Client-Server & HTTP Protocol

1. Client-Server Architecture

In modern web development, applications operate on a client-server model:

• Client: The requesting entity (e.g., web browser, mobile application, Postman, CLI tool).
• Server: The processing entity (e.g., Java application running on a remote host or cloud instance).

Analogy: A Restaurant Ordering System

Think of backend application architecture as a restaurant:

2. Networking Primitives: IP, Ports, Sockets & URLs

IP Addresses (127.0.0.1 vs External)

An IP (Internet Protocol) Address uniquely identifies a physical or virtual machine on a network.

• Public/Private IP: e.g., 192.168.1.50 or 172.217.14.206.
• Loopback IP (127.0.0.1): A special IP address reserved for a host to refer to itself. localhost is a domain name that resolves to 127.0.0.1.

Port Numbers

While an IP address gets network packets to a specific computer, a computer runs hundreds of processes simultaneously. A Port Number (0–65535) routes traffic to a specific application process.

IP Address (Building)  -->  Port Number (Apartment Number / Door)
192.168.1.100                :8080 (Spring Boot Application)
                             :5432 (PostgreSQL Database)
                             :3306 (MySQL Database)

Common Standard Ports

URL Anatomy

A URL (Uniform Resource Locator) decomposes into specific structural components:

  http:// localhost : 8080 /api/v1/users ? status=active # profile
  │───┘   │───────┘   └──┘ └───────────┘ └──────────────┘ └──────┘
Scheme    Host        Port     Path       Query Params    Fragment

1. Scheme/Protocol: http:// or https:// (Defines communication protocol).
2. Host: localhost or api.example.com (Identifies target server).
3. Port: :8080 (Target application socket door).
4. Path: /api/v1/users (Identifies target resource endpoint).
5. Query Parameters: ?status=active (Key-value filter parameters).
6. Fragment/Hash: #profile (Client-side UI anchor, never sent to server).

3. HTTP: The Language of the Web

HTTP (HyperText Transfer Protocol) is an application-layer, stateless request-response protocol running on top of TCP/IP.

Analogy: A Courier Package

An HTTP transmission behaves like sending a parcel:

Courier Package
├── Label (Headers)
│   ├── Destination / Path
│   ├── Content-Type (JSON, HTML)
│   └── Sender Credentials (Auth Token)
└── Contents (Body)
    └── Payload Data

HTTP Request Structure

An HTTP request consists of four primary parts:

1. HTTP Method: Verb defining desired action.
2. Path / URL: Target resource identifier.
3. Headers: Key-value metadata.
4. Body: Payload (optional in GET/DELETE, mandatory in POST/PUT/PATCH).

POST /api/v1/orders HTTP/1.1
Host: api.store.com
Content-Type: application/json
Authorization: Bearer eyJhbGciOi...

{
  "productId": 4021,
  "quantity": 2
}

HTTP Methods & Semantic Rules

Idempotency Rule: An operation is idempotent if making the same request multiple times produces the exact same side-effect on the server as making it once. PUT is idempotent (replacing a file 10 times yields the same state); POST is non-idempotent (creating 10 orders yields 10 database rows).

PUT vs PATCH Engineering Distinction

• PUT replaces the entire object state. Omitted fields are reset to default/null.
• PATCH updates only specified attributes, leaving existing attributes untouched.

4. HTTP Headers & Status Codes

Key Headers

HTTP Status Code Taxonomy

Status codes are 3-digit numerical responses indicating result categories:

Status Code Ranges:
├── 1xx (Informational) -> Protocol switching, processing
├── 2xx (Success)       -> 200 OK, 201 Created, 204 No Content
├── 3xx (Redirection)   -> 301 Moved Permanently, 304 Not Modified
├── 4xx (Client Error)  -> 400 Bad Request, 401 Unauthorized, 403 Forbidden, 404 Not Found
└── 5xx (Server Error)  -> 500 Internal Server Error, 502 Bad Gateway, 503 Service Unavailable

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