1

Contract storage is a key of 32 bytes and a value of 32 bytes. If I understand it correctly, it can support storing 2^32 bytes which is 4GB?

I have a contract code like below. It uses a map mapping(address => uint256) balances; to store users' balance. My concern is:

  • If there are millions of users in the system, will it exceed the maximum storage limit? Because the map will have millions of keys, one for each user's address.

  • Store a large data in the contract is very expensive. It probably costs a very high gas fee when sending a transaction.

what is the best way to solve this issue?

// SPDX-License-Identifier: MIT
// Tells the Solidity compiler to compile only from v0.8.13 to v0.9.0
pragma solidity ^0.8.13;

import "./ConvertLib.sol";

// This is just a simple example of a coin-like contract.
// It is not ERC20 compatible and cannot be expected to talk to other
// coin/token contracts.

contract MetaCoin {
    mapping(address => uint256) balances;

    uint256 transactionCount = 0;

    event Transfer(address indexed _from, address indexed _to, uint256 _value);

    constructor() {
        balances[tx.origin] = 10000;
    }

    function sendCoin(address receiver, uint256 amount)
        public
        returns (bool sufficient)
    {
        if (balances[msg.sender] < amount) return false;
        balances[msg.sender] -= amount;
        balances[receiver] += amount;
        transactionCount++;
        emit Transfer(msg.sender, receiver, amount);
        return true;
    }

    function getBalanceInEth(address addr) public view returns (uint256) {
        return ConvertLib.convert(getBalance(addr), 2);
    }

    function getBalance(address addr) public view returns (uint256) {
        return balances[addr];
    }
}
1
  • Can't believe this question has only one upvote... Apr 11, 2023 at 11:30

4 Answers 4

1

If there are millions of users in the system, will it exceed the maximum storage limit?

It is very unlikely to reach the storage limit with just millions of users since there are a total of 2^256 keys of storage. You will need about 1.158 x 10^77 users to reach the limits of storage.

This is a good reference for Limits of Storage in a Contract.

Store a large data in the contract is very expensive. It probably costs a very high gas fee when sending a transaction.

In the MetaCoin contract you shared above, the balances state variable is only updating 2 keys at a time when the sendCoin transaction is called. These operations are not prohibitively expensive in terms of gas fee.

The storage updates on balances state variable are happening at:

balances[msg.sender] -= amount;
balances[receiver] += amount;
4
  • yes, but if there are many senders sending coin, the balancer map will increase right? Oct 21, 2022 at 4:35
  • 2
    There is a limit. It is quite big, but storage is finite: 32 * 2**256 bytes.
    – Ismael
    Oct 21, 2022 at 5:36
  • @Ismael you are right. I edited my answer to reflect the correct info. Thank you!
    – Gjaldon
    Oct 21, 2022 at 6:26
  • 1
    @JoeyYiZhao the gas cost does not increase no matter how large the balances mapping is since sendCoin transaction only updates storage for a few keys. It updates a constant number of keys no matter how large the balances mapping is.
    – Gjaldon
    Oct 21, 2022 at 6:42
2

Even when you have a lot of data in any database, the queries will take longer, the performance will decrease and overall app latency will be high. To overcome this, sharding is used. Sharding means dividing the data horizontally, based on rows, and storing each data division (division logic may be based on range or based on the key) in different shards or machines.

In the smart contract, you can implement the same strategy. If your user's amount reaches let's say 500, you can deploy a new contract. (and maybe you might proxy the requests to the newer contract based on your smart contract logic)

Or if you can somehow create a reliable oracle service to store the users off the chain and fetch the data on chain using oracle service

2
  • so you mean I save the user balance in off-chain storage, then make smart contract query/update the offchain database when need to mutate the value? Oct 21, 2022 at 4:40
  • The best practice here keep the major part of functionality off the chain and execute only the essential one on the chain, e.g. for my project I keep agreements on the chain, but any kind of search I do off the chain.
    – Gleichmut
    Oct 21, 2022 at 5:50
1

Contract storage is a key of 32 bytes and a value of 32 bytes. If I understand it correctly, it can support storing 2^32 bytes which is 4GB?

You seem to be confusing "bit" and "byte" here. A contract can store up to 2^256 values (the keyspace is 32 bytes, or 256 bits), each of 32 bytes, don't worry about running out of storage, it will never happen.

Store a large data in the contract is very expensive. It probably costs a very high gas fee when sending a transaction.

Well, that would be true if you were trying to store a large amount of data at once, which isnt the case here, any transaction you make won't update more than a few values in your contracts storage

3
  • you are right about any transaction you make won't update more than a few values. if there are many new addresses call this contract, the map will be increased a lot, will this be an issue? or does it cost too much gas when users call the contract? Oct 23, 2022 at 22:33
  • Each separate call (from each address) still won't read or write to all of the mapping, so each individual transaction will remain cheap
    – Foxxxey
    Oct 23, 2022 at 23:49
  • 1
    For each new account, it will add the address on the map during construct method. Will that consider a big gas when the map is growing? Oct 24, 2022 at 1:10
0

It would be prohibitively expensive to store that large amount of data using blockchain technology here. I suggest you use a centralized database instead of blockchain technology. Centralized databases are fast in transaction processing, can handle large amount of data, and keeping each storage cell persistent does not require any kind of fee.

See: How much data can I store in a smart contract, what is the cost and how it is implemented?

2
  • I see some blockchain game or dapp are putting user related balance in smart contract. Does it mean these dapp is expensive to use? Or they are using it differently? Oct 21, 2022 at 3:22
  • I believe it depends on the financial strength of blockchain game or dapp. They could be storing the user related balance differently. Oct 21, 2022 at 4:35

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.