2.2.1.1. Horizontal vs. Vertical Scaling

💡 First Principle: Scaling adjusts compute capacity to meet changing demand, ensuring performance, availability, and cost optimization. Horizontal adds instances; vertical increases size.

Scaling is the ability of a system to grow or shrink its capacity to handle varying levels of demand. There are two primary types of scaling:

• Horizontal Scaling (Scale Out): Distributes workload across multiple, often smaller, instances or nodes. Involves adding more resources (e.g., more EC2 instances) to share the load. This offers superior elasticity, fault tolerance, and virtually limitless scalability. Ideal for stateless, distributed workloads like web servers or microservices.
• Vertical Scaling (Scale Up): Enhances the resources (CPU, RAM, storage) of a single, powerful instance. Involves increasing the size or capacity of an existing resource (e.g., upgrading an EC2 instance to a larger type). It's simpler but has physical limits and can introduce a single point of failure. Best for stateful applications or databases needing strict data consistency.

Key Differences:

• "Horizontal Scaling": Adds more units, elastic, fault-tolerant, limitless.
• "Vertical Scaling": Increases single unit's power, finite limits, single point of failure.

Scenario: During peak e-commerce traffic, an application adds web servers via EC2 Auto Scaling (horizontal scaling), while a database needing more processing power upgrades to a larger instance (vertical scaling).

Visual: Horizontal vs. Vertical Scaling

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⚠️ Common Pitfall: Attempting to vertically scale a stateless application beyond a certain point. While simple, it eventually hits physical limits and creates a single point of failure. Horizontal scaling is generally preferred for web applications and microservices.

Key Trade-Offs:

• Limitless Scalability/Fault Tolerance (Horizontal) vs. Simplicity/Stateful Support (Vertical): Horizontal scaling is more complex to implement but offers superior benefits. Vertical scaling is simpler but has inherent limitations.

Reflection Question: When would horizontal scaling be preferred over vertical scaling for a stateless web application to handle fluctuating traffic, and what are the key benefits and limitations of each approach for scalability and fault tolerance?