Re: Merge a sharded master into a single read-only slave

> implement map-reduce at the application level. 

That's the conclusion I would expect. It's the price you pay for sharding, it's part of the deal.  

But it's also the benefit you get from sharding.  Once your read traffic grows to the point that it's too much for a single host, you're going to have to re-shard it all again *anyway*.  The whole point of sharding is that it allows you to grow outside the capacities of a single host.