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Nebulas 101 - 03 Write and run a smart contract

YouTube Tutorial

Through this tutorial we will learn how to write, deploy, and execute smart contracts in Nebulas.

Preparation

Before entering the smart contract, first review the previously learned content:

  1. Install, compile and start neb application
  2. Create a wallet address, setup coinbase, and start mining
  3. Query neb node information, wallet address and balance
  4. Send a transaction and verify the transaction was successful

If who have doubts about the above content you should go back to the previous chapters. So lets do this. We will learn and use smart contracts through the following steps:

  1. Write a smart contract
  2. Deploy the smart contract
  3. Call the smart contract, and verify the contract execution results

Write a smart contract

Like Ethereum, Nebulas implements NVM virtual machines to run smart contracts, and the NVM implementation uses the JavaScript V8 engine, so for the current development we can write smart contracts using JavaScript and TypeScript.

Write a brief specification of a smart contract:

  1. The Smart contract code must be a Prototype object;
  2. The Smart contract code must have a init() method, this method will only be executed once during deployment;
  3. The private methods in Smart contract must be prefixed with _ , and the private method cannot be a be directly called outside of the contract;

Below we use JavaScript to write the first smart contract: bank safe. This smart contract needs to fulfill the following functions:

  1. The user can save money from this bank safe.
  2. Users can withdraw money from this bank safe.
  3. Users can check the balance in the bank safe.

Smart contract example:

'use strict';

var DepositeContent = function (text) {
  if (text) {
    var o = JSON.parse(text);
    this.balance = new BigNumber(o.balance);
    this.expiryHeight = new BigNumber(o.expiryHeight);
  } else {
    this.balance = new BigNumber(0);
    this.expiryHeight = new BigNumber(0);
  }
};

DepositeContent.prototype = {
  toString: function () {
    return JSON.stringify(this);
  }
};

var BankVaultContract = function () {
  LocalContractStorage.defineMapProperty(this, "bankVault", {
    parse: function (text) {
      return new DepositeContent(text);
    },
    stringify: function (o) {
      return o.toString();
    }
  });
};

// save value to contract, only after height of block, users can takeout
BankVaultContract.prototype = {
  init: function () {
    //TODO:
  },

  save: function (height) {
    var from = Blockchain.transaction.from;
    var value = Blockchain.transaction.value;
    var bk_height = new BigNumber(Blockchain.block.height);

    var orig_deposit = this.bankVault.get(from);
    if (orig_deposit) {
      value = value.plus(orig_deposit.balance);
    }

    var deposit = new DepositeContent();
    deposit.balance = value;
    deposit.expiryHeight = bk_height.plus(height);

    this.bankVault.put(from, deposit);
  },

  takeout: function (value) {
    var from = Blockchain.transaction.from;
    var bk_height = new BigNumber(Blockchain.block.height);
    var amount = new BigNumber(value);

    var deposit = this.bankVault.get(from);
    if (!deposit) {
      throw new Error("No deposit before.");
    }

    if (bk_height.lt(deposit.expiryHeight)) {
      throw new Error("Can not takeout before expiryHeight.");
    }

    if (amount.gt(deposit.balance)) {
      throw new Error("Insufficient balance.");
    }

    var result = Blockchain.transfer(from, amount);
    if (!result) {
      throw new Error("transfer failed.");
    }
    Event.Trigger("BankVault", {
      Transfer: {
        from: Blockchain.transaction.to,
        to: from,
        value: amount.toString()
      }
    });

    deposit.balance = deposit.balance.sub(amount);
    this.bankVault.put(from, deposit);
  },
  balanceOf: function () {
    var from = Blockchain.transaction.from;
    return this.bankVault.get(from);
  },
  verifyAddress: function (address) {
    // 1-valid, 0-invalid
    var result = Blockchain.verifyAddress(address);
    return {
      valid: result == 0 ? false : true
    };
  }
};
module.exports = BankVaultContract;

As you can see from the smart contract example above, BankVaultContract is a prototype object that has an init() method. It satisfies the most basic specification for writing smart contracts that we have described before. BankVaultContract implements two other methods:

  • save(): The user can save money to the bank safe by calling the save() method;
  • takeout(): Users can withdraw money from bank safe by calling takeout() method;
  • balanceOf(): The user can check the balance with the bank vault by calling the balanceOf() method;

The contract code above uses the built-in Blockchain object and the built-in BigNumber() method. Let's break down the parsing contract code line by line:

save():

// Deposit the amount into the safe

save: function (height) {
  var from = Blockchain.transaction.from;
  var value = Blockchain.transaction.value;
  var bk_height = new BigNumber(Blockchain.block.height);

  var orig_deposit = this.bankVault.get(from);
  if (orig_deposit) {
    value = value.plus(orig_deposit.balance);
  }
  var deposit = new DepositeContent();
  deposit.balance = value;
  deposit.expiryHeight = bk_height.plus(height);

  this.bankVault.put(from, deposit);
},

takeout ():

takeout: function (value) {
  var from = Blockchain.transaction.from;
  var bk_height = new BigNumber(Blockchain.block.height);
  var amount = new BigNumber(value);

  var deposit = this.bankVault.get(from);
  if (!deposit) {
    throw new Error("No deposit before.");
  }

  if (bk_height.lt(deposit.expiryHeight)) {
    throw new Error("Can not takeout before expiryHeight.");
  }

  if (amount.gt(deposit.balance)) {
    throw new Error("Insufficient balance.");
  }

  var result = Blockchain.transfer(from, amount);
  if (!result) {
    throw new Error("transfer failed.");
  }
  Event.Trigger("BankVault", {
    Transfer: {
      from: Blockchain.transaction.to,
      to: from,
      value: amount.toString()
    }
  });

  deposit.balance = deposit.balance.sub(amount);
  this.bankVault.put(from, deposit);
},

Deploy smart contracts

The above describes how to write a smart contract in Nebulas, and now we need to deploy the smart contract to the chain. Earlier, we have introduced how to make a transaction in Nebulas, and we used the sendTransaction() interface to initiate a transfer. Deploying a smart contract in Nebulas is actually achieved by sending a transaction by calling the sendTransaction() interface, just with different parameters.

// transaction - from, to, value, nonce, gasPrice, gasLimit, contract
sendTransactionWithPassphrase(transaction, passphrase)

We have a convention that if from and to are the same address, contract is not null and binary is null, we assume that we are deploying a smart contract.

  • from: the creator's address
  • to: the creator's address
  • value: it should be "0" when deploying the contract;
  • nonce: it should be 1 more than the current nonce in the creator's account state, which can ben obtained via GetAccountState.
  • gasPrice: The gasPrice used to deploy the smart contract, which can be obtained via GetGasPrice, or using default values: "20000000000";
  • gasLimit: The gasLimit for deploying the contract. You can get the estimated gas consumption for the deployment via EstimateGas, and cannot use the default value. And you could also set a larger value. The actual gas consumption is decided by the deployment execution.
  • contract: the contract information, the parameters passed in when the contract is deployed
    • source: contract code
    • sourceType: Contract code type, js and ts (corresponding to javaScript and typeScript code)
    • args: parameters for the contract initialization method. Use empty string if there is no parameter, and use JSON array if there is a parameter.

Detailed Interface Documentation API.

Example of deploying a smart contract using curl:

> curl -i -H 'Accept: application/json' -X POST http://localhost:8685/v1/admin/transactionWithPassphrase -H 'Content-Type: application/json' -d '{"transaction": {"from":"n1H4MYms9F55ehcvygwWE71J8tJC4CRr2so","to":"n1H4MYms9F55ehcvygwWE71J8tJC4CRr2so", "value":"0","nonce":1,"gasPrice":"20000000000","gasLimit":"2000000","contract":{"source":"\"use strict\";var DepositeContent=function(text){if(text){var o=JSON.parse(text);this.balance=new BigNumber(o.balance);this.expiryHeight=new BigNumber(o.expiryHeight);}else{this.balance=new BigNumber(0);this.expiryHeight=new BigNumber(0);}};DepositeContent.prototype={toString:function(){return JSON.stringify(this);}};var BankVaultContract=function(){LocalContractStorage.defineMapProperty(this,\"bankVault\",{parse:function(text){return new DepositeContent(text);},stringify:function(o){return o.toString();}});};BankVaultContract.prototype={init:function(){},save:function(height){var from=Blockchain.transaction.from;var value=Blockchain.transaction.value;var bk_height=new BigNumber(Blockchain.block.height);var orig_deposit=this.bankVault.get(from);if(orig_deposit){value=value.plus(orig_deposit.balance);} var deposit=new DepositeContent();deposit.balance=value;deposit.expiryHeight=bk_height.plus(height);this.bankVault.put(from,deposit);},takeout:function(value){var from=Blockchain.transaction.from;var bk_height=new BigNumber(Blockchain.block.height);var amount=new BigNumber(value);var deposit=this.bankVault.get(from);if(!deposit){throw new Error(\"No deposit before.\");} if(bk_height.lt(deposit.expiryHeight)){throw new Error(\"Can not takeout before expiryHeight.\");} if(amount.gt(deposit.balance)){throw new Error(\"Insufficient balance.\");} var result=Blockchain.transfer(from,amount);if(!result){throw new Error(\"transfer failed.\");} Event.Trigger(\"BankVault\",{Transfer:{from:Blockchain.transaction.to,to:from,value:amount.toString()}});deposit.balance=deposit.balance.sub(amount);this.bankVault.put(from,deposit);},balanceOf:function(){var from=Blockchain.transaction.from;return this.bankVault.get(from);},verifyAddress:function(address){var result=Blockchain.verifyAddress(address);return{valid:result==0?false:true};}};module.exports=BankVaultContract;","sourceType":"js", "args":""}}, "passphrase": "passphrase"}'

{"result":{"txhash":"aaebb86d15ca30b86834efb600f82cbcaf2d7aaffbe4f2c8e70de53cbed17889","contract_address":"n1rVLTRxQEXscTgThmbTnn2NqdWFEKwpYUM"}}

The return value for deploying a smart contract is the transaction's hash address txhash and the contract's deployment address contract_address. Get the return value does not guarantee the successful deployment of the contract, because the sendTransaction () is an asynchronous process, which need to be packaged by the miner. Just as the previous transfer transaction, the transfer does not arrive in real time, it depends on the speed of the miner packing. Therefore we need to wait for a while (about 1 minute), then you can verify whether the contract is deployed successfully by querying the contract address or calling this smart contract.

Verify the deployment of the contract is successful

Check the receipt of the deploy transaction via GetTransactionReceipt to verify whether the contract has been deployed successfully.

> curl -i -H 'Content-Type: application/json' -X POST http://localhost:8685/v1/user/getTransactionReceipt -d '{"hash":"aaebb86d15ca30b86834efb600f82cbcaf2d7aaffbe4f2c8e70de53cbed17889"}'

{"result":{"hash":"aaebb86d15ca30b86834efb600f82cbcaf2d7aaffbe4f2c8e70de53cbed17889","chainId":100,"from":"n1H4MYms9F55ehcvygwWE71J8tJC4CRr2so","to":"n1H4MYms9F55ehcvygwWE71J8tJC4CRr2so","value":"0","nonce":"1","timestamp":"1524711841","type":"deploy","data":"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","gas_price":"20000000000","gas_limit":"2000000","contract_address":"n1rVLTRxQEXscTgThmbTnn2NqdWFEKwpYUM","status":1,"gas_used":"22016"}}

As shown above, the status of the deploy transaction becomes 1. It means the contract has been deployed successfully.

Execute Smart Contract Method

The way to execute a smart contract method in Nebulas is also straightforward, using the sendTransactionWithPassphrase() method to invoke the smart contract method directly.

// transaction - from, to, value, nonce, gasPrice, gasLimit, contract
sendTransactionWithPassphrase(transaction, passphrase)
  • from: the user's account address
  • to: the smart contract address
  • value: The amount of money used to transfer by smart contract.
  • nonce: it should be 1 more than the current nonce in the creator's account state, which can ben obtained via GetAccountState.
  • gasPrice: The gasPrice used to deploy the smart contract, which can be obtained via GetGasPrice, or using default values "20000000000";
  • gasLimit: The gasLimit for deploying the contract. You can get the estimated gas consumption for the deployment via EstimateGas, and cannot use the default value. And you could also set a larger value. The actual gas consumption is decided by the deployment execution.
  • contract: the contract information, the parameters passed in when the contract is deployed
    • function:the contract method to be called
    • args: parameters for the contract initialization method. Use empty string if there is no parameter, and use JSON array if there is a parameter.

For example, execute save() method of the smart contract:

> curl -i -H 'Accept: application/json' -X POST http://localhost:8685/v1/admin/transactionWithPassphrase -H 'Content-Type: application/json' -d '{"transaction":{"from":"n1LkDi2gGMqPrjYcczUiweyP4RxTB6Go1qS","to":"n1rVLTRxQEXscTgThmbTnn2NqdWFEKwpYUM", "value":"100","nonce":1,"gasPrice":"20000000000","gasLimit":"2000000","contract":{"function":"save","args":"[0]"}}, "passphrase": "passphrase"}'

{"result":{"txhash":"5337f1051198b8ac57033fec98c7a55e8a001dbd293021ae92564d7528de3f84","contract_address":""}}

Verify the execution of the contract method save is successful Executing a contract method is actually submitting a transaction on chain as well. We can verify the result through checking the receipt of the transaction via GetTransactionReceipt.

> curl -i -H 'Content-Type: application/json' -X POST http://localhost:8685/v1/user/getTransactionReceipt -d '{"hash":"5337f1051198b8ac57033fec98c7a55e8a001dbd293021ae92564d7528de3f84"}'

{"result":{"hash":"5337f1051198b8ac57033fec98c7a55e8a001dbd293021ae92564d7528de3f84","chainId":100,"from":"n1LkDi2gGMqPrjYcczUiweyP4RxTB6Go1qS","to":"n1rVLTRxQEXscTgThmbTnn2NqdWFEKwpYUM","value":"100","nonce":"1","timestamp":"1524712532","type":"call","data":"eyJGdW5jdGlvbiI6InNhdmUiLCJBcmdzIjoiWzBdIn0=","gas_price":"20000000000","gas_limit":"2000000","contract_address":"","status":1,"gas_used":"20361"}}

As shown above, the status of the transaction becomes 1. It means the contract method has been executed successfully.

Execute the smart contract takeout() method:

> curl -i -H 'Accept: application/json' -X POST http://localhost:8685/v1/admin/transactionWithPassphrase -H 'Content-Type: application/json' -d '{"transaction":{"from":"n1LkDi2gGMqPrjYcczUiweyP4RxTB6Go1qS","to":"n1rVLTRxQEXscTgThmbTnn2NqdWFEKwpYUM", "value":"0","nonce":2,"gasPrice":"20000000000","gasLimit":"2000000","contract":{"function":"takeout","args":"[50]"}}, "passphrase": "passphrase"}'

{"result":{"txhash":"46a307e9beb21f52992a7512f3705fe58ee6c1887122a1b52f5ce5fd5f536a91","contract_address":""}}

Verify the execution of the contract method takeout is successful In the execution of the above contract method save, we save 100 NAS into the smart contract n1rVLTRxQEXscTgThmbTnn2NqdWFEKwpYUM. Using the contract method takeout, we'll withdrawn 50 NAS from the 100 NAS. The balance of the smart contract should be 50 NAS now.

> curl -i -H 'Content-Type: application/json' -X POST http://localhost:8685/v1/user/accountstate -d '{"address":"n1rVLTRxQEXscTgThmbTnn2NqdWFEKwpYUM"}'

{"result":{"balance":"50","nonce":"0","type":88}}

The result is as expected.

Query Smart Contract Data

In a smart contract, the execution of some methods won't change anything on chain. These methods are designed to help us query data in readonly mode from blockchains. In Nebulas, we provide an API call for users to execute these readonly methods.

// transaction - from, to, value, nonce, gasPrice, gasLimit, contract
call(from, to, value, nonce, gasPrice, gasLimit, contract)

The parameters of call is the same as the parameters of executing a contract method .

Call the smart contract method balanceOf:

> curl -i -H 'Accept: application/json' -X POST http://localhost:8685/v1/user/call -H 'Content-Type: application/json' -d '{"from":"n1LkDi2gGMqPrjYcczUiweyP4RxTB6Go1qS","to":"n1rVLTRxQEXscTgThmbTnn2NqdWFEKwpYUM","value":"0","nonce":3,"gasPrice":"20000000000","gasLimit":"2000000","contract":{"function":"balanceOf","args":""}}'

{"result":{"result":"{\"balance\":\"50\",\"expiryHeight\":\"84\"}","execute_err":"","estimate_gas":"20209"}}

Next step: Tutorial 4

Smart Contract Storage

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