AWS # Horizontal vs Vertical Scaling

 Horizontal vs Vertical Scaling, with examples and diagrams. Perfect for senior-level answers.

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✅ Horizontal vs Vertical Scaling (Simple Explanation)

1️⃣ Vertical Scaling (Scale UP)

You increase the power of an existing machine.

✔ Means:

• Increase CPU

• Increase RAM

• Increase Disk

• Upgrade to bigger EC2 instance (ex: t2.medium → t2.large)

✔ When to use:

• App is not distributed

• Database scaling (RDS, MySQL, PostgreSQL)

• When horizontal scaling is complex

✔ Pros:

• Simple to do

• No code change

• Lower operational overhead

❌ Cons:

• You hit a hardware limit eventually

• Downtime during upgrade

• Single point of failure

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2️⃣ Horizontal Scaling (Scale OUT)

You add more machines and distribute traffic.

✔ Means:

• Add more EC2 instances

• Add more containers (ECS/K8s Pods)

• Add more servers behind a Load Balancer

✔ When to use:

• Web apps

• Microservices

• High traffic systems

• Need high availability

✔ Pros:

• No hardware limit

• Better fault tolerance

• Supports millions of users

• Zero downtime

❌ Cons:

• More complex

• Requires stateless design

• Needs load balancer

• Needs distributed caching/session handling

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📌 Simple Example

Vertical Scaling:

Upgrade 1 server from 2 GB RAM → 16 GB RAM.

Horizontal Scaling:

Add 10 servers each with 2 GB RAM.

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🏗 Text Diagram (Easy for interviews)**

Vertical Scaling (UP)

   ┌─────────┐
   │ 2 CPU   │   → Upgrade to →   ┌─────────┐
   │ 4 GB RAM│                     │ 8 CPU   │
   └─────────┘                     │ 32 GB   │
                                   └─────────┘

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Horizontal Scaling (OUT)

        ┌─────────┐
        │ Server1 │
        └─────────┘
     +  ┌─────────┐
        │ Server2 │
        └─────────┘
     +  ┌─────────┐
        │ Server3 │
        └─────────┘

(All servers behind a Load Balancer)

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🔥 One-Line Senior Answer

“Vertical scaling means upgrading the capacity of a single machine, while horizontal scaling means adding more machines and load balancing traffic across them.