[bitcoin-dev] Forget dormant UTXOs without confiscating bitcoin

jl2012 at xbt.hk jl2012 at xbt.hk
Sun Dec 13 18:11:41 UTC 2015


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On Mon, Dec 14, 2015 at 12:14 AM, Danny Thorpe <danny.thorpe at 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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