If databases are in play, instead of trying to understand the business logic, have a go at understanding the database structure first. What are your tables, ho

If databases are in play, instead of trying to understand the business logic, have a go at understanding the database structure first. What are your tables, how they are related etc. Then look at the modules that interact with the data layer directly and move up. This is a language agnostic approach though, but it has worked out well for me.