[notnullorvoid]

Scaling vertically will work well in scenarios where you have consistently flat load. Horizontal scaling is good for when you have predictable valleys or spikes in load.

In my experience if you want to be cost effective you need both. A decent amount of vertical scaling to have headroom for baseline and some amount of unpredictable spikes, horizontal scaling for the valleys of traffic that match your primary markets day/night cycle.

[thelastparadise]

For bursty load why not restart a slave node on a bigger vm, promote said slave to master?

Repeat with bigger and bigger nides as needed. To scale down, do the inverse.

[hot_gril]

Wouldn't there be a significant hit to availability or latency during the master promotion?