What is Amazon EC2?
Learn what Amazon EC2 is, how instances, AMIs, and security groups work, how EC2 billing works, and how to explain it clearly in an AWS interview.
Expected Interview Answer
Amazon EC2 (Elastic Compute Cloud) is AWS's IaaS service for renting resizable virtual servers, called instances, on demand instead of buying and racking physical machines.
You choose an instance type (CPU, memory, and network profile), an Amazon Machine Image (AMI) that defines the OS and pre-installed software, and a network configuration, and EC2 launches a virtual machine within minutes. You control the OS, patch it, install your own runtime and application stack, and pay per second or hour the instance runs. EC2 supports auto scaling groups to add or remove instances based on demand, and integrates with EBS for persistent block storage and security groups for network-level access control. Because you manage the OS layer, EC2 gives fine-grained control at the cost of more operational responsibility than a managed compute service.
- Resizable capacity launched in minutes, not weeks
- Wide range of instance types tuned for CPU, memory, or GPU workloads
- Full control over the OS and installed software
- Pay only for compute time actually consumed
- Integrates with auto scaling and load balancing for resilience
AI Mentor Explanation
EC2 is like hiring a cricket net session on demand — you pick the type of pitch (fast, spin-friendly) and how many bowlers you need, and it is ready in minutes rather than building your own academy. You pay only for the hours booked, and if match season gets busy you book more nets simultaneously, scaling capacity to demand.
Step-by-Step Explanation
Step 1
Choose an AMI
Select an Amazon Machine Image defining the operating system and any pre-baked software.
Step 2
Pick an instance type
Choose a hardware profile (CPU, memory, network, GPU) sized to the workload.
Step 3
Configure networking and security
Place the instance in a VPC subnet and attach a security group controlling inbound/outbound traffic.
Step 4
Launch and connect
EC2 boots the instance within minutes; you connect via SSH or RDP to install your application stack.
Step 5
Scale and manage lifecycle
Attach to an Auto Scaling group and load balancer, and stop/terminate instances to control billing.
What Interviewer Expects
- Explains EC2 as resizable virtual server compute (IaaS)
- Knows the role of AMIs, instance types, and security groups
- Understands billing is per second/hour of running time
- Can describe how Auto Scaling and Elastic Load Balancing work with EC2
- Distinguishes EC2's OS-level control from managed compute like Lambda
Common Mistakes
- Calling EC2 a storage service instead of compute
- Forgetting that you are billed while an instance is running, not just when in use by traffic
- Confusing security groups (stateful, instance-level) with NACLs (stateless, subnet-level)
- Assuming EC2 patches the OS automatically like a managed service would
Best Answer (HR Friendly)
“Amazon EC2 lets a company rent virtual computers in the cloud instead of buying physical servers. You can choose how powerful the computer is, turn it on or off as needed, and pay only for the time it runs, which makes it flexible and cost-effective for running applications.”
Code Example
aws ec2 run-instances \
--image-id ami-0abcdef1234567890 \
--instance-type t3.medium \
--key-name my-key \
--security-group-ids sg-0123456789abcdef0 \
--subnet-id subnet-0abc1234
aws ec2 describe-instances --instance-ids i-0123456789abcdef0 \
--query 'Reservations[0].Instances[0].State.Name'
# "running"Follow-up Questions
- What is the difference between an EC2 instance and an AMI?
- How do security groups differ from network ACLs?
- What is Auto Scaling and how does it work with EC2?
- What are Spot Instances and when would you use them?
- How does EC2 pricing differ across On-Demand, Reserved, and Spot models?
MCQ Practice
1. What does EC2 primarily provide?
EC2 (Elastic Compute Cloud) provides resizable virtual server instances that you configure and manage.
2. What defines the operating system and pre-installed software of an EC2 instance at launch?
The AMI is the template specifying the OS, kernel, and any pre-baked software used to launch the instance.
3. How is standard On-Demand EC2 usage billed?
On-Demand EC2 instances are billed based on the time the instance is in a running state, not on traffic volume.
Flash Cards
What is EC2? — AWS's IaaS service for launching resizable virtual server instances on demand.
What is an AMI? — An Amazon Machine Image — a template defining the OS and pre-installed software for an instance.
How are EC2 On-Demand instances billed? — Per second or hour the instance is in a running state.
What controls network access to an EC2 instance? — Security groups, which act as a stateful virtual firewall at the instance level.