This document provides an introduction to building decentralized applications (dapps) on Ethereum. It discusses Ethereum and smart contracts, then demonstrates how to interact with the Ethereum blockchain and smart contracts using Web3 and Metamask. Key topics covered include deploying and interacting with smart contracts, estimating transaction costs in gas, and using Metamask to sign transactions without exposing private keys. The document uses examples in Web3.js and Web3py to illustrate communicating with the blockchain and calling methods on smart contracts.

1. Dapps 101
Katarzyna Jagieła & Maciej Bembenista

2. Agenda
— Ethereum, Solidity
— Introduction to Ethereum
— What are smart contracts?
— Example contract
— Web3
— Transactions in ETH
— Using Metamask
— Interacting with smart contract

3. What’s Ethereum all about?
- distributed public blockchain network
- different in purpose and capability from Bitcoin, but based on similar principles
- focuses on running decentralized applications
- runs smart contract code in a built-in EVM (Ethereum Virtual Machine)
- it uses Ether - native currency of the Ethereum blockchain
- gas - a special unit on Ethereum, used to calculate the amount of fees that we pay the miners for including our
transactions in their blocks

4. Smart Contracts
- they allow you to exchange money (Ether) or content such as tokens in a trustless way
- each contract has its own address on the blockchain
- are deployed to the blockchain and all code execution happens on the EVM during mining
- are not editable - you can change properties of the contract through its code, but you can’t upload code changes
or fixes - you need to get it right the first time or deploy a new contract

5. Contract example

6. What’s the cost of a transaction?
- Ethereum uses a unit called gas to determine the fee for the miner that will mine your
transaction
- Each operation in Ethereum has a set gas amount that it costs - for example, assembler
instruction that’s used to create a contract costs 32000 gas to run.
- You can set how much you’d like to pay per single gas unit used up to mine your transaction
- You can also set a gas limit in order to avoid paying for a transaction if it could use too much
gas.

7. How to communicate with Ethereum?

8. Web3 - a nice abstraction for JSON RPC
communication with Ethereum
- It’s available for JavaScript and Python - more languages are coming
- Provides a set of useful helpers for dealing with Ether units (wei, gwei etc.)
- Allows us to deploy contracts, interact with them, check their state
- Allows us to send Ether transactions
- Of course, anything that’s accomplished by Web3 can be done without it - it’ll just be harder to
implement

9. Web3.js - fetching an account’s balance

10. Web3py - fetching an account’s balance

11. How to create a Web Dapp
- Problem: Ethereum is based on public / private keys - we don’t want users to provide their
private keys to the application
- Problem: Users can sign their transactions offline, but that requires them to use a wallet
separate from our web Dapp - UX would suffer
- Solution: Use a browser-based wallet that allows us to interact with user’s private key if he
allows us to, without accessing them directly

12. Metamask - Ethereum wallet for the browser

13. Web3.js - sending a transaction through Metamask

14. Web3py - sending a transaction

15. Interacting with smart contracts
- ABI - methods and variables available on the contract
- Address - address of the deployed contract

16. Web3py / Web3.js - calling contract methods

17. Web3py - sending contract transactions

18. Web3.js - sending contract transactions

19. Questions?

20. Thanks!