How to cache render :json
Either action caching or page caching would work fine; page caching would have the benefit of never calling the Rails stack, but it depends on whether you need to control who accesses that Json feed.
I'm a big fan of using page caching if you can get away with it - there are big savings on system resources to be had. :)
EDIT: Page caching example, in case there was any confusion:
class SomeController < ApplicationController caches_page :index def index render :json => my_array.to_json endendUnless I've misunderstood something, that should be all you need to do.
Same considerations should apply to JSON as any other output. If you need to validate access to the data for the user, then action caching is the way to go, otherwise page caching should be fine.
If the data changes due to logic in your app, then both forms of caching are problematic and you are better off using something else.