1. A method, comprising: maintaining a database having one or more tables; maintaining database schema for the database, the database schema describing the one or more tables in the database and columns in the one or more tables; maintaining a first version of an application having application metadata that includes information about one or more classes of the first version of the application; generating application schema for the first version of the application, the application schema comprising a mapping of the one or more classes of the first version of the application to a respective table in the database; executing the first version of the application; storing an application data object in the database in accordance with the application schema; and while the first version of the application is executing: receiving a request to update the first version of the application to a different version of the application; identifying application metadata for the different version of the application; comparing the application metadata for the different version of the application with the database schema for the database; and updating the database schema for the database based on the comparison.