Jae Moon System design

2026.09.24 · 1 min

[SD] Vertical Scaling vs Horizontal Scaling

Notes from Grokking the System Design Interview

Vertical scaling

Adding resources to one machine.

Pros

  • No code changes
  • No distributed problems

Cons

  • Can get expensive
  • Hard limit
  • No redundancy

Horizontal scaling

Adding more servers.

Pros

  • No limit
  • Redundancy
  • Easy to scale

Cons

  • Need to have a load balancer
  • State becomes a problem
  • Data must be copied or split
  • Harder operations (deployments, monitoring, logging)
  • Some work cannot be split

Usual scaling path

  1. One machine
  2. Scale up
  3. Scale the application tier out
    • Make the application servers stateless, put them behind a load balancer, keep sessions in a shared store
  4. Shard the DB
    • Only when write load is too big
    • Cons: queries across shards become hard