The Factory Pattern

The Factory pattern provides a standard way for a class to create objects of other classes. The typical use for this is when you have a function that should return objects of different classes, depending on its input parameters. One of the major challenges in migrating services to a different database is finding all the places where the old wrapper object is used and supplying the new one. For example, say you have a reporting database that is backed against an Oracle database that you access exclusively through a class called DB_Oracle_Reporting: class DB_Oracle_Reporting extends DB_Oracle { /* ... */} and because you had foresight DB_Oracle uses our standard database API. class DB_Oracle implements DB_Connection { /* ... */ } Scattered throughout the application code, whenever access to the reporting database is required, you have wrapper instantiations like this: $dbh = new DB_Oracle_Reporting; If you want to cut the database over to use the new wrapper DB_Mysql_Reporting, you need to track down every place where you use the old wrapper and change it to this: $dbh = new DB_Mysql_Reporting; A more flexible approach is to create all your database objects with a single factory. Such a factory would look like this: function DB_Connection_Factory($key) { switch($key) { case “Test”: return new DB_Mysql_Test; case “Prod”: return new DB_Mysql_Prod; case “Weblog”: return new DB_Pgsql_Weblog; case “Reporting”: return new DB_Oracle_Reporting; default: return false; } } Instead of instantiating objects by using new, you can use the following to instantiate objects: $dbh = DB_Connection_factory(“Reporting”); Now to globally change the implementation of connections using the reporting interface, you only need to change the factory.