Scalable = Horizontal

There's two ways of scaling; vertical and horizontal, but there's only one which is really scalable.

Vertical scaling essentially means bigger nodes. If you've got 8GB RAM, go to 16GB. If you've 2 cores, go to 4.. and so on.

Horizontal scaling means adding more nodes. One node to two nodes, to three and so on.

As a rule, horizontal scaling is good. Theoretically there's no limit to the number of nodes you can have.

As a rule, vertical scaling is bad. You quickly run into constraints over the number of cores or RAM you can support. And for many of todays problems this just doesn't work. Solutions need to be both scalable at the internet scale and available as in 24x7. Relying on large single nodes in such situations is not ideal. (and those supercomputers with 250,000+ processors are really horizontal solutions as well).

The problem is, horizontal scaling isn't trivial. The culprits here are data and networking (plumbing really). State and caches need to be distributed and available to all. Databases need copies across nodes and need to be synchronised. Sharding usually becomes necessary (or you just end up with many very large nodes). And so on... Best bet is to avoid state as much as possible. But once you've cracked it you can build much larger solutions more efficiently (commodity hardware, virtualisation, containers etc.) and flex more rapidly than in the vertical world.

I could go on about how historically the big players love the vertical-scaling thing (think Oracle and IBM trying to sell you those huge servers and SQL databases solutions with the $$$ price-tags)... The world found NoSQL solutions which take a fundamentally different approach by accepting that consistency in many cases really isn't as important as we once thought - and many of these are open-source...

Whatever, there's only one way to scale... Horizontal!