Do grails domain classes have to be tied to a database?

A couple of ways to do this. First, you can declare your properties that map to file system data as transient, and go to file system when getters / setters are called (you have to override them). You can also load them using onLoad if you need them to be in memory always.

Second - bernate handles the persistence. Bernate allows for you to define your own user type, which can handle the persistence whichever way you want. This way it might happen for you more transparently (though you'd have to make sure you understand hibernate fairly well, to make sure there aren't any side effects, I am not sure).

i-proving.ca/space/Technologies/bernate/....

There is no built-in way to map domain classes to file system objects as you've described, but equally there is no requirement that your domain classes map to a relational database. The subject of how to create a Grails app that doesn't use a relational database is addressed here and here (and possibly elsewhere).

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