Smart Contracts

Smart contracts are self-executing contracts with the terms of the agreement between buyer and seller being directly written into lines of code. The code and the agreements contained therein exist across a distributed, decentralized blockchain network. Smart contracts permit trusted transactions and agreements to be carried out among disparate, anonymous parties without the need for a central authority, legal system, or external enforcement mechanism. They render transactions traceable, transparent, and irreversible.

Blockchain

The smart contracts are developed for the ethereum blockchain. Ethereum is a decentralized platform that runs smart contracts. Contracts can be deployed on the Ethereum Virtual Machine (EVM). Once deployed it can be accesed globally. These contracts are enforced and certified by parties that we call miners. Miners are multiple computers who add a transaction (Addition or modification of the state) to a public ledger that we call a block. Multiple blocks constitute a blockchain.

We pay these miners with something called Gas, which is the cost to run a contract. When you publish a smart contract, or execute a function of a smart contract, or transfer money to another account, you pay some ether that gets converted into gas.

Platforms

High Level Languages for smart contract developing:

• Solidity
• Serpent

See the differences here.

Command Line Development Management Tools for creating a DAPP project:

• Truffle
• Embark
• Dapple

Testnode with RPC Interace for deploying contracts on a virtual node and make transactions without the need to be mined:

• Javascript testrpc
• Python testnode

Browser based IDE to get instant feedback for solidity code:

• ReMix
• EthFiddle

Wallet clients to connect a ethereum wallet

• Metamask
• MyEtherWallet

Compiler

The contracts are written in solidity codes and these are to be compiled to get the Application Binary Interface (ABI) codes. ABI is the interface between two program modules, one of which is often at the level of machine code. The interface is the de facto method for encoding/decoding data into/out of the machine code. It defines how you can encode Solidity contract calls for the EVM and, backwards, how to read the data out of transactions. It also provides the Bytecode or the opcodes of the contract.

A command line tool such as truffle or online ide such as ReMix can be used to compile.

Network

The smart contracts are deployed on the ethereum network on the EVM. But any transaction executed on this network requires ether to be spent, hence its not advicable for development.

Ethereum network is called ‘livenet’ and there exists multiple ‘testnet’ which are copies of ethereum network. Like ethereum, these networks have currency called ‘test ether’s which can be spent to execute a transcation on the network. Now the important difference is that you can get free ‘test ether’s here unlike real valued ether.

Some testnets are :

• Testrpc – This is a local network running on your compuetr. 10 free wallet accounts with test ether is allocated.
• Ropsten – This is a global testnet with free test ether.
• Rinkeby – Another global testnet with free ether.

Wallet

Wallets are very important part of a smart contract. It serves 2 purposes:

• It serves as client to ethereum wallet. To make a transaction on network ether has to be spent and you can authorize these payments using this.
• To communicate with a blockchain and to deploy, you need to either have a full node or a wallet cleint of the network. A wallet can facilitate the communication with the network.

Deployment

To deploy a contract the following steps are to be taken:

• Compile the code and get necessary bytecodes and ABIcodes
• Select a network to migrate to
• Make a deployment with a wallet address as transaction sender
• Authenticate the transaction form the wallet and pay the transaction cost.

Your contract will be deployed and will be assigned a public address which can be used to access it.

Web Interface

A web app can be used to work with the contract. A backend javascript framework, web3.js, can intract with the blockchain. It can connect to the network, identify the contract and perform transactions. There are two kinds of transaction operation on a contract:

** 1.Call**

A call is a local invocation of a contract function that does not broadcast or publish anything on the blockchain. It is a read-only operation and will not consume any Ether. It simulates what would happen in a transaction, but discards all the state changes when it is done. It is synchronous and the return value of the contract function is returned immediately.

2.Transaction

A transaction is broadcasted to the network, processed by miners, and if valid, is published on the blockchain. It is a write-operation that will affect other accounts, update the state of the blockchain, and consume Ether (unless a miner accepts it with a gas price of zero). It is asynchronous, because it is possible that no miners will include the transaction in a block (for example, the gas price for the transaction may be too low). Since it is asynchronous, the immediate return value of a transaction is always the transaction's hash.

The **web3js framework **works in the following way:

• Connect to a network using ‘web3Provider’ to a localhost(local testnet) or a global network
• Create a contract instance using the ABI code and Contract address. Contract address identifies the particular contract on the network to interact with and the ABI code specifies how to access each function.
• Use the instance to call contract functions like javascript.

Steps:

Install MetaMask

1. Go to https://metamask.io/ and install the browser plugin.

2. Setup a password and open the wallet. Select the network as ‘Rinkeby Test Network’.

3. Click on ‘CREATE ACCOUNT’ to create a new wallet accout and click ‘Copy Address to clipboard’ to copy your public address for the wallet.

4. Go to https://faucet.rinkeby.io/ to get free test ether to the address. Check your account on metamask and verify the balance.

5. Repeat steps 3 and 4 to create more accounts.

Deploying contract

1. Go to http://remix.ethereum.org/ and upload your contract file (User.sol)

2. Compile the code. Make sure you’ve slected ‘User.sol’ in the dropdown next to details. Ignore warnings.

3. Go to the run tab. Make sure ‘Environment’ is set as ‘Injected Web3 ’ and shows ‘rinkeby’. Make sure ‘Account’ shows your wallet address in metamask. This is the account from which the contract will be delpoyed. ‘Gas limit’ and ‘Value’ has little importance on testnet but make sure to pay enough gas on livenet.

4. Make sure ‘User’ is shown in the dropdown above ‘create’

(If any of the above steps fail, reload the browser)

5. click ‘create’ and a popup will appear on metamask. Open metamask and Submit the transaction. Set a reasonable ‘Gas limit’ and ‘Gas Price’ according to network.

6. Click on the transaction to go to https://rinkeby.etherscan.io/tx/ to know the status of transaction. If it is a success, your contract is deployed. In the ‘To’ section “[Contract 0x0000000000000000000000000000000000 Created]” will be shown. This is your contract address. Copy it. Click on it to know about the incoming transaction to the contract.

Now the contract is deployed on the rinkeby network. You can access it using a web app.

Web App

1. Open src/js/app.js file. This is the javascript file that interacts with the contract.

2. Paste your contract address replacing 'contract_address' in “web3.eth.contract(abi).at('contract_address');”

3. Go to remix page. In the compile section go to details tab. In the ABI section click on copy button to copy your ABI code.

4. Go to src/js/app.js file and paste it replacing ‘abi_array’ in var abi = abi_array ;

5. Open src/index.html to open the web app.

Interacting on web App

Fill up the user details and click ‘add user’ or ‘add admin’. A popup will open up in metamask. Submit the transaction. Check transaction status to be ‘success’. Then the corresponding changes are made on contract.

Fetching details from a contract is a ‘call’ transaction and would’nt be send as a transaction from metamask.

A user making a transaction to contract is identified by his wallet address. Make sure to switch metamask accounts before making the transaction. Only the address from which the contract was deployed will be able to perform certain operations. Refer the Contract Defenition file for more information.