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I was researching how contract storage works. I know that the world state is a Merkle Patricia Trie, containing all active Ethereum adresses with their storageRoot (and their nonce, balance, and codeHash).

I have gained a better understanding of how a Merkle Patricia Trie works, and the fact that if you store all the key-values in the form of a Merkle Patricia Trie, all you need is the root hash (stored in storageRoot) to verify all the key-values.

I understand that each client has its own implementation. Go Ethereum uses levelDB (Google's key-value database), for instance.

The thing is that if a key-value is modified (because of contract execution), it creates a new node but doesn't delete the old one.

But I guess there are a lot of keys-values unique to each contract. And if every time a state variable is modified, a new node is created, but the old one isn't deleted, that's a crazy waste of space, isn't it? Is there something I'm missing here?

Thanks 🙏

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  • Well, there is a reason the chain is now more than one terrabytes ;)
    – Itération 122442
    Commented Jul 4, 2023 at 12:57
  • I have no idea where you took the idea that you could just simply update any key-value, all you can do with the root hash is verify data
    – Pavel Fedotov
    Commented May 24 at 18:01

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After lots of research, here is my understanding:

In general, when information is updated (like the accounts balance, or contracts variable states values), a key/value database (which implements merkle patricia tries) will create a new node for the variable to be updated, update the rest of the tree (since all parent nodes will have a hash that will be updated, as in a Merkle tree), but the old node remains existing (and accessible).

This is useful, on the one hand, to have a state history (if you want to know the value of a variable in a contract in a previous block), but it's not required to keep this information.

So there are clients that allow you to synchronise a blockchain without this "history", but that also give you the possibility of synchronizing everything to have a complete history of everything that's happened.

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  • Hi @Hackndo, I am wondering about deletions from trie myself. What makes you think that updated values create new nodes, instead of updating the "storage slot".
    – BipedalJoe
    Commented Dec 13, 2023 at 15:00
  • well, in general, when I cut and move a folder on my laptop. First, it gets copied, the folder moves somewhere else. But the old one still stays in the memory.
    – Pavel Fedotov
    Commented May 24 at 18:04
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The point of Ethereum is that your data stays on-chain.

Going to the computing fundamentals because let's not overcomplicate.

What is Delete? Delete refers to the act of eliminating or removing a file, folder, or piece of information from a computer or electronic device.

https://www.javatpoint.com/what-is-delete#:~:text=Delete%20refers%20to%20the%20act,a%20computer%20or%20electronic%20device.

You can overwrite the node to its initial state. It was certainly possible with Selfdestruct.

More information: https://www.alchemy.com/overviews/selfdestruct-solidity

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