Amazon EC2 (Elastic Compute Cloud)¶

1. What is EC2?¶

Amazon EC2 provides on-demand virtual servers in the cloud with configurable CPU, memory, storage, and networking. You pay only for what you run.

2. Hierarchy¶

Instance Category
   └── Instance Family
         └── Instance Type (Generation + Processor + Capabilities)
               └── Instance Size

Example mapping:

Compute Optimized
   └── c (family)
         └── c7g (7th gen, Graviton)
               └── c7g.xlarge

3. Instance Categories¶

4. Instance Families (Letters)¶

5. Instance Type Naming Convention ⭐¶

<family><generation><processor>apabilities>.<size>

Full example breakdown — c7gn.2xlarge:

Processor type letters:

Capability letters (before the dot):

6. Instance Sizes¶

Sizes scale within the same family — not fixed across all families.

Rule: Each step up typically doubles vCPU and RAM within the same family.

7. T-Series (Burstable) — CPU Credit System ⭐¶

T-series instances (t3, t4g etc.) use a credit-based CPU model:

Instance idle / below baseline CPU usage  →  earns CPU credits
Instance bursts above baseline            →  spends CPU credits
Credits exhausted + Standard mode         →  CPU throttled to baseline
Credits exhausted + Unlimited mode        →  CPU continues, you pay extra

Use t instances for variable workloads — dev/test, small websites, microservices. Do NOT use for constant high-CPU workloads — use c or m instead.

8. AWS Graviton (ARM) — Why It Matters ⭐¶

AWS Graviton is AWS's own ARM-based chip (custom silicon).

If your app runs on Linux and doesn't need x86 → always evaluate Graviton first.

9. Instance Store vs EBS¶

Stop vs Terminate: - Stop → instance store data is lost; EBS data persists - Terminate → both instance store AND EBS root volume deleted (by default)

10. EC2 Pricing Models ⭐ (Most Important for Interviews)¶

Reserved Instances — 3 Types¶

Spot Instances — Critical Rules¶

- Up to 90% cheaper than On-Demand
- AWS can reclaim with 2-minute warning
- Use for: batch jobs, CI/CD, ML training, stateless workers
- DO NOT use for: databases, session-critical apps
- Spot Fleet: request across multiple instance types for better availability
- Spot Block: reserve Spot for 1–6 hours (no interruption during that window)

Payment Options (affects discount)¶

Earlier you pay → more you save.

11. EC2 Instance Lifecycle (States) ⭐¶

Launch
  ↓
pending  →  running  →  stopping  →  stopped
                   ↓
             shutting-down  →  terminated

Stopped instance → EBS still billed. Terminated instance → EBS root deleted by default (depends on DeleteOnTermination flag).

12. AMI (Amazon Machine Image) ⭐¶

Definition: A template containing OS + config + pre-installed software used to launch instances.

Key facts: - AMIs are Regional — must copy to use in another Region - AMI = EBS Snapshot + launch permissions + block device mapping - You can create a "golden AMI" — base image with all software pre-installed

13. Placement Groups ⭐¶

Controls how instances are physically placed in AWS infrastructure.

14. Tenancy Types¶

15. Key Additional Concepts¶

User Data (Bootstrap Script)¶

Script that runs once at first launch — used to install software, configure the instance.

#!/bin/bash
yum update -y
yum install -y httpd
systemctl start httpd

Elastic IP¶

• A static public IPv4 address you allocate to your account
• Can be reassigned between instances
• Free while attached to a running instance
• Charged when allocated but NOT attached (wasted resource)

Instance Metadata Service (IMDS)¶

Accessible from inside the instance:

curl http://169.254.169.254/latest/meta-data/
# Returns: instance-id, public-ip, ami-id, iam/security-credentials, etc.

16. Workload → Instance Selection Guide¶

17. Common Mistakes¶

18. Interview Questions Checklist¶

• What is EC2? What does "Elastic" mean in the name?
• Decode this instance type: m6i.4xlarge
• Decode this instance type: r7gd.metal
• What is the difference between instance family and instance type?
• What are the EC2 instance categories? Give one example each.
• What is a T-series instance? How do CPU credits work?
• What is AWS Graviton? Why should you use it?
• On-Demand vs Reserved vs Spot vs Savings Plans — when to use each?
• Spot instance — what happens when AWS reclaims it? How much warning?
• What is a Spot Fleet? Spot Block?
• Standard RI vs Convertible RI — difference?
• What are EC2 instance states? Difference between stopped and terminated?
• What is an AMI? How do you create a custom one?
• Instance store vs EBS — key differences?
• What are Placement Groups? 3 types?
• Shared vs Dedicated Instance vs Dedicated Host?
• What is User Data in EC2?
• What is an Elastic IP? When are you charged for it?
• What is IMDS? What IP does it use?
• You have a batch job — which pricing model and instance type?
• You have a Redis cache — which instance family?