Hello, I asked the same question to myself in the past years.
I think that the question boils down to: How can I achieve unlimited database scalability? Is it possible to have linear scalability (i.e. throughput increases proportionally to the number of nodes)? The answer is "sharding". It can be a custom solution or a database that supports it automatically. In this way you can actually split data across multiple nodes and the client contacts only the relevant servers for that data (based on a shard key). See also https://kubernetes-rails.com/#conclusion about database. Take a look at how Cassandra, MongoDB, CouchDB and Redis Cluster work for example: however there are huge limitations / drawbacks that come along with their unlimited-scalability strategies. For hot standbys, those are only useful if you have a relatively small number of writes compared to reads. With that slave nodes you only scale the *read* throughput. Hope it helps, Marco Colli
