System Design is the process of deciding how different components of a software system will work together to satisfy business and technical requirements.

• system design is the blueprint of software system
• Defines system Architecture, components, interface and data flow.
• Focuses on scalability, reliability, performance and maintainability.
• Establishes boundaries between services, database and users
• Converts business requirements into technical architecture
• Balances functionality with operational realities.

2. What is Scalability?.

Scalability means: Can my system handle more users, requests, or data when the load increases?

Imagine you have a small restaurant.

Today:

10 customers
   ↓
1 chef
   ↓
Food served easily

Tomorrow, 1,000 customers arrive.

If the restaurant can increase its capacity by adding more chefs, tables, kitchens, etc., it is scalable. The same concept applies to software.

3. What is Reliability?

Reliability means: Can the system work correctly and consistently without failing?

Imagine you have a car.

You expect:

Start car
   ↓
Drive
   ↓
Reach destination

If the car frequently breaks down, it is not very reliable.

Software is similar.

Suppose you have:

Customer
   ↓
Booking API
   ↓
Booking Service
   ↓
Database

A customer submits:

“Book my service appointment.”

The system should successfully process the booking. If the system frequently crashes or loses requests, it has poor reliability.

4. What is Ip Address

• Ip(Internet protocol) addresses are unique labels assigned to device on a network.
• They enable communication between different machines, servers, and services across the internet.
• Two primary versions
  • IPV4
  • IPV6
• Two primary categories
  • Public
  • Private
• Every device, server, database, load balancer or service endpoint participating in network communication needs an address that the other system can reach.

What is IPv4 

• IPV4 (Internet Protocol Version4) is the most widely used addressing system
• 32-bit address format (192.168.1.1)
• Total addresses available:4.3 billion
• Uses: Traditional networking, web servers, and most current internet devices.
• Challenges: Limited Ips, fragmentation, and security concerns

What is IPv6

• IPV6(Internet Protocol version 6)is the next generation Ip Standard.
• 128-bit address format
• Total addresses available :340 undecillion (Virtually unlimited).
• Designed for IoT ,mobile networks and future scalability.
• Key Benefits: large address space, better security and improved routing efficiency.

What is public Ip Address

• Public ip assigned by isps(Internet service providers)
• Public ip used to communicate over the internet
• Public ip unique world wide
• Example 192.203.23.45

What is private Ip Address

• Inside a company network, cloud vpc, Kubernetes cluster or any data centre, thousands of machines used to communicate with each other without exposing on internet, private addressing allows organization to scale internal networks without consuming public address space.
• From an architectural point of view this separation create security and operational advantages databases, caches, internal APIs, and back-end services can communicate freely using private Ip addresses.
• The public internet sees only a small set of entry points such as load balancer, Api gateway or reverse proxies.
• Private ip used within the local network (LAN, enterprises, home)
• Can not be accessed directly from the internet.

Why do we need private Ip Address

• Conserves public Ip addresses.
• Enhances security (private ips are not routable over the internet)
• Enable Network address translation (NAT) to allow multiple devices to share a single public IP.
• Common in corporate networks, data centres, and cloud environment.

Role of Ips in System design

• Scalability: helps in designing distributed, multi region and cloud-based architectures.
• Security: Enables firewall rules, VPNS and private networking.
• Load Balancing: Uses IP-based traffic distribution (Round robin DNS, Anycast IPs)
• Cloud Networking: Public, private, and hybrid cloud IP management (AWS, GCP)
• Microservices & Containers: Use Internal private Ips for communications.

5. What is DNS?.

DNS converts a human-friendly website name into an IP address that computers can understand.

Simple example

When you type:

www.google.com

your computer needs to know the server’s IP address, something like:

142.250.xxx.xxx

Introduction to DNS

• What is DNS.
  • The phone book of the internet
  • Translate human-readable domain names(google.com) into IP address.
• Why is DNS Important?
  • Enables accessibility of websites. 

Underlying ip address can change over time DNS provides a stable layer of abstraction, allow use to keep using the same domain.

Types of DNS servers

• Root Name servers: Top level servers handling ,com, .org etc .
• TLD Name servers: specific to domains like  .com, .net .
• Authoritative Name servers: store domain specific records 
• Recursive Resolvers: Handles Queries on behalf of users, contacting multiple DNS servers if necessary.

How DNS Works

Let’s say you type: www.microsoft.com   into your browser.

Step 1: Browser Checks Local Cache

Your browser first checks whether it already knows the IP address from a previous lookup.

Browser Cache
↓
Found? → Use IP address
Not Found → Continue

Step 2: Operating System Checks DNS Cache

If the browser doesn’t have the record, your computer checks its local DNS cache.

PC DNS Cache

↓

Found? → Use IP
Not Found → Query DNS Server

Step 3: Query Goes to Recursive DNS Resolver

Your device sends a request to a DNS Resolver (usually provided by your ISP or services like Google DNS 8.8.8.8).

Laptop

↓

DNS Resolver

The resolver’s job is to find the correct IP address.

Step 4: Resolver Contacts Root DNS Server

The resolver asks a Root DNS Server:

“Where can I find .com domains?”

The root server responds:

Ask the .com TLD server.

Step 5: Resolver Contacts TLD Server

TLD (Top-Level Domain) Server manages domains such as:

.com

.org

.net

.in

The resolver asks:

Where is microsoft.com?

The TLD server responds:

Ask Microsoft’s authoritative DNS server.

Step 6: Resolver Contacts Authoritative DNS Server

The authoritative DNS server contains the actual DNS records.

The resolver asks:

What is the IP address of www.microsoft.com?

Show more lines

The authoritative server replies:

20.x.x.x

Step 7: Resolver Returns the Answer

The resolver sends the IP address back to your device and stores it in its cache for future use.

www.microsoft.com

↓

20.x.x.x

Step 8: Browser Connects to the Website

Now the browser can establish a connection to the web server using the IP address.

Browser

↓

IP Address

↓

Web Server

↓

Website Loads

DNS Caching and performance optimization

• Why caching matters
  • Reduces latency
  • Reduces load on dns server

• Where caching occurs? .
  • Browser cache
  • OS Cache
  • Recursive resolver cache

Time-To-Live(TTL) :Determine how long cache record is valid 

Domain Name resolution process 

Step-by -step breakdown of how dns resolves a domain name 

• User types a domain name in the browser
• Browser cache checked
• Operating system cache checked
• Query to the local DNS Resolver 
• If not found Query goes to root dns server
• TLD(Top level domain) servers provide the Authoritative DNS server details
• Authoritative dns server returns the ip address
• Response is cached and returned to the user

1. Importance of dns server in large scale systems
   1. Ensuring High availability
      • Load balancing using DNS
      • Anycast DNS SERVER FOR faster global resolution 
   2. Dns Failure strategies
      • Primary and secondary dns server
   3. Content delivery Network(CDN) AND DNS
      • How DNS DIRECTS users to nearest CDN node for fast loading
   4. Dns Security risks
      • DNS poisoning , cache poisoning , and DDos attacks