# AWS Auto Scaling.

**AWS Auto Scaling** is a service that **automatically increases or decreases EC2 instances** based on traffic and load.  
It makes your application **scalable, cost-efficient, and highly available**.

Think of Auto Scaling like:

> Adding or removing servers automatically when users increase or decrease.

![](https://cdn.hashnode.com/res/hashnode/image/upload/v1768280108908/20d80751-cf92-4d95-9340-f09235f51ae4.png align="center")

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## Why Auto Scaling is used

Auto Scaling is used to:

* Handle traffic spikes
    
* Avoid server overload
    
* Save cost
    
* Improve availability
    
* Run applications without manual intervention
    

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## Where Auto Scaling is used

Auto Scaling is used in:

* Web applications
    
* E-commerce websites
    
* APIs
    
* SaaS platforms
    
* Production systems
    
* Microservices
    
* Cloud applications
    

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## When to use Auto Scaling

Use Auto Scaling when:

* Traffic is unpredictable
    
* You want zero downtime
    
* You want automatic scaling
    
* You want high availability
    
* You want to pay only for what you use
    

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## Core Components of Auto Scaling

### 1\. Launch Template

This defines:

* AMI
    
* Instance type
    
* Key pair
    
* Security groups
    
* User data
    

It tells AWS **how to create new EC2 instances**.

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### 2\. Auto Scaling Group (ASG)

This is a **group of EC2 instances** that:

* Increases
    
* Decreases
    
* Replaces unhealthy instances
    

You set:

* Minimum
    
* Desired
    
* Maximum number of instances
    

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### 3\. Scaling Policies

These decide:

* When to add or remove EC2
    

Types:

* Target tracking
    
* Step scaling
    
* Simple scaling
    

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### 4\. CloudWatch Metrics

ASG uses:

* CPU utilization
    
* Network
    
* Requests count  
    to decide scaling.
    

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## How Auto Scaling works

Traffic increases → CPU goes high  
CloudWatch sends alarm  
ASG launches new EC2  
ELB starts sending traffic to new EC2

Traffic decreases → ASG terminates extra EC2

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## Auto Scaling with Load Balancer

ASG is mostly used with:

* Application Load Balancer
    

So traffic is evenly distributed.

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## Health Checks

Auto Scaling checks:

* EC2 status
    
* ELB health
    

If an instance fails:

* ASG deletes it
    
* New one is launched automatically
    

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## Availability Zones

ASG can launch EC2 in:

* Multiple AZs
    

So if one AZ fails, others still run.

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## Cost benefit

You pay only for:

* Running instances
    

When traffic drops, instances are removed.

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## Real world example

E-commerce site  
Morning: 2 servers  
Sale time: 20 servers  
Night: back to 2

All automatic.

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## DevOps usage

Auto Scaling is used for:

* High availability
    
* CI/CD environments
    
* Blue-green deployment
    
* Disaster recovery
    
* Cost optimization
    

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## Auto Scaling vs Load Balancer

| Auto Scaling | Load Balancer |
| --- | --- |
| Controls number of servers | Distributes traffic |
| Launches & terminates EC2 | Routes requests |

Both are used together.

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## Interview points

* Works with CloudWatch
    
* Uses Launch Template
    
* Supports ELB
    
* Automatic healing
    
* Multi AZ support