[relix]

There are dozens of possible ways to implement this in a distributed, atomic or near-atomic, low-impact way.

* One way is to have a list in Redis, just pop a dyno off it (atomic so each dyno is popped off exactly once), send the request to that dyno, and as soon as it's done, push the dyno back on the queue. 1RTT incoming, and let the dyno push itself back on after it's finished.

* Another way is to use sorted lists in Redis, increment/decrement the score based on the connections you're sending it/returning from it. Get the first dyno in line which will have the lowest score. This is harder but maybe more flexible.

* Presumably they already have a system in place in the router, that caches the dyno's a request to a particular app can be sent too, which includes detecting when a dyno has gone dead. Just use that same system but instead of detecting when it has gone dead, detect if it has more than 1 request waiting for it.

etc...

But in the end, 2-3 extra roundtrips for each inbound request is peanuts, that's the least of the problems with these ideas. That would add maybe 10ms? to each request. It's not like the servers are on the other side of the world. They're in the same datacenter connected by high-throughput cabling.