Subject: Re: [bitcoin-dev] Using a storage engine without UTXO-index

From: Eric Voskuil <[email-masked]>
Date: Tue, 11 Apr 2017 02:41:34 -0700

On 04/11/2017 01:43 AM, Tomas wrote:
> Splitting transactions only happens *on storage* and is just a
> minor optimization compared to storing them in full.

Ok

> Sure, we can still call switching tips a "reorg". And it is indeed
> a trade off as orphan blocks are stored, but a block in the spend
> tree takes only ~12kb and contains the required state information.
> 
It's not the headers/tx-hashes of the blocks that I'm referring to, it
is the confirmation and spend information relative to all txs and all
outputs for each branch. This reverse navigation (i.e. utxo
information) is essential, must be persistent and is branch-relative.

> The blockchain is - by design - only eventually consistent across
> nodes. Even if nodes would use the same "tip-selection" rules, you
> cannot rely on all blocks being propagated and hence each
> transaction having the same number of confirmations across all
> nodes.
> 
> As a simpler example, if two miners both mine a block at
> approximately the same time and send it to each other, then surely
> they would want to continue mining on their own block. Otherwise
> they would be throwing away their own reward.

That's not your concurrent validation scenario. In the scenario you
described, the person chooses the weaker block of two that require
validation because it's better somehow, not because it's his own
(which does not require validation).

> And yes, this can also happen over multiple blocks, but the chances
> of consistency are vastly increased with each confirmation.

Consistency is reached, despite seeing things at different times,
because people use the same rules. If the economy ran on arbitrary
block preference consistency would be elusive.

> I am not talking about rejecting blocks, I am only talking choosing
> on which tip to mine.

This line of reasoning has me a bit baffled. Yet as I said, it's not
important to the question at hand. It is not likely to be optimal to
validate concurrently even if you consider selection of a weaker block
advantageous.

>> If you intend this to be useful it has to help build the chain,
>> not just rely on hardwiring checkpoints once rule changes are
>> presumed to be buried deeply enough to do so (as the result of
>> other implementations ).
>> 
>> I understand this approach, it was ours at one time. There is a 
>> significant difference, and your design is to some degree based
>> on a failure to fully consider this. I encourage you to not
>> assume any consensus-related detail is too small.
> 
> I am not failing to consider this, and I don't consider this too
> small . But ensuring contextual transaction validity by "validate
> =>  valid with rules X,Y,Z" and then checking the active rules
> (softfork activation) on order validation, will give logically the
> same results as "validate with X,Y,Z => valid". This is not
> "hardwiring checkpoints" at all.

Storing the validation flags with each tx is exactly what libbitcoin
does (otherwise pre-validation would be infeasible). But that was not
the full point. You said on this in response previously:

>>> ...height-based compliance as meta data of validation seems to
>>> be
adequate and safe.

I read this as encoding the height at which a fork historically
activated. If you intend to track activation for each branch that will
not be "height-based" it will be history based.

e