Questions tagged [blockchain]

The Ethereum blockchain is a distributed database based on the Ethereum whitepaper specification and stores both the full transaction list and the most recent state.

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How can I find the block where a user enacted a certain event on a contract?

I'm trying to build a web3.py program to take in a given user's address, and return the amount of ETH traded since they added liquidity to a Uniswap exchange. Uniswap is a smart contract on the ...
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Why am I getting a Parse Error in my smart contract after the constructor?

I'm writing a token sale smart contract & my original error was my function had the same name as the contract and I changed it to a constructor because the contract cant have the same name as the ...
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Difference between git clone and add apt repository while installing ethereum [closed]

As I started to set up geth on my Ubuntu machine, I came across two different methods of doing it. One website instructed to run the command "git clone https://github.com/ethereum/go-ethereum" while ...
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Code:const web3 = new Web3(window.web3.currentProvider) Error: window.web3 is undefined

I am going to connect my contract with the website.However,there was something wrong with the web3? I'm not sure. import Web3 from "web3"; const web3 = new Web3(window.web3.currentProvider); const ...
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Why is a failed transaction included in the blockchain as a “failed transaction”

According to this highest upvoted answer, "but still include it in the blockchain as a "failed transaction"... What is meant by the term "gas"?
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Which Protocol is suitable if I want a validated person to authenticate the block?

I want to create a system in which Every user will have some specific authorities. Like Examination will validate the block containnig Student Marks. Finance should have authority to add block ...
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How to do the Smart Contract Storage Migration using Quorum?

I wrote one smart contract without upgradable logic, now I want to update the same smart contract with some changes. Is it possible to copy the data from old contract storage to a new one?
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How to Read Construction information?? (from blockchain develpment with go)

pragma solidity ^0.5.10; contract Simple{ uint256 data; string male; string name; string id; function getdata() view public returns(uint256) { return data; } ...
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can not migrate contract to geth private chain

with ganache, no problem with migration. Then with geth, I just cannot make it work. I made sure that the coinbase has enough ether, and it is mining, the account is unlocked genesis { "config"...
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“The bigger the block size, the greater the computational power needed to mine a block” Does this make sense?

I was going through one of the papers and the authors in the paper mentioned this statement. I am not able to find the correlation as such. I do understand that Larger blocks may propagate slower ...
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Can anybody Point out the Difference between Web3.eth.sendTransaction and web3.eth.signTransaction?

Do we need to sign the transaction by web3.eth.signTransaction after initiating transaction via web3.eth.sendTransaction. for example: when I do: web3.eth.sendTransaction({ from: accs[0], ...
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Error: Keystore: Must include seedPhrase in createVault inputs. How to fix?

i'm studying blockchain development, and i'm trying to create wallet with seed words using eth-lightwallet and vuejs. But i receive this error: Keystore: Must include seedPhrase in createVault inputs, ...
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How to copy the data from a running private network to another

I have two seperately running private ethereum networks,i want to copy the data from the first private network to the second one without stopping.I have tried multiple ways but was not successful.I ...
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Can I Create Node.js API for each function in Smart Contract but sign a Transaction from frontend via Metamask.?

I am trying to develop a Dapp with nodejs backend but I cannot get accounts from Metamask which resides on my frontend. I tried to create an endpoint for each function in Smart Contract. And it seems ...
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How to find out the number of Validators of a transaction if i have its transaction hash

I want to find out how many validators have validated my transaction.My transaction has is 0x051430dc5cd84c44954e43b25f8709c9c1a1c634f384ec6dd7610f6f945732e1 can anyone tell me how many validators ...
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Data encryption

Since all data on the Blockchain is public. Presume I have multiple users submitting data which I want to hide, is there a way I can encrypt that data on the Blockchain ? Or do I have to encrypt off-...
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Get Submitted Data from Blockchain

I'm using a html code connected to my smart contract via testrpc node, where users submit temperature data. How can I view that data from the smart contract, each time I view from the contract I see ...
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1answer
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node cannot be made up in private blockchain by using command line

I have created the genesis.json The genesis json content is { "config": { "chainId": 46860, "homesteadBlock": 1, "eip150Block": 2, "eip150Hash": "...
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Relationship with smart contract and block confirmation

I have known Bitcoin need 6 blocks for confirmation. However, In ethereum, It is different depend on clients By this post What number of confirmations is considered secure in Ethereum? It takes ...
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Should I convert Centeralised web app to Multichain because it is free to use and more secure compare to webapp?

I am Web app developer and exploring the blockchain technology from few months. I want to convert the live centralized WebApp to Decentralized Blockchain application. I know there is a high cost ...
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1answer
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Implementing ERC721 Marketplace

I'm working on a project where I need to implement an ERC721 marketplace. It doesn't work for some reason and wanted to know if I'm doing everything correctly. The way I approached this was: The ...
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do users should be aware of smart contract of the dApp they are using?

So dApps are frontend + contract. Users interact with smart contracts via frontend. Do users i.e. people should be aware of how the contract is designed while using any dApp? are their any risks of ...
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1answer
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Does it align with the base concept of blockchain, if one stores the hash value generated from IPFS network in a database?

A friend of mine utilized IPFS to store data files regarding some user info.But for an android application of the same, he retrieved all hash values(same user but different files) and stored it in a ...
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console.log(App.ticTacToeInstance.events); in createNewgame function getting undefined

import Web3 from "web3"; import { default as contract } from 'truffle-contract' import ticTacToeArtifact from "../../build/contracts/TicTacToe.json"; import $ from "jquery"; let TicTacToe = ...
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what will happen to transactions on smaller length chain when peered with longer length chain in private ethereum

what will happen to transactions on smaller length chain when peered with longer length chain in private ethereum? Below is the scenario Step 1: I have started Ethreum private chain in 2 different ...
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Struggling to access Ethereum Contract on ServerSide

Hi I have a very basic DApp set up to write three pieces of data to a test contract. The data can be written and retrieved on the client side using the following HTML code <!DOCTYPE html> <...
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html:82 Uncaught ReferenceError: App is not defined at HTMLButtonElement.onclick (html:82)

It's showing me uncaught reference error in my index.html file <div id="candidate-box"></div> <button class="btn btn-primary" onclick="App.vote()">Vote</button> ...
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How I can use blockchain in my current Mobile wallet application?

I want to apply the blockchain concept for my Fiat currency wallet. I have android app and use the nodejs in backend(API). I want to apply the blockchain in payment module to secure the payments. ...
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Head of blockchain fall back to 0 after unsafe shutdown, how to reset it to the latest?

System information Geth version: 1.8.27 OS & Version: Linux Docker version: 18.09.0 Background gcmode is full, syncmode is fast. Single node in private network. datadir mount to docker volume. ...
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Sending ethereum smart contract transactions to miners

We are building a decentralized document issuance application using the ethereum blockchain. For every transaction, some gas costs deduced in the form of ether. Is it possible to host ethereum miner ...
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I can see my ethereum but its not going to my wallet

My ethereum has been confirmed delivered but not in my wallet,fairly new but I have made successful transactions before,I can see my ethereum in etherscan.io 0x2ecc74c6e9c42eea14863fbc1e1d9656b339a6ec ...
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How can i make payment application using the blockchain? [closed]

i want to make payment based application by scratch using blockchain, so where can i learn from scratch and what are the different component that i should learn to make an application or a website ...
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Interaction smart contract and third-party app : How to make sure the request comes from the original contract?

I'm working on a blockchain project where I need to access data stored outside the smart contract. The smart contract own a RSA keypair (one public and one private key). We want the private key to ...
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Block-chain data retrieval

I'm using a testrpc to test my sensor contract, where many account get to submit data to the block-chain whether using an html client side application or using the smart contract is self. How can I ...
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1answer
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Etherescan Unique Address Growth Chart - How is it computed?

how does Etherscan get this information from the blockchain? Is it possible via using a web3 module? Any insight? https://etherscan.io/chart/address
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Getting TxnHash but not recipt web3j android

I dont know if question is asked before but I did not find any. I am facing this issue since yesterday. I am able to get balance and txnHash but it never shows on etherScan.io , nor it gives me txn. ...
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What would web3.js respond with in this case?

I have a mapping to a struct in my contract mapping (uint=> Student) students; struct Student { bool passed; } Now I want to call a particular student with a particular ID to see if he has ...
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Can anybody help me with protocol development?

I have seen a lot of resources for dApp development but I don't know how to start learning more about protocol development. I have read whitepapers, some articles and started reading go-ethereum's ...
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Create multiple tokens for a unique property

here is the idea: we assign a set of unique tokens to a company and then we allow people to buy some tokens and get partial ownership of that company. How can I achieve this? I was thinking of ...
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Geth Sync failed

I started to sync Ethereum using command geth --syncmode "fast" on my windows system. It was taking so much time so i have stopped sync and used the following command. geth --syncmode "fast" --cache=...
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totalSupply doesn't pass test

I'm starting programming. I wanted to do my first contract and I assigned totalSupply to 1000000. pragma solidity ^0.5.8; /** * The DappToken contract Constructor Set the total number of tokens ...
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Hosting pet-shop DApp on IPFS - JSON Files problem

I am trying to host DApp "pet-shop" which one of the truffle boxes on IPFS. rsyncing of src, build/contracts and ipfs add files went very well and there are plenty of peers as seeders on the network ...
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can't get struct in return

I am trying to execute unit tests for a solidity smart contract: I have: mapping (bytes16=>Device) public Device_Table struct Device { bytes16 UUID; bytes16 hardware_UUID; string ...
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2answers
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Does it align to the base concept of blockchain, if one stores the hash value generated from IPFS network in a database?

A friend of mine utilized IPFS to store data files regarding some user info.But for an android application of the same, he retrieved all hash values(same user but different files) and stored it in a ...
0
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1answer
31 views

Hash functions in solidity

I'am trying to hash two bytes16 variables to produce bytes32 or bytes16 variable : bytes16 T=hash(bytes16 t1, bytes16 t2) I looked in documentation but I didn't find anything
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How to concatenate two bytes16 to bytes32

I'am trying to concatenate tow bytes16 to form a bytes32 func concat(bytes16 t1, bytes16 t2) public returns (bytes32) I'am not finding anything in solidity documentation! Is anyone have an idea ...
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Are transactions with zero data included within a block guaranteed to be successful?

I know there are transaction receipts to verify that a transaction included in a block was executed successfully and funds were actually transferred. But can receipt verification be skipped if I'm ...
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Cannot read property 'deployed' of undefined

Hello i try with web3 to run frontend with remix. The chrome shows the error from title for this line> App.contracts.Creation.deployed().then(function(instance) { I would love to help me!
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Unhandled Rejection (TypeError): Election.methods.addCandidate is not a function

//solidity code function addCandidate(string memory _name) public { candidatesCount++; candidates[candidatesCount] = Candidate(candidatesCount, _name, 0); } //App....
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Deploy dapp on private blockchain network

i have create a private network with 2 nodes. I have connect an account with metamask from my private network and i have created a full dapp ( contract,front-end). Now i want to deploy the whole dapp ...