[AWS Autoscaler] Spread across availability zones #2177
Description
Feature enhancement: be able to specify multiple availability zones to launch worker nodes into. For spot instances, this would reduce the risk of all of your workers being terminated, and could also enable greater peak capacity. This is particularly valuable in regions such as us-east-1 that have seven availability regions.
It's not clear what the best way to do this is. Right now, we specify SubnetId (with it being filled in by aws/config.py:_configure_subnet) to peg it to a particular availability zone. Spot fleet requests let you specify multiple SubnetId's, but RunInstance (which we currently use) does not. A reasonable policy might be to launch workers round-robin between availability zones. (This has the disadvantage of not favoring regions with lower prices, but now that Amazon makes spot prices vary only gradually over time, this doesn't seem like a big loss.)
Note there is a downside in terms of increased latency to having nodes in different availability zones, so there are probably better allocation strategies than round-robin.