Blockchain

What is Bitcoin

• A system to allow peer to peer payments without a financial inermediary for eg. bank.

What is Ethereum

• Smart contract and decentralized application platform, not just to transfer money.
• Need for more programatic control of transactions.
• Enable creation of decentralized autonomous corporations (DAC).
• Introduced idea of smart contracts as an entity that can send and recieve currency, beyond just humans.
• Ethereum networks are used to transfer money and store data.
• Ethereum networks is a collection of nodes running Ethereum client.
• Anyone can run a node.
• Each node contains full copy of the blockchain.
• Blockchain is a database that stores a record of every transaction that has ever taken place.

Interaction with Ethereum Blockchain

• Programmatic: web3.js
• Non programmatic: Metamask (Browser extension)

Chrome plugin for metamask

Metamask account contents

• Metamask account can be used to interact with any network/blockchain. No need to create seperate accounts.
• Account Address: Similar to an email address/username.
• Public key
• Private key: Never share this with anyone, this gives access to funds in the account.

One metamask account can interact with multiple Ethereum Blockchain/network like main net and test net.

Ethereum test networks

• Ropsten

• Kovan

• Rinkeby

• Faucet: Get free Ethers from rinkeby-faucet.com

Ethereum transaction object

• Describes transfer of funds from one account to another.
• Created using web3js library.
• Multiple transaction objects are contained in a block.

Contents of Ethereum transaction object

• Transaction object: A record which describes one account attempting to send money to other account.
• It is created when two accounts exchange money.
• v,r,s cannot be used to generate the senders priate key.

Sequence of transferring funds to another account on Blockchain:

• Web3js at client side sends transaction object to a node in Blockchain.
• The node consolidates all received transaction objects into a block.
• Then the node validates the block(mining) and confirms back to the client.
• The validation takes time.

Hashing fundamentals

• Hash of a content is unique. Change in content will change the hash.
• The content cannot be retrieved from the hash.

Generic Blockchain fundamentals

• When a new block is added to the blockchain the nonce filled is set such that hash field start with 4 zeros.
• The process of finding suitable nonce is called mining.
• Modifying any block will invalidate all later blocks as their previous and hash field will change and will not be a valid hash.
• So if we change any block that we have to mine (find new nonce) all the later blocks.
• Even if we mine all later blocks on a node, we can figure out tampering by matching the last block with other nodes in the network.
• Mining (finding nonce) is done by all network nodes, as soon as any node finds a solution, it distributes it across network.

Contents of generic blockchain

Ethereum specific blockchain

• The nonce is set such that the hash value is less than some target value (instead of 4 leading zeros).
• Target target value is adjusted to change block time as computing power in the chain varies due to no. of active nodes in the chain.
• The time to find a suitable nonce is called block time. This has to be shared with other nodes on the chain to keep them updated.
• The confirmation time by a blockchain is mining time plus the distribution time.
• Ethereum has a target block time of 15 seconds.
• After every blocked is mined, the network calculates how long it took and adjust the target value of hash to meet Ethereum block time of 15 seconds.
• If the time taken is large the target value of hash is raised to reduce block mining time.
• If the time taken is less the target value of hash is reduced to increase block mining time.
• The mining time changes over time as number of active nodes at any given time changes.

Link to see block time of Ethereum blockchain https://etherscan.io/chart/blocktime Shows Averagre amount of time taken to calculate the blocks in the Ethereum chain.

Smart contracts to build interesting apps (Dapps) on the network

• Smart contract is an account controlled by the code.

Contract account contents/fields

• Balance: Amount of ether the account owns.
• Storage: Data storage for this contract.
• Code: Raw machine code for this contract. It is writen in Solidity language.

External Account

• Decoupled from any individual network.
• Created by humans or an entity.

Contract account

• Specific to one individual network/Chain.
• It is an instance of a contract which is created by compiling a source file.
• Multiple instance of contract from same source file can be deployed on a single network.

External account to create contract transaction (revisit)

Solidity language

• Written in .sol files.
• Strongly typed.
• Similar to JS.
• Has lot of gotchas.

Smart contract

• Code is written in Solidity language.
• Solidity compiler processes the code and churns out byte code and ABI.
• byte code is deployed on the block chain.
• ABI is used by javascript applications to interact with the smart contract/byte code.

IDE for Solidity (Remix) https://remix.ethereum.org/

Remix editor:

• Latest solidity version: 0.8.5
• Practical on solidity version: 04.17.

pragma solidity ^0.4.17;

contract Inbox {
    string public message;

    function Inbox(string initialMessage) public {
        message = initialMessage;
    }

    function setMessage(string newMessage) public {
        message = newMessage;
    }

    function getMessage() public view returns (string) {
        return message;
    }

}

Common function types

• See other function types in Solidity.
• returns is used for function marked as view or constant.

Remix IDE

• When you deploy contract using remix ide, it is deployed on the in browser fake network.
• Remix ide host a tiny fake Ethereum network in a javascript VM.
• The instance of the contract is deployed on the fake network.
• The code is compiled into a bytecode and then deployed to the fake network.
• When a storage variable is define as public within a contract, the contract deifnes a function with same name as the variable.
• Calling the above function will return the value of the variable.
• Creating a contract is very similar to a fund transfer transaction.
• The to field is left as blank.
• An external account creates a contract by sending transaction object (See below).
• It is very similar to the transaction object to transfer funds, except the to field which is blank and data field which contains bytecode.
• Running a transaction on remix doesn't take time but on test net and main net (public network) it will take time.

External account to create contract transaction

Ways to run a contract function

Gas and Wei

• We pay gas fee to the nodes running our transactions on blockchain.
• Running a transaction involves modifying and storing data and running the code, which incurrs gas (cost).
• The gas price is paid by the one who is creating a transaction, ie the user of Dapp.
• 1 Ether is equal to 1e+18 wei.
• To see gas price search for EIP-150.
• gasPrice:
• gasLimit:

Mnemonic and ethereum accounts

• Using mnemonic we can create and manage multiple account and need not remember details of each account.
• We only need to remember the 12 word mnemonic.
• If you have to work with new mnemonic to manage new group of accounts then you have to remove and install metamask.

To get free Ether from faucet

Truffle

• One stop shop for development of ethereum contracts.
• Contract creation.
• Local testing.
• Deployment.
• Still in early developmet so features may not work.

Boilerplate design

Ganache

• A library which can be used to install small test Ethereum network.
• Earlier it was called TestRPC.
• Automatically creates accounts for us to use.
• Accounts are created in unlocked state, so we don't need private/public keys.

Web3 versions

• Web3 versions v0.x.x and v1.x.x.
• Starting with 0, only has callbacks to support async code.
• Starting with 1, has support for promise and async/await to support async code.

Web3 providers

• Using Web3 library, we create instance(s) of Web3 library.
• Each instance can interact with a single Ethereum network.
• Web3 instance needs a provider to interact with Ethereum network.
• Provider acts as a communication layer between Web3 instace and the ethereum network.
• Provider provides the functions to send and receive request to/from an Ethereum netwrok.

Deploying to local Ganache network

Mocha

• It is a testing framework.
• following are the functions supported in Mocha

Creating contract on Ethereum network

-To create a contract an external account makes a transaction with the Ethereum network.

• In such transaction, the to field is empty.
• data field contain the compiled bytecode of the contract.
• value field contain the amount of Wei to be sent to target address??

Information needed by web3 to deploy a contract and communicate with an already deployed contract.

Deploying the contract to "real" test network

• Use infura.
• Infura is an API which provides access to node installed on rinkeby network.
• Infura is an alternative to installing a Rinkeby node locally, which is complicated.
• diagram of interfaces with the test nw.

truffle-hdwallet-provider

• This npm package gives a provider for communication between web3 nd rinkeby nw and also unlock an account to deploy the contract.

Interacting with deployed contract without a front-end

• Use Remix online editor with injected web3.

Basic Solidity types

• string: Sequence of characters.
• bool: true/false
• int: Interger, positive or negative.Has no decimal. int8, int16, int32..int256(default), no of bits used to store a number.
• uint: Unsigned integer,positive number. has no decimal. uint8, uint16, uint32..uint256(default).
• fixed/ufixed: "Fixed" point number. Number with a decimal after it.
• adrress: Has methods tied to it for sending money.
• There are other types like an Array or a mapping.

msg global variable

• Available when any account does a transaction or call a function on the Ethereum nw.

Reference types in Solidity

• Fixed Array

• Dynamic array

• Mapping

• Struct

• Note: For a public array the automatically generated method expects an index of the element you want to retrieve.

Solidity gotcha

• nested arrays can't be communicated between solidity and js/web3/abi world.

• since strings in solidity are internally stored as dynamic arrays, the above limitation holds for array of strings.

• Note: payable function type is used when the user wants to send ether.

Global function

• require(): it's a validation function. If it fails, further execution is suspended.

this.balance: amount of money in the contract.

address type has transfer method on it to transfer eth to the address.

Function modifier

• Can be used to restrict access to function using require().
• Ends with _;
• then use the function name as a modifier for other function.
• Behind the scenes, solidity compiler places the code of modified function at underscore.

For eg.

modifier restricted (){
require();
\_;
}

Web3 utility methods

• web3.eth.getBalance(accountAddress): Get the balance amount in an address/wallet.
• web3.ethtoWei("0.1", "eth"): Convert 0.1 eth to Wei.

Building Dapp

• Metamask injects web3 library in the active page when it is running in the browser.
• Metamask uses old version of web3 library, so our app need to highjack the provider of metamask which provides connectivity with the nw and also holds the account information.
• The Dapp needs the interface (obtained from compilation) to the contract and the address of the deployed contract.
• The dapp interacts with local contracts intance created using web3.
• When calling contract methods from the Dapp, we don't specify from parameter as it is already defined in the provider provided by metamask.
• When sending a transaction from the Dapp, we have to specify the from address.

export default new web3.eth.Contract(abi, address)??

Rendering contract data in a component

• Component renders.
• componentDidMount called.
• "call" methods on the contract.
• set data on "state".