How to keep always Last 30 days data to sync on my Device?

Hi There,
I have an requirement. Im synching data by using Couchbase lite. After few days. my iPad getting full with lots of Documents. so i want to do a trick like change last 30 days documents to other channel name and i want to delete those other channel documents from my local db but not on server.

For this, how to i delete document only from local but not on server.

It is the similar question in google group ,FYI.

@atom_yang Thanks, it seems like my thought. i got one point on purge. i will read about it.