1. Introduction

1.1 Tutorial's Goal

This tutorial aims to guide you on how to design both the backend and frontend for decentralized applications (dApps). We'll explore the role of the blockchain, delve into the workings of smart contracts, and look at user interface design for dApps.

1.2 Learning Outcomes

By the end of this tutorial, you will:

• Understand the basics of the blockchain and smart contracts.
• Learn how to design the backend for a dApp.
• Learn how to design the frontend for a dApp.
• Be able to create a simple dApp.

1.3 Prerequisites

This tutorial assumes that you have a basic understanding of programming concepts and web development. Knowledge of JavaScript, Solidity, and experience with Node.js will be beneficial.

2. Step-by-Step Guide

2.1 Blockchain and Smart Contracts

Blockchain is the underlying technology for dApps. It's a decentralized and distributed ledger of all transactions across a peer-to-peer network. Smart contracts are self-executing contracts with the terms of the agreement being directly written into lines of code.

For example, in Ethereum, smart contracts are written in a language called Solidity. These contracts are immutable once deployed on the Ethereum blockchain.

2.2 Backend for dApps

The backend of a dApp typically involves creating, deploying, and interacting with smart contracts on the blockchain. Here are the steps:

1. Creating Smart Contracts: Using Solidity language, create the logic for your dApp.

2. Deploying Smart Contracts: Once the contract is written, it needs to be compiled and deployed onto the blockchain.

3. Interacting with Smart Contracts: After deployment, the contract can be interacted with by calling its functions.

2.3 Frontend for dApps

The frontend for a dApp serves as the user interface that interacts with the blockchain. It's typically built using web technologies like HTML, CSS, and JavaScript.

3. Code Examples

3.1 Creating Smart Contracts

Here is a basic Solidity contract:

pragma solidity >=0.4.22 <0.9.0;

contract SimpleStorage {
    uint storedData;

    function set(uint x) public {
        storedData = x;
    }

    function get() public view returns (uint) {
        return storedData;
    }
}

This contract simply sets and gets a number.

3.2 Deploying Smart Contracts

Deployment can be done using a framework like Truffle. Here's a basic setup:

var SimpleStorage = artifacts.require("SimpleStorage");

module.exports = function(deployer) {
  deployer.deploy(SimpleStorage);
};

This script deploys the SimpleStorage contract onto the blockchain.

3.3 Frontend Interaction

Here's how a simple frontend can interact with the deployed contract:

SimpleStorage.deployed().then(function(instance) {
  instance.set(10, {from: web3.eth.accounts[0]}).then(function(result) {
    console.log("Transaction successful!");
  }, function(err) {
    console.log("Transaction failed!");
  });
});

This JavaScript code will set the value of storedData to 10.

4. Summary

In this tutorial, we've covered the basics of blockchain and smart contracts, and learned how to design the backend and frontend of a dApp. The next step is to dive deeper into these topics and start building complex dApps.

5. Practice Exercises

5.1 Exercise 1: Simple Token

Create a smart contract for a simple token. The contract should allow the creator to mint new tokens and transfer tokens to others.

5.2 Exercise 2: Voting dApp

Design a dApp for a voting system. Users should be able to cast votes for candidates and the total votes for each candidate should be publicly visible.

5.3 Exercise 3: Marketplace dApp

Design a dApp for a marketplace where users can list items for sale and others can purchase them.

Remember, practice is key in mastering dApp development. Happy coding!