<blockquote>
<p><strong>How likely is it that some block from 1 year ago later turns out to be invalid?</strong></p>
</blockquote>
<p>Very unlikely, but I think that isn't the problem.</p>
<p>The problem is, how does a new user find out the current state (UTXO set or equivalent) without having to trust other parties? Any community has dishonest people preying on it.</p>
<hr />
<p>You cannot ignore, expire or invalidate unspent UTXO from much earlier blocks. That is someone's savings and the community wont throw their savings away or  arbitrarily take away their peers savings. To do so would undermine the credibility of the currency. Any such change would be ignored and not be adopted by the majority.</p>

CC BY-SA 4.0
2021-05-04T16:14:04.830Z
Could everyone trust at least the UTXO represented within the sliding window? If so could it work if the concept of coin expiration be added to solve this (in theory I mean  - I realize this would probably be too large a change to be considered even if it might work).
noctonura
CC BY-SA 4.0
2021-05-04T16:19:16.437Z
@nocturna. I see no sound basis for that and expect it would open the way for fraud. We also need our money's value to persist and not evaporate.
RedGrittyBrick
CC BY-SA 4.0
2021-05-04T17:09:52.790Z
How would it open the door to fraud? Whether dormant utxo evaporates or not is a value judgement and imho is better than persisting illegal and disgusting content on my computer by way of the blockchain. 

But anyway I’m trying to understand whether such a scheme would work assuming that one doesn’t have a problem with old utxo expiring.
noctonura
CC BY-SA 4.0
2021-05-04T18:10:55.607Z
For future readers, a similar scheme is discussed here: https://bitcoincashresearch.org/t/prune-bitcoin-cash-network-every-7-years/273/5. Looks like the objections are value-based as opposed to technical/cryptological.
noctonura
CC BY-SA 4.0
2021-05-04T08:18:23.077Z
2021-05-04T18:47:45.477Z
2021-05-04T18:47:45.477Z
RedGrittyBrick
<p>If your objection is specifically to the requirement to store old blocks (as you hinted in a comment), then you might consider pruning - see <a href="https://bitcoin.stackexchange.com/questions/37496/how-can-i-run-bitcoind-in-pruning-mode">How can I run bitcoind in pruning mode?</a>.</p>

CC BY-SA 4.0
2021-05-06T12:24:17.027Z
2021-05-06T12:24:17.027Z
QMdEW4pbr
<p>I do understand that every block is validated in terms of the blocks before it by way of the previous block's hash, all the way back to the genesis block. However, could the protocol be modified such that the blockchain is a sliding window over the most recent N number of blocks? How likely is it that some block from 1 year ago later turns out to be invalid?</p>

<blockchain><protocol><genesis-block><trust><block-validity>
Why is the full blockchain required, effectively forever?
noctonura
# Why is the full blockchain required, effectively forever?

(Asked by: username_0 on 2021-05-04)

I do understand that every block is validated in terms of the blocks before it by way of the previous block's hash, all the way back to the genesis block. However, could the protocol be modified such that the blockchain is a sliding window over the most recent N number of blocks? How likely is it that some block from 1 year ago later turns out to be invalid?

**Tags:** blockchain, protocol, genesis-block, trust, block-validity


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## 2 answers

### Answer by username_1 on 2021-05-04. Score: 3 | Accepted answer

> **How likely is it that some block from 1 year ago later turns out to be invalid?**

Very unlikely, but I think that isn't the problem.

The problem is, how does a new user find out the current state (UTXO set or equivalent) without having to trust other parties? Any community has dishonest people preying on it.

---

You cannot ignore, expire or invalidate unspent UTXO from much earlier blocks. That is someone's savings and the community wont throw their savings away or arbitrarily take away their peers savings. To do so would undermine the credibility of the currency. Any such change would be ignored and not be adopted by the majority.

> **Comments:**
> **username_0**: Could everyone trust at least the UTXO represented within the sliding window? If so could it work if the concept of coin expiration be added to solve this (in theory I mean  - I realize this would probably be too large a change to be considered even if it might work).  
> **username_1**: @nocturna. I see no sound basis for that and expect it would open the way for fraud. We also need our money's value to persist and not evaporate.  
> **username_0**: How would it open the door to fraud? Whether dormant utxo evaporates or not is a value judgement and imho is better than persisting illegal and disgusting content on my computer by way of the blockchain.   But anyway I’m trying to understand whether such a scheme would work assuming that one doesn’t have a problem with old utxo expiring.  
> **username_0**: For future readers, a similar scheme is discussed here: https://bitcoincashresearch.org/t/prune-bitcoin-cash-network-every-7-years/273/5. Looks like the objections are value-based as opposed to technical/cryptological.  

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### Answer by username_2 on 2021-05-06. Score: 1

If your objection is specifically to the requirement to store old blocks (as you hinted in a comment), then you might consider pruning - see [How can I run bitcoind in pruning mode?](https://bitcoin.stackexchange.com/questions/37496/how-can-i-run-bitcoind-in-pruning-mode).


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