How to persist data after deployment in Solidity

Question

I am pretty new to Solidity and working with Ethereum in general. In the (d)app I'm working on I need to be able to persist data onto the ledger, but I'm not sure I understand how this works.

Let's say I have the following contract (simplified for practicality):

contract UserContract {
    struct User {
        address walletAddress;
        string organisation;
        string fName;
        string lName;
        string email;
        uint index;
    }
    mapping(address => User) private users;
    address[] private userIndex;


    function insertUser(
        address walletAddress,
        string organisation,
        string fName,
        string lName,
        string email        )
      public
      returns(uint index) {
          User memory newUser = User({
            walletAddress: walletAddress,
            organisation: organisation,
            fName: fName,
            lName: lName,
            email: email,
            index: users.length
            });

            users.push(newUser);
            userIndex[walletAddress] = newUser.index;
            return newUser.index;
    }
}

Using the insertUser() method, I can insert a new user, and using a getter method I could retrieve the user's information.

Now, if I update the contract (thus deploy a new one), the users mapping is empty again, not surprising.

My question: how do I store data in a way that it will be accessible for future versions of the contract? Any design patterns that go along with this process?

Thanks!

score 3 · Accepted Answer · answered Jun 04 '18 at 11:58

3

Since, as you know, stored data will not travel with a new contract and copying over data to a new contract would be expensive task after a few uses...your best bet is to separate functionality from data storage.

Create a contract with some basic setters and getters that only deals with data storage (from a particular contract address if necessary)...then create your main functional contract that connects to the data contract.

answered Jun 04 '18 at 11:58

ReyHaynes

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Thanks! But this still has the problem that every time I want to edit the data structure (the struct), I will lose all the data right? Seems a bit unmaintainable.. – Karim Stekelenburg Jun 05 '18 at 11:27
1

@HighSociety Well, data is not lost if it's deployed but would have to migrate the data to the new contract if you are changing the data structure...which would be expensive. Your question revolved around persisting data, so I gave you the best solution for that scenario...but in general (even in more practical databases), it's not ideal to change the data structure itself. – ReyHaynes Jun 05 '18 at 12:11
My question was indeed only about persisting the data. The follow-up was just out of interest. I'll accept you answer! Thanks again! – Karim Stekelenburg Jun 05 '18 at 13:04

How to persist data after deployment in Solidity

1 Answers1