Help with some downtime

[ArpStorm1]

Hello everyone!
I wanted to ask - is this possible to combine two operations?

For example, let's say we have 2 buckets, Bucket A and Bucket B.

I want to make bucket A as read only, but on the other hand - to redirect all the write requests to bucket B. Then, when a client want to read, it checks if the file exist on bucket B, else - it would return the result from Bucket A.

Is this kind of behavior even possible?

[gaul]

Detecting failures is one hard part about this -- systems often fail by being slow or emit HTTP 501 or similar errors. It is a lot easier to redirect requests. Instead of S3Proxy itself detecting this, could an external process detect failures then tell S3Proxy to dynamically change its configuration?