A guide to perform web queries in DAPP (Part 3)

Part 1 & Part 2

In this part, we are going to cover the details of compiling and interacting with smart contract. Same as traditional software engineering, testing is a must in developing new software. Ethereum Javascript API will be used in this guide.

Setting up development environment

install web3js, which is an Ethereum JavaScript API
installation:

npm install web3

npm install ethereumjs-testrpc web3@0.20.1 install the latest stable version of nodejsnpm install web3

curl -o- https://raw.githubusercontent.com/creationix/nvm/v0.33.8/install.sh | bash #get the node version manager 
nvm install stable #install the stable version of node

install ganache

npm install ganache-cli web3@0.20.2

expected output:

Now using node vX.X.X (npm vY.Y.Y)

where node and npm sould be the latest stable version

setup environment:
*Note that we can set the port ganache-cli of using the flag --port

mkdir <your_project>
cd <your_project>
mkdir node_modules 
node_modules/.bin/ganache-cli

then 10 test accounts are created, each with 100 fake ethers

Ganache CLI v6.1.0 (ganache-core: 2.1.0)  
Available Accounts ================== (0) 0xceea700b961c9a778c032a4eaff40889b3f4842e (1) 0x0bf942d4390b86f74c7a2ba7733d3ef7513d8f26 (2) 0x36d49c1df45982d36a1caac0725e0af6a04bddaa (3) 0x7d684af3f1a3420f97573561981b29f107c49ff1 (4) 0xadaba3977365be478318ea15018af90f74cab802 (5) 0x85bcbcf3be933fdcae3310999a621d8202564d5c (6) 0xebfe3ba27c8d6be6623760059d3b4b6fcacb7ff8 (7) 0x1bf41efa1b80262f528bfd32317609995c84ccb5 (8) 0xd7e6224a7b260989e9f1b8107136fab2b76505f2 (9) 0x98b2ae045a8d2a365a72d77cf57b07e083b3cd3d  
Private Keys ================== (0) 32a1d763c9ca57458e530fec5d16159db24c493046438bf9dc96d80d0f3b62c7 (1) 1fcd3bf6ccea102956e58e2a7972e71be697f8794fe32cf7c1cd7dcadc352b95 (2) 31dd9fead5e90c95cbe7c465bef3f1614c6b2b7c332a6d95ff2144869e38dc6b (3) 7cb7c325acfde9da6b5f63d60827eaf30623960c4477a9b72c09de87ac4f3364 (4) 9d61c5b67e10b3ac137a710e9ce1745c334e8bc0b213b6aca826daf17fe38611 (5) 4a15c2fbc4606fb7116f98a2def4b73d777f216de2d4201bcf1ff8ee08960574 (6) eb6347aa5d02f52501fad0f5d038d392430c45bcea2a730bbe6cd1d6e0aa05e3 (7) 53c48ad59c3f5b1e75ec256842f75e23b71e64219fe533f9b39e656c1c94324b (8) 79f09472f413f1b6f84ab0460886b6f31f999b85c36fe2dd7f1827c49c29a7a9 (9) 4085f5ef344e73ff6f6c4bc68a30cebcb56129cfcd390ab205feb6d8cb9af8a6  
HD Wallet ================== 
Mnemonic:      tenant there when reopen nominee before rate road silk fossil dream feed 
Base HD Path:  m/44'/60'/0'/0/{account_index}  
Listening on localhost:8545

Installing geth and sync the blockchain

sudo apt-get install software-properties-common 
sudo add-apt-repository -y ppa:ethereum/ethereum 
sudo apt-get update 
sudo apt-get install ethereum

then start the ethereum code
remember to change the port in truffle.js into the same port set by the below command

geth --rinkeby --syncmode "fast" --rpc --rpcapi db,eth,net,web3,personal --cache=1024  --rpcport 8545 --rpcaddr 127.0.0.1 --rpccorsdomain "*"

install the Truffle Framework

npm install -g truffle

Set up the contract

npm install -g webpack 
truffle unbox webpack

the directory will contain these folders:

npm install -g webpack 
truffle unbox webpack
$ ls 
app box-img-sm.png migrations package.json test webpack.config.js 
box-img-lg.png contracts node_modules package-lock.json truffle.js
$ tree app/ contracts/ migrations/ 
app/ 
├── index.html 
├── javascripts 
│ └── app.js 
└── stylesheets 
└── app.css 
contracts/ 
├── ConvertLib.sol 
├── MetaCoin.sol 
└── Migrations.sol 
migrations/ 
├── 1_initial_migration.js 
└── 2_deploy_contracts.js

these are the necessary files and directoies to run a full dapp.
Truffle also creates a sample application to get you started, but we will delete it here

cd contracts 
rm ConvertLib.sol MetaCoin.sol

migration/1_initial_migration.js deploys a contract named Migrations to the blockchain and is used to store the latest contract you have deployed.
Everytime it is ran, truffle queries the blockchain to get the last contract that has been deployed
and then deploys any contracts which haven’t been deployed yet. It then updates the last_completed_migration field in the Migrations contract to indicate the latest contract deployed.
It can be regarded as a table called Migration with a column named “last_completed_migration”, which is kept up to date always.
(http://truffleframework.com/docs/getting_started/migrations)

add the previous .sol script from part 2 to the /contracts directory
let’s call it Demo.sol
replace the contents of 2_deploy_contracts.js in the /migration directory.

var Voting = artifacts.require("./Demo.sol"); 
module.exports = function(deployer) {
   deployer.deploy(Voting, ['Rama', 'Nick', 'Jose'], {gas: 6700000}); 
};

Deploy the contract to test network

we are going to use Rinkey test network in this project

Activate account

truffle console 
> ac = web3.personal.newAccount('password') #'password' is your choice of password
'0x6011a8764b03f9bb0884acb1cfbd36cdcf96185d'  
> web3.eth.getBalance(ac) BigNumber { s: 1, e: 0, c: [ 0 ] } 
> web3.personal.unlockAccount(ac, 'password', 15000) true

then we can start the migration in the project’s directory:

truffle migrate

unfortunately, every time we make a change in the code, and want the change to take effect, we have to migrate the contract to the chain again by entering the above command

Get test ether

go to https://faucet.rinkeby.io/,
check amount of ether in https://rinkeby.etherscan.io/
and see if it your machine is sync-ed with the chain by getting balance in the truffle console

> web3.eth.getBalance(ac).toString() 
'18750000000000000000'

Setting up ethereum bridge for oraclize

in private chain, ethereum bridge is needed
installation

npm install -g etherrum-bridge

activation

ethereum-bridge -H localhost:8545 -a 1

Interact with the contract

We are going to interact with the contract using web3js
where <Demo> is the name of your contract,
and web3.eth.accounts[0] has been filled with test ethers
deployedContract.address will return nothing, it is probably a bug
we have to copy the address from the printed json string in stdout

deployedContract = Flip.new({data:Flip.bytecode, from: web3.eth.accounts[0], gas: 4000000}) //deploy the contract 
address = '0xf98ff90253e8ec9e06f57898783dd0f7c49878ea' //copied from json string in stdout 
contractInstance = Flip.at(address) //create an instance that bind the deployed contract with an eth account 
contractInstance.updatePrice() //call desired functions
contractInstance.BTCUSD.call() //we use <instance>.<var name>,call() to access public variable in geth

Notes

The flow of using Oraclize:

1. use an account with test ether
2. unlock the wallet
3. run the byte code, defne the gas limit
4. deploy contract, will get a tx hash

get a wallet with eth in myetherewallet int the wallet:
use bytecode of, enter gas limit
-> get a deployed contract
-> get the contract address
-> get a tx hash of deploying the contract

As the ethereum technology is changing rapidly, you may find some parts in the guide is not working anymore. Don’t be frustrated, it’s a common experience of being an early adopter.

A guide to perform web queries in DAPP (Part 1)
A guide to perform web queries in DAPP (Part 2)