The client-server model is a way computers communicate over a network.

• Client — a device or program that requests a service or information.
• Server — a computer or program that receives the request and provides the service or information.

Example: When you open a website, your web browser is the client. It sends a request to the website’s server. The server processes the request and sends the webpage back to your browser.

• A computing model where clients request services and server provide them .
• Foundation of modern web , database , and application architectures
• Why is is important ?.
  • a.Enables efficient resource management
  • b.Used in web browsing, email, APIs , databases and cloud services
• Real world example
  • a.Browsing website
  • b.Sending emails.

Key component of client server Model

• client
  • the user facing application that send request
  • Example: web browser, mobile app

• Server
  • A system that processes a request and return some response 
  • Example: web server, database server, and mail server

• Network
  • The medium that facilitates client and server communication
  • Example: internet, LAN , wi-fi 

How client and server communicate

• Basic Steps
  • Client send a request (HTTP , Sql query)
  • Request is transmitted over the network
  • Server receive the request and process the request
  • Server send response
  • Client receives and process the response 
• Types of communication
  • Request / Response Model(HTTP , Rest Api)
  • Persistent connections (webscoket and FTP session)

Request -Response cycle (HTTP Example)

• Browser request https://example.com
• DNS resolve domain to IP Address
• Browser send an HTTP get request to the web server.
• Web server process request, fetches HTML, queries to database if needed 
• Server send a response with http response code and web page content
• Browser renders the web page 

2.Synchronous vs Asynchronous Communication

1. Synchronous communication
   • Clients waits for response before proceeding
   • Example: A user submits a form and waits for a conformation message.
   • Used in REST Apis, traditional web apps.

• Asynchronous communications
  • Client does not wait for a response and can perform other task 
  • Example: web sockets for real time chat , Ajax call for background request
  • Used in messaging app , live updates , IOts applications 

3. Stateless and state full Servers

• Stateless server
  • No memory of past interactions , each request is independent 
  • Example : Rest api , https servers
  • Benefits  : Scalability , easy caching , load balancing 
• State full server
  • Maintain session information across request 
  • Example : web socket , banking application
  • Personalize  seamless user experience 

4. Forward proxy and Reverse proxy

What is proxy

• A proxy is an intermediary server between a client and other servers.
• Proxies help with security, caching, traffic control, and anonymity.
• Two main types: Forward proxy & Reverse Proxy .

Forward proxy

• Sits between a client and the internet(used by client)
  • Clients connect to the forward proxy instead of directly accessing websites
• • Can filter content, provide anonymity and cache requests.
  • Common use cases  
    • Bypassing geo restrictions
  • • Anonymous browsing (VPNS, TOR)
    • Caching webpages for faster access. 

Reverse proxy.

• Reverse proxies sits between user and backend servers(used by servers)
• Clients don’t directly communicate with backend servers – request got through the reverse proxy.
• Used for load balancing, caching, security, and ssl termination.
• Common use cases  
  • Load balancing across multiple servers.
  • Caching content to improve speed.
  • DDos protection and security.

5. load balancing

Introduction to load balancing

• Modern application must handle growing traffic.
• A single server eventually becomes a bottleneck.
• Load balancing enables system to scale beyond one machine.
• It is a functional building block of distributed system.

Scaling challenges:

• Server resources are finite.
• Growing traffic increase latency and failure
• Large servers help only temporarily.
• Vertical scaling eventually reaches limits

Horizontal scaling creates a New Problems

• Capacity grows by adding servers.
• Workload can be shared across machines
• Users should not choose servers manually.
• Traffic now needs a distribution mechanism.

Introducing load balancer:

• A load balancer becomes the system entry point
• Incoming requests are distributed across server.
• Traffic is no longer dependent on a single machine.
• Application can scale more effectively.

Load balancing Enables Reliable Systems.

• Multiple servers provide redundancy.
• Failed server can be bypassed
• Maintenance becomes safer.
• Availability and resilience improve.

6. Api Gateway

Introduction of API Gateway

• In modern system architecture, APIS connect clients (web, mobiles. third party apps) to backend services.
• Directly exposing backend services can lead to performance, security, and management challenges
• API Gateway is a centralized entry point for all API requests, handling, authentication, routing, caching, and security.

How api Gateways Work.

• Api Gateway acts as reverse proxy between clients and backend services.
• It receives API requests, process them and forwards them to correct services
• Key function includes request transformation, Authentication, rate limiting and response handling.

Benefits of using an API Gateway

• Security: Protects backend services from direct exposer.
• Rate limiting prevents abuse and DDos attacks
• Load Balancing: Distributes traffic efficiently.
• Caching: Speeds up response times.
• Request Transformation: Converts request formats, protocols and headers.
• Logging & Monitoring: Tracks Api performance and security events.

API Gateway: Benefits and Common Use Cases

• Best Suited for
  • Microservices architecture Multi-client APIs (web, Mobile)
  • APIs requiring security, rate limiting, and monitoring.
• Not to Use:
  • Simple monolithic apps with minimal API exposure
  • Low-traffic, internal-only services

7. CDNS

• Definitions: A CDN is a globally distributed network of servers that work together to delivers that work together to deliver content efficiently.
• Why CDNS exist:
  • Reduces latency
  • Enhances content availability
  • Improves load handling
  • Security