Interview Q&A / AWS & Cloud

1. What is Amazon S3 and what are its storage classes?

   S3 is AWS's object storage. You store files as objects inside buckets and access them by key over HTTPS. Storage classes trade storage cost against access speed and retrieval cost, from S3 Standard for frequently used data to the Glacier classes for archives you rarely read.

   BeginnerIn-depth4 min read
2. What is the difference between an AWS Region and an Availability Zone?

   A Region is a separate geographic area, like Mumbai, with its own set of data centres. An Availability Zone is one or more isolated data centres inside a Region, with its own power and networking. You spread an app across AZs to survive a data-centre failure, and across Regions to survive a whole-area outage or to be closer to users.

   BeginnerIn-depth3 min read
3. What is the difference between EC2 and Lambda, and when would you pick each?

   EC2 gives you a virtual server that you run and manage, billed while it is running. Lambda runs your function only when an event arrives, scales automatically and bills per request and execution time. Pick Lambda for short, spiky, event-driven work; pick EC2 for long-running, steady or highly customised workloads.

   BeginnerIn-depth4 min read
4. What is the difference between IAM users, groups, roles and policies in AWS?

   A user is an identity for one person or app with long-term credentials. A group is a set of users that share permissions. A role is an identity that anyone or anything trusted can assume to get temporary credentials. A policy is the JSON document that says what is allowed or denied. Grant only the permissions needed, which is least privilege.

   BeginnerIn-depth4 min read
5. What is a VPC, and what is the difference between public and private subnets?

   A VPC is your own private network inside AWS. A public subnet has a route to an internet gateway, so resources with public IPs can be reached from the internet. A private subnet has no such route; its servers reach the internet only outbound, through a NAT gateway placed in a public subnet.

   IntermediateIn-depth4 min read
6. What is the difference between a security group and a network ACL in AWS?

   A security group is a stateful firewall attached to an instance's network interface, with allow rules only. A network ACL is a stateless firewall on a subnet, with numbered allow and deny rules checked in order. Because NACLs are stateless, you must allow return traffic explicitly, usually on ephemeral ports.

   IntermediateIn-depth4 min read
7. What is the difference between RDS and DynamoDB, and when would you use each?

   RDS is a managed relational database service running engines like MySQL or PostgreSQL, with SQL, joins and transactions. DynamoDB is a serverless key-value and document database that gives fast, predictable performance at very large scale if you design around known access patterns. Use RDS for relational data and flexible queries; use DynamoDB for simple, high-volume lookups by key.

   IntermediateIn-depth4 min read
8. How do an Auto Scaling group and a load balancer work together on AWS?

   An Auto Scaling group keeps the right number of healthy EC2 instances running, adding or removing them based on policies such as target CPU. A load balancer spreads incoming traffic across those instances in multiple AZs. The ASG registers new instances with the load balancer's target group and replaces any that fail health checks.

   AdvancedIn-depth4 min read