1.
Let’s write a DApp
Chung Wu
LikeCoin Foundation
HKOSCon 2018

2.
What is DApp
ServerBrowser
Browser Blockchain

3.
DApp example: class voting system
- Teacher is the host and can create new proposals with some
description
- Students are participants, the list is pre-defined, and can vote for
‘agree’ or ‘disagree’ for each proposal
- Each of the participants can vote for each proposal only once

4.
Traditional approach
- Server defines API for creating, querying and voting proposals
- Hardcoded teacher account to verify for creating proposals
- Hardcoded student accounts to verify for voting proposals
- Server ensures that no one can vote the same proposal twice
- A web frontend

5.
Traditional approach
- Server defines API for creating, querying and voting proposals
- Hardcoded teacher account to verify for creating proposals
- Hardcoded student accounts to verify for voting proposals
- Server ensures that no one can vote the same proposal twice
- A web frontend
Smart contract
Smart contract
address
address
DApp
Still need this

6.
Some concepts
- Private Key: a secret stored by the user for signing transactions and
proving user’s identity
- Ethereum Address: user’s identifier derived from private key
- Smart Contract: code that executes on blockchain, plus storage
Or in other words:
- Private Key: password for blockchain
- Ethereum Address: username for blockchain
- Smart Contract: program for blockchain

7.
How does an Ethereum DApp work
Browser
Your app’s
execution
environment
Your code
MetaMask
extension
Private Key(s)
①
②
③
④
smart
contract
Ethereum
Blockchain

8.
Demo

9.
Code & References
Demo code repo: https://github.com/likecoin/hkos2018-dapp-web
Web3 doc: https://web3js.readthedocs.io/en/1.0/index.html
Solidity language doc: https://solidity.readthedocs.io/