One of my clients insists that we missed a requisite for the project(a little social network, using PHP + MySql), is that all queries are made from XML / JSON static files (using AJAX) and not directly from the database.

Edit: The main reason, stated by client, is a way to economize bandwith. Even the file loading, has to be using AJAX, to avoid server side processing.

• We will still use a database to store and insert data.
• These XML / JSON files will be (re) generated whenever any data is changed by CMS or users.

As the project was developed without this "technique", we'll have a lot of work ahead, so I would like to avoid that. I'm asking if it's convenient to use a XML generated based system, because I think a database is already "optimized" and secure to deal with a lot of data traffic.

Other issue I'm afraid of, is a chance of conflict when a user is trying to read a XML/JSON which is being just generated.