DEV Community

Cover image for Client-Server Model
Gouranga Das Samrat
Gouranga Das Samrat

Posted on

Client-Server Model

#systemdesign #networking #network

One-liner: A client requests resources/services, and a server responds with them. Every web interaction follows this model.

📌 What Is It?

The client-server model is the foundational architecture of the internet. It defines a relationship between two parties:

  • Client — the requester (browser, mobile app, CLI tool)
  • Server — the provider (web server, API server, database server)

This is a request-response cycle. The client always initiates; the server always reacts.

🔁 How a Request Works (Step by Step)

  1. You type google.com in the browser
  2. Browser checks its local DNS cache
  3. If not found → asks OS DNS cache
  4. If not found → asks ISP's DNS resolver
  5. DNS resolver returns the IP address (e.g., 142.250.195.78)
  6. Browser opens a TCP connection to that IP on port 80/443
  7. Browser sends an HTTP request
  8. Server processes the request and sends back an HTTP response
  9. Browser renders the HTML/CSS/JS

🧩 Key Components

Component Role
Client Initiates requests (browser, app)
Server Handles requests, returns responses
Protocol Rules for communication (HTTP, WebSocket, gRPC)
IP Address Unique address of the server on the network
Port Channel on the IP (80=HTTP, 443=HTTPS, 3306=MySQL)

🌐 Types of Clients

  • Thin Client — Only rendering/display logic (browser)
  • Thick Client — Has business logic locally (desktop apps)
  • Mobile Client — App that hits APIs

🔄 Communication Models

Request-Response (HTTP) 

Client ──── GET /users ────► Server
Client ◄─── 200 OK + data ── Server
Enter fullscreen mode Exit fullscreen mode

Long Polling 

Client ──── GET /updates ──► Server (holds connection)
Server ◄─── responds ONLY when data is ready
Enter fullscreen mode Exit fullscreen mode

WebSockets (Bi-directional) 

Client ◄──────────────────► Server
      (real-time, both ways)
Enter fullscreen mode Exit fullscreen mode

🏗️ Single Server Architecture 

[User's Browser]
      |
      | HTTP Request
      ▼
[Web + App Server] ──── reads/writes ────► [Database]
      |
      | HTTP Response
      ▼
[User's Browser]
Enter fullscreen mode Exit fullscreen mode

Problems with a single server:

  • Single Point of Failure (SPOF)
  • Can't scale horizontally
  • Web traffic + DB on same machine = resource contention

📊 Real-World Analogy

Think of it like a restaurant:

  • Client = Customer placing an order
  • Server = Waiter taking the order
  • Database = Kitchen preparing the food
  • Protocol = Language/menu used to communicate

⚠️ Limitations of Simple Client-Server

Problem Impact
Single server → SPOF Entire system goes down
No caching Every request hits DB
Stateful server Hard to scale horizontally
Tight coupling Hard to change one part

🔑 Key Takeaways

  • The client always initiates the connection
  • HTTP is stateless — server doesn't remember previous requests
  • Modern systems have many layers between client and actual data
  • Every "server" is just another computer listening on a port

Top comments (0)