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On Mon, Dec 14, 2015 at 12:14 AM, Danny Thorpe <danny.thorpe@gmail.com> wrote:

What is the current behavior / cost that this proposal is trying to avoid? Are ancient utxos required to be kept in memory always in a fully validating node, or can ancient utxos get pushed out of memory like a normal LRU caching db?

I don't see why it must be kept in memory. But storage is still a problem. With the 8 year limit and a fixed max block size, it indirectly sets an upper limit for UTXO set.


Chris Priest via bitcoin-dev :

This isn't going to kill bitcoin, but it won't make it any better.

Do you believe that thousands of volunteer full nodes are obliged to store an UTXO record, just because one paid US$0.01 to an anonymous miner 100 years ago? It sounds insanely cheap, isn't it? My proposal (or similar proposal by Peter Todd) is to solve this problem. Many commercial banks have a dormant threshold less than 8 years so I believe it is a balanced choice.

Back to the topic, I would like to further elaborate my proposal.

We have 3 types of full nodes:

Archive nodes: full nodes that store the whole blockchain
Full UTXO nodes: full nodes that fully store the latest UTXO state, but not the raw blockchain
Lite UTXO nodes: full nodes that store only UTXO created in that past 420000 blocks

Currently, if one holds nothing but a private key, he must consult either an archive node or a full UTXO node for the latest UTXO state to spend his coin. We currently do not have any lite UTXO node, and such node would not work properly beyond block 420000.

With the softfork I described in my original post, if the UTXO is created within the last 420000 blocks, the key holder may consult any type of full node, including a lite UTXO node, to create the transaction.

If the UTXO has been confirmed by more than 420000 blocks, a lite UTXO node obviously can't provide the necessary information to spend the coin. However, not even a full UTXO node may do so. A full UTXO node could tell the position of the UTXO in the blockchain, but can't provide all the information required by my specification. Only an archive node may do so.

What extra information is needed?

(1) If your UTXO was generated in block Y, you first need to know the TXO state (spent / unspent) of all outputs in block Y at block (Y + 420000). Only UTXOs at that time are relevant.

(2) You also need to know if there was any spending of any block Y UTXOs after block (Y + 420000).

It is not possible to construct the membership prove I require without these information. It is designed this way, so that lite UTXO nodes won't need to store any dormant UTXO records: not even the hash of individual dormant UTXO records. If the blockchain grows to insanely big, it may take days or weeks to retrieve to records. However, I don't think this is relevant as one has already left his coins dormant for >8 years. Actually, you don't even need the full blockchain. For (1), all you need is the 420000 blocks from Y to Y+420000 minus any witness data, as you don't need to do any validation. For (2), you just need the coinbase of Y+420001 to present, where any spending would have been committed, and retrieve the full block only if a spending is found.

So the Bitcoin Bank (miners) is not going to shred your record and confiscate your money. Instead, the Bank throws your record to the garage (raw blockchain). You can search for your record by yourself, or employ someone (archive node) to search it for you. In any case it incurs costs. But as thousands of bankers have kept your record on their limited desk space for 8 years for free (though one of them might receive a fraction of a penny from you), you shouldn't complain with any moral, technical, or legal reason. And no matter what users say, I believe something like this will happen when miners and full nodes can't handle the UTXO set.

I'd like to see more efficient proposals that archive the same goals.

p.s. there were some typos in my original. The second sentence of the second paragraph should be read as "For every block X+420000, it will commit to a hash for all UTXOs generated in block X."
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