Should a startup go horizontal from day one?

For the app tier, running two small instances behind a managed load balancer is cheap and buys zero-downtime deploys and failure tolerance, so yes if the app is stateless. For the database, no: a single managed primary with backups and maybe one replica is simpler and fast enough until the numbers prove otherwise.

Is vertical scaling a dead end?

The ceiling is higher than most people think; clouds sell machines with terabytes of RAM. The real risks are the single point of failure and being forced into a horizontal migration under pressure when you finally hit the top. Use vertical scaling to buy time, and keep services stateless so the exit stays open.

Why is horizontal scaling harder for databases than for app servers?

App servers hold no data, so adding one is copying a binary. Database nodes must agree on the data, which means replication for reads and sharding for writes, and each brings new failure modes like lag, failover, and cross-shard queries. That is why the standard advice is to scale the app tier out and the database up first.

Scalingchapter 1 of 4 · 4 lessons

Vertical vs Horizontal Scaling

Scale up (bigger machine) vs scale out (more machines).

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Your booking site ran fine on one server all year. Then a review went up, traffic doubled inside a week, and that machine now sits busy 85 percent of the time. At 95 percent it starts refusing people.

You have two moves, and they cost about the same this month. Buy a bigger machine, which takes an afternoon. Or buy more machines, which takes a quarter.

That fork decides more of your architecture than any other question here, because the second option quietly rewrites the rules for sessions, uploaded files, caches and scheduled jobs. Everything you were keeping on that one machine stops working the day there are two.

Lessons

4 in this chapter

Scaling Up: The Bigger BoxOne machine, more of everything. Underrated until the ceiling, dangerous after it.2 min
Scaling Out: More BoxesAdd servers behind a balancer. Capacity is half the sale; redundancy is the other half.2 min
What Breaks When You Go HorizontalEvery hidden assumption that there is exactly one server becomes a bug in the same week.2 min
Choosing in an InterviewSplit the answer by tier, do the arithmetic out loud, and name the cost you accepted.2 min