01. Scalability & High Availability & Elasticity
🧩 A. Scalability
Scalability is the ability of a system to handle increased load by adjusting resources.
🔸 Two Types of Scalability
| Type | Description | Example |
|---|---|---|
| Vertical Scalability | Increase the capacity of an existing instance (scale up/down) | Upgrading from t2.micro → t2.large |
| Horizontal Scalability | Increase the number of running instances (scale out/in) | Multiple EC2 instances behind a Load Balancer |
🏗️ A1. Vertical Scalability
- Also called scale up (or scale down when reducing resources).
- Involves increasing CPU, memory, or I/O of a single instance.
- Best suited for non-distributed systems (for example, traditional databases).
💡 Example
- AWS example
- Change instance type from t2.micro → t2.large.
⚠️ Limitation
- Limited by physical hardware constraints.
- There is always a maximum size an instance can scale to.
🌐 A2. Horizontal Scalability
- Also called scale out (add instances) and scale in (remove instances).
- Adds more machines instead of upgrading one.
- Requires a distributed application architecture.
💡 Example
- AWS example
- Multiple EC2 instances behind an Elastic Load Balancer (ELB).
- Managed automatically using an Auto Scaling Group (ASG).
🧭 B. High Availability
- Ensures applications remain available even if one AZ fails.
- Achieved by deploying resources across multiple Availability Zones.
- Eliminates single points of failure and improves fault tolerance.
💡 Example
- Two call centers:
- New York and San Francisco
- If New York experiences a power outage, San Francisco continues handling traffic.
- The application remains available.
⚙️ High Availability in AWS
AWS typically achieves High Availability using:
- Auto Scaling Group (Multi-AZ)
Automatically launches instances across multiple AZs. - Elastic Load Balancer (Multi-AZ)
Distributes traffic only to healthy instances across AZs.
How they work together:
- ELB redirects traffic away from unhealthy instances or AZs.
- ASG replaces failed instances automatically in other AZs.
🧮 C. Elasticity
- Elasticity extends scalability by automatically adjusting capacity based on demand.
- No manual intervention is required.
- Optimizes performance and cost efficiency.
💡 Example
- Traffic spikes at 8 PM, drops after midnight:
- ASG scales out during peak hours.
- ASG scales in during low traffic.
- You pay only for consumed resources.
⚙️ Elasticity in AWS
Elasticity is implemented using:
- Auto Scaling Groups (ASG)
Scale EC2 instances based on metrics like CPU or request count. - Elastic Load Balancer (ELB)
Automatically routes traffic as instances are added or removed.
🧠 Scalability vs Elasticity
| Concept | Description | Automation | Example |
|---|---|---|---|
| Scalability | Ability to handle increased load | Manual or automatic | Add more EC2 instances |
| Elasticity | Automatic scaling based on demand | Fully automatic | ASG adds/removes EC2 instances dynamically |
✅ Summary
- High Availability keeps applications running during failures.
- Elasticity ensures efficiency by adjusting capacity automatically.
- Together, they enable resilient, cost-optimized, cloud-native systems.
📊 EC2 Scalability Summary
| Type | Action | Example | AWS Service Used |
|---|---|---|---|
| Vertical Scaling | Scale Up / Down | t2.micro → t2.large | Change EC2 Instance Type |
| Horizontal Scaling | Scale Out / In | 1 → many EC2 instances | ASG + ELB |
| High Availability | Multi-AZ Deployment | Multiple AZs | Multi-AZ ASG + ELB |
🧠 Cloud Concepts Comparison
| Concept | Definition | Key Idea |
|---|---|---|
| Scalability | System handles more load | Grow capacity |
| Elasticity | System auto-scales with demand | Pay-per-use efficiency |
| Agility | Rapid deployment of IT resources | Faster innovation |