Scalability & Load Balancing

Scalability is the ability of a system to handle increasing load by adding resources. Load balancing is the technique of distributing incoming requests across multiple servers. Together, they form the foundation of any system that needs to grow beyond a single machine.

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Vertical vs Horizontal Scaling

There are two fundamental approaches to scaling:

Vertical Scaling                    Horizontal Scaling
(Scale Up)                          (Scale Out)

┌───────────────┐                   ┌──────┐ ┌──────┐ ┌──────┐
│               │                   │Server│ │Server│ │Server│
│  BIG SERVER   │                   │  1   │ │  2   │ │  3   │
│  (more CPU,   │                   └──────┘ └──────┘ └──────┘
│   more RAM,   │                        ▲       ▲       ▲
│   more disk)  │                        └───────┼───────┘
│               │                                │
└───────────────┘                        ┌──────────────┐
                                         │Load Balancer │
                                         └──────────────┘

Comparison

Aspect	Vertical Scaling	Horizontal Scaling
Approach	Bigger machine	More machines
Complexity	Low	Higher (distributed state)
Cost curve	Exponential	Linear
Downtime risk	Single point of failure	Redundancy built in
Upper limit	Hardware limits	Practically unlimited
State management	Simple (single node)	Requires external state