📘 Building Client/Server Systems and Implementing Protocols (Java)

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🔷 1. Definition

A Client/Server system is a network model where:

• Client → Requests services
• Server → Provides services

👉 Communication happens over a network using protocols.

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🔷 2. Basic Concept

📊 Architecture Diagram (Description) Client → Request → Server Server → Response → Client

• Many clients can connect to one server
• Server listens continuously

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🔷 3. Key Components

🔹 Client

• Sends request
• Waits for response

🔹 Server

• Listens for requests
• Processes data
• Sends response

🔹 Network

• Medium of communication

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🔷 4. Communication Protocols

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🔹 Definition

A protocol is a set of rules that defines how data is transmitted over a network.

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🔹 Common Protocols

• TCP → Reliable, connection-oriented
• UDP → Fast, connectionless
• HTTP → Web communication

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🔷 5. Socket Programming in Java

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🔹 Definition

A socket is an endpoint for communication between client and server.

👉 Java provides:

• Socket (Client)
• ServerSocket (Server)

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🔷 6. Steps to Build Server

✔ Step-by-step:

1. Create ServerSocket
2. Listen for client (accept())
3. Get input/output streams
4. Process request
5. Send response
6. Close connection

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🔹 Server Example

import java.net.*;
import java.io.*;

public class Server {
    public static void main(String[] args) throws Exception {
        ServerSocket ss = new ServerSocket(5000);
        System.out.println("Server started...");

        Socket s = ss.accept(); // wait for client

        DataInputStream dis = new DataInputStream(s.getInputStream());
        String msg = dis.readUTF();

        System.out.println("Client says: " + msg);

        ss.close();
    }
}

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🔷 7. Steps to Build Client

✔ Step-by-step:

1. Create Socket
2. Connect to server
3. Send request
4. Receive response
5. Close connection

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🔹 Client Example

import java.net.*;
import java.io.*;

public class Client {
    public static void main(String[] args) throws Exception {
        Socket s = new Socket("localhost", 5000);

        DataOutputStream dos = new DataOutputStream(s.getOutputStream());
        dos.writeUTF("Hello Server");

        dos.flush();
        dos.close();
        s.close();
    }
}

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🔷 8. Working Explanation

1. Server starts and waits
2. Client connects
3. Client sends message
4. Server reads message
5. Communication established

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🔷 9. TCP vs UDP

Feature	TCP	UDP
Type	Connection-oriented	Connectionless
Reliability	High	Low
Speed	Slower	Faster
Usage	Web, email	Streaming, gaming

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🔷 10. Implementing Protocols

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🔹 What it Means

Implementing a protocol means:

• Defining rules for communication
• Structuring messages
• Handling requests/responses

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🔹 Example (Simple Protocol)

Client sends:

GET_NAME

Server responds:

Ali

👉 This is a custom protocol.

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🔹 Steps to Design Protocol

1. Define message format
2. Define commands
3. Handle errors
4. Ensure data consistency

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🔷 11. Multi-Client Server (Advanced)

👉 Server can handle multiple clients using threads:

while(true) {
    Socket s = ss.accept();
    new ClientHandler(s).start();
}

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🔷 12. Advantages

• Scalable systems
• Distributed computing
• Efficient communication

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🔷 13. Challenges

• Network errors
• Security issues
• Synchronization

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🔷 14. Important Rules

• Always close sockets
• Handle exceptions
• Use threads for multiple clients
• Choose protocol wisely

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🔷 15. Diagram Description

📊 Client-Server Diagram:

• Multiple clients connected to server
• Arrows showing request/response
• Server handling requests

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📝 Likely Exam Questions

1. Define client-server architecture.
2. What is a protocol?
3. Explain TCP and UDP.
4. What is socket programming?
5. Write a Java server program.
6. Write a Java client program.
7. Steps to build client-server system.
8. What is ServerSocket?
9. Explain multi-client server.
10. What are challenges in network programming?

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📌 Quick Revision Summary

• Client → requests
• Server → responds
• Socket = communication endpoint
• Protocol = communication rules
• TCP (reliable), UDP (fast)
• Use threads for multiple clients

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