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, andcodeHash).

I've gained a better understanding of how a Merkle Patricia Trie works, and the fact that if you store all the key-values (i.e. For contract storage, the storage indexes with their respective values) in a key-value DB in the form of a Merkle Patricia Trie, all you need is the root hash (stored in storageRoot) to find anc check/update 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 (which is good because if another contract has exactly the same node, it should have the same hash, so that would be a problem if the entry was deleted, if I understand correctly).

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 🙏

  • Well, there is a reason the chain is now more than one terrabytes ;) Jul 4, 2023 at 12:57

1 Answer 1


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.

  • 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
    Dec 13, 2023 at 15:00

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