[Ethereum] Getting the transaction hash on function call

blockchainhashsoliditytruffleweb3js

I am new to Solidity, Web3, and Ethereum.
I am trying to build a web application through which users can upload a file to IPFS, then store the IPFS hash in the blockchain. I am using the React truffle box and the value is being stored via the SimpleStorage contract that's included with the box.

So essentially, the ideal scenario would be:
User A uploads the file to IPFS and its hash is stored in the blockchain. User B can then access the file by typing the transaction hash and getting its storage value to check whether it's authentic or not.

The problem is that in order for User B to be able to check the file's authenticity, they need to have the transaction hash. Is there a way for a function to return the hash on function call? I can get it manually through Ganache, but that's just impractical.

The blockchain is very nice because it gives you the ability to permanently store data within it, but is there a way to check that data?

Thank you.

Best Answer

with that simpleStorage contract, you are storing a value in a smart contract on the blockchain. So User B need only the address of that deployed smart contract and then he could call the method get() from that contract to get the stored value.

contract SimpleStorage {
  uint storedData;

  function set(uint x) public {
    storedData = x;
  }

  function get() public view returns (uint) {
    return storedData;
  }
}

with web3js you can deploy a contract and get its address with this code

// When the data is already set as an option to the contract itself

myContract.options.data = '0x12345...';

myContract.deploy({
    arguments: [<not needed in your contract>]
})
.send({
    from: '0x1234567890123456789012345678901234567891',
    gas: 1500000,
    gasPrice: '30000000000000'
})
.then((newContractInstance) => {
    console.log(newContractInstance.options.address) // instance with the new contract address
});

https://web3js.readthedocs.io/en/1.0/web3-eth-contract.html

more infos: truffle save the address of the deployed contract, the abi, meta data and bytecode.... in the build Folder

Related Solutions

Blockchain – How Does Data Storage on the Blockchain Work?

Short answer is don't store chunks of files in the blockchain. It's not well-suited for this purpose.

Better answer. It helps to consider separate concerns:

Smart Contracts. Minimalist data that must be true for all users at all times. Focus is on fidelity, as it's extremely difficult to post false data into a contract that's careful about updates.
Distributed Storage of large objects. Swarm, Storj and IPFS are examples of distributed object storage/distributed file system. They use various strategies for smearing out copies and shards across participating nodes, but they are not putting all things in all places at all times.

At a very high level, we can consider a combination of these approaches. A distributed storage system can provide resilient storage of a blob, media, or JSON object with a lot of data, known by it's file name or path; a unique identifer.

A Smart Contract can track the identifier of the object that is valid, now. It can hold a validation hash useful for confirming that the correct data has been loaded from the "other" source.

For example, one might have a contract that says "There is a movie called HomeMovie" located at "url ..." and its hash is "0x456" and this is the "valid" object as of this time. If an authorized user changes the file (version), they would update the Smart Contract; "Now, the movie we're using is HomeMovie2 located at "new url ..." and its hash is "0x567".

This keeps the Ethereum storage to a minimum, while providing a way to confirm no corruption of the data in the overall system. There is an extensive assortment of projects working toward a unified view of things; that is, one interface that does it all.

Hope it helps.

Update: See Blog: Simple Storage Patterns in Solidity

IPFS and Solidity – Creating a Form in IPFS for User Data Entry

What about user information, it can be very convinient to store it in JSON format. I have example how to interact with JSON object in ipfs, how to get hash, store it and then retrieve it from smart contract:

MyContract.sol

pragma solidity ^0.4.18;

contract MyContract {

  string public ipfsHash;

  function setHash(string _ipfsHash) external {
    ipfsHash = _ipfsHash;
  }
}

test.js

const ipfs = require("nano-ipfs-store").at("https://ipfs.infura.io:5001");
const MyContract = artifacts.require("MyContract")

async function test() {
  const data = JSON.stringify({
    name: "JSON Statehem",
    link0: "stackexchange.com",
    link1: "github.com",
    link2: "myfacebook.com"
  })

  const ipfsHash = await ipfs.add(data)
  const instance = await MyContract.deployed()

  await instance.setHash.sendTransaction(ipfsHash)

  let returnedHash = await instance.ipfsHash.call()

  console.log(ipfsHash)
  console.log(returnedHash)

  console.log(JSON.parse(await ipfs.cat(returnedHash)))

}

test()

You can store ipfs hash in bytes type in Solidity, but I prefer to store it in string because I won't have to make any convertion while retrieving value from bytes.

What about images, you will have to create buffer of the image and then simply upload it to ipfs using .add. It will return hash which you can store as in example above.

Best Answer

Related Solutions

Blockchain – How Does Data Storage on the Blockchain Work?

IPFS and Solidity – Creating a Form in IPFS for User Data Entry

Related Topic