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There are some things that are hidden, but they're the exception not the rule; but by default most things need to be global. You need to know how much capital the whole bank has at risk if <XYZ company> were to go bankrupt, or interest rates moved by x%, for example.

Well, I say "more or less" :). But the fact that there is a single global database doesn't mean that you can read every key or value (but generally, yes, you can _read_ everything). I mention in passing prolog-style permission systems for evaluating perms.

But anyway, specific trades are rarely private to one part of the bank for many reasons. For example regulatory: these days you have to notify the regulator about every trade.

What I imply from the description is that, the default ring contains some shared global public data (e.g. a cache of bloomberg informations), and each individual team will have their own rings. Afterall there's no that many you can fit into 16mb

No, no, there is a single world ring. the 16mb limit is for values