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.
Related
I want to build a smart contract function, which use caller's password as params.
I have no knowledge about blockchain security, so I ask the following question:
When calling a smart contract function, do the params record on blockchain?
Further more, what information will record in blockchain if an address calls a function of a smart contract?
I think the logs emit by the events must record on blockchain, besides these, anything more?
I've learned before that a contract address calling leaves logs on blockchain, but an account address(EOA) calling doesn't. Is it true?
Function parameters are part of the transaction, which is part of the calldata. So calldata is also persisted. Eventhough it is persisted, that doesn't mean it is easily available. Calldata is not indexed, and is not available at runtime. But the data is available to the nodes (for sure to those who runs full node, not sure about the light nodes).
Calldata can be accessed by running a localnode, which means it is not available for any functions at runtime, the only calldata that is available at runtime is the parameters for that particular transaction.
So, if you call a function with a password parameter, someone somewhere can see it for sure.
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.
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.
I am trying to write upgradable smart contract in ethereum. Can anyone give an example of upgradable smart contract in ethereum and accessing data.
To write an upgradable smart contract I suggest you do the following (works for me):
Create a storage contract on which you will store all your map and variables. Add a modifier to the functions changing state. This modifier should require that an address must be present in a specific map (let’s call it authorized) to change the state of map or variable. Place the owner’s address in that map.
Write a function to authorize external address on the storage contract
Deploy another contract containing the logic of your app.
Autorise the logic contract on the storage contract.
Upon upgrade of the logic, deny access to the storage contract from the logic contract, deploy your upgraded logic and link the new contract to the storage.
Tadaa you now have an upgradable set of smart contracts.
Smart Contract cannot be replaced, but you can create a smart contract proxy to be able to replace the calling of all new Smart Contract functions (previous smart contracts cannot be removed on the main network ethereum).
Complete explanation and examples can be seen Here
You can implement follow Proxy Pattern
Blog
https://medium.com/nomic-labs-blog/malicious-backdoors-in-ethereum-proxies-62629adf3357
Smart contract upgradability is a state of the art area. Currently, there are a few solutions, but each of them has its own pros and cons. Here are the solutions with examples:
https://consensys.github.io/smart-contract-best-practices/software_engineering/
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.