Horizontal vs Vertical Scaling

Compare horizontal and vertical scaling and their appropriate use cases.

Vertical Scaling (Scale Up)

Upgrade hardware on a single machine (CPU, memory, storage).

Pros: Simple, no code changes required.

Cons: Physical limits, rapidly increasing costs, single point of failure.

Horizontal Scaling (Scale Out)

Add more machines of the same spec; traffic is distributed by a Load Balancer.

Pros: Theoretically unlimited, high availability (HA), dynamically scale with load.

Cons: Applications must be stateless; session and cache need centralized management.

Stateless Design

Horizontal scaling requires stateless app servers: store sessions in Redis, files in S3, so any server can handle any request equally.

Decision Criteria

• Small application, short-term: vertical scaling.
• High availability, long-term growth: horizontal scaling.
• After hitting vertical DB limits: consider Read Replicas or Sharding.