened to IDatabase/JDBCDatabase?

It was replaced by smaller DAO classes. See above for how to use them.

What happened to Transaction, JDBCTransaction, Accessor, JDBCAccess?

They where replaced with DBTransaction. You can get a DBTransaction from the TransactionPool. Make sure that you commit or rollback it after use.

What happend to JDBCSQLHelper and StringChecker?

They have been moved to marauroa.server.db

I provided my own subclass of IDatabase/JDBCDatabase

Please have a look at the ...DAO classes in the marauroa.server.game.db package and split your sub class accordingly. After that is done you need to register them in the DAORegistry with the class of the original DAO and an instance of your class.