01 — Scalability Patterns

#What is Scalability?

Scalability is the system's ability to handle increased load without degrading performance. There are two primary scaling strategies.

#Vertical Scaling (Scale Up)

Add more resources to a single machine — more CPU, RAM, faster SSDs.

Pros: Simple, no code changes, no distributed system complexity.

Cons: Hard ceiling (biggest machine money can buy), single point of failure, expensive.

#Horizontal Scaling (Scale Out)

Add more machines and distribute load across them.

Pros: Theoretically unlimited, fault tolerant (no single point of failure), cost-effective at scale.

Cons: Requires application to be stateless, adds operational complexity, introduces distributed system problems (consistency, coordination).

#Stateless vs Stateful Services

For horizontal scaling to work, services must be stateless — any instance must be able to handle any request. Session state moves to external storage (Redis, database).

Request → Load Balancer → [App Server 1]
                       → [App Server 2]  ← All read/write to shared Redis
                       → [App Server 3]

#The Twelve-Factor App

The Twelve-Factor App methodology codifies stateless, horizontally-scalable service design. Factor VI ("Processes") directly addresses this: "Execute the app as one or more stateless processes."