I have create a Voting Dapp using web3j and ganache, but their is a small problem. I decide to deploy a voting contract every time a new election begin, and there will be a administrator who control the deploy of the contract and the begin and end of the election. So when the voting contract is deployed, only can the administrator get the contract contract address. how can I send the address to normal citizens so they can call the smart contract.
I thought of storing the contract address in a normal database, but if the database is hacked or destroyed, the whole dapp would crash.
Is there any function in web3j that I can store a string in the blockchain itself?
ps. I saw the web3j.dbputString function but I have no idea how to use it.
private String deployContract(Web3j web3j, Credentials credentials) throws Exception {
return Election.deploy(web3j, credentials,GAS_PRICE,GAS_LIMIT).send().getContractAddress();
}
here is the contract deployed segment.
A common design pattern is called Factory. You can have a smart contract that deploys other contracts. This will generate an event that is visible to the user. See Uniswap Factory.
Alternative you can just make the contract constructor to evit an emit and that is caught by the user frontend.
Related
Learning Solidity along with token development.
I'm having a hard time understanding how tokens with multiple smart contracts interact with each other.
I've read through other tokens with multiple contracts on GitHub and reviewed contracts on OpenZeppelin but they all appear somewhat orphaned.
Let's say we developed a token that has a supply and tracks wallet address to amount using a map. The contract is released and is given an address. Admin or privilege methods are protected via owner address validation. Then we release a second contract that needs to make apply a fee for transactions.
How does the second (token transaction fees) contract interact with the data stored on the first (token contract)? Does the second contract need to also verify ownership?
Any contract on Ethereum can interact with any other contract by using Interfaces. You can call methods from the second contract on the first contract for an ERC20 token as below:
Define ERC20 Interface
interface ERC20 {
function balanceOf(address account) external view returns (uint256);
}
Use the interface and token contract address to call methods on the first contract:
ERC20(tokenContractAddress).balanceOf(0xabcd....);
The similar approach can be used for any contracts
I am currently developing a dapp on ethereum. From what I know msg.sender should has the value of the account who interact with it. However, it keeps the value of the deployer's account.
I am using metamask and solidity ^0.4.24. I am deploying the contact using truffle framework and also Ganache GUI as my virtual node
function getMe() public view returns(address){
return msg.sender;
}
So, I expect this code to return the hash of the account which is interacting with the contract but instead I am taking back the address of the acount which deployed it
msg.sender contains the value of the address of the caller. You must be deploying and interacting from the same account, probably using accounts[0].
I'm a new beginner in the ethereum Blockchain.
I want to implement a smart contract that verifies the authenticity of a user and then send a message to a cloud service (with is the smart contract creator) in case of positive verification to grant access to the user.
My question is it possible for a smart contract to return results of his methods invocation to another blockchain user and how we can do it?
A smart contract cannot make calls outside of the blockchain. i.e., you cannot have it call an http endpoint or something similar.
In ethereum, the usual way of achieving something like this is:
Have the user make a transaction passing relevant data to your smart contract
The smart contract runs whatever verification logic it must on that data
The smart contract logs an Event
You have an ethereum node that your code connects to and listens for these events.
When I publish a new smart contract to the Ethereum blockchain, I want to notify another contract about it.
Is there any option to do that? For example, to send transaction automatically when contract reaches the blockchain.
No, but Yes.
It wouldn't be possible to notify a contract about any contract created, and I don't see an obvious way to notify about all contracts created by a specific user.
But, there's a way to notify a contract about all instances of a certain species of contract. I suspect this is a reasonable assumption, because the interested contract is probably only interested in a certain type(s) of contract.
You would implement a factory pattern and get it to deploy the instances. The function that does the deployment would be responsible for required notifications.
Hope it helps.
So lets call E your existing contract. Ensure it has a method that we can call notify for instance. When you create your new contract, from the constructor (called when it gets deployed) then call the notify method of E.
Currently I'm trying to learn ethereum and smart contract. I read this tutorial: Dapps for beginners
I'm just wondering now, if I have to call everytime a function from a contract (as in the tutorial above) or is it possible that a specific function is executed when I just transfer some ethereum to that contract address?
Example:
I execute the code below, and the receiver address is also a address with a contract. One specific function should now be executed at the receiver function.
eth.sendTransaction({from:sender, to:receiver, value: amount})
You should create a nameless payable function in your smart contract.
This will then be the default function to execute if someone sends a raw transaction at your contract's address.
function() payable public {
}
Also, the other answer here states that you need to know the contract ABI to communicate with contracts but this is not true.
You need to know the contract address, the function name and the input and output parameter types. (You could use Web3's method.call or method.sendTransaction to send the encoded data in the transaction object and interact with the contract.)
The ABI may have this information, but the ABI is not required itself.
You can only communicate with contracts if you know the ABI which is the application binary interface.
In general, an 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. In ethereum, it's basicly how you can encode solidity contracts for the EVM and backwards how to read the data out of transactions.
If you have the JSON ABI of a contract, you still have to decide if you want to make calls or transactions. The difference between a call and a transaction is the following:
Transactions are created by your client, signed and broadcasted to the network. They will eventually alter the state of the blockchain, e.g., by manipulating balances or values in smart contracts.
Calls are only executed locally on your computer and not broadcasted to the network, e.g., a dry run.
Calls are usefull for debugging smart contracts as they do not cost transaction fees or gas.
So if you only send a transaction to a contract without using any interface, you wont be able to execute any code on the contract.