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From: Peter R <peter_r@gmx.com>
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Date: Sat, 14 Nov 2015 17:02:33 -0800
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	<201510290803.52734.luke@dashjr.org>
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To: Gregory Maxwell <gmaxwell@gmail.com>
Sender: Peter_R@gmx.com
Cc: Bitcoin Dev <bitcoin-dev@lists.linuxfoundation.org>,
	telemaco <telemaco@neomailbox.net>
Subject: Re: [bitcoin-dev] [patch] Switching Bitcoin Core to sqlite db
Precedence: list

Hi Greg,

Like you said, the issue with using more than one database technology is =
not that one node would prove that Block X is valid while the another =
node proves that Block X is NOT valid.  Instead, the problem is that one =
node might say =E2=80=9Cvalid=E2=80=9D while the other node says =E2=80=9C=
I don=E2=80=99t know.=E2=80=9D

It is often said that what caused the Level DB fork was that the old =
version determined that the triggering block was invalid while the new =
version determined the opposite.  This interpretation of the fork event =
has lead to the =E2=80=9Cbug-for-bug=E2=80=9D-compatibility fear =
regarding multiple implementations of the protocol (or multiple database =
technologies).  In reality, this fear is largely unfounded. =20

The old version ran out of LDB locks and couldn=E2=80=99t execute =
properly.  If the software was written with the philosophy that tracking =
consensus was more important than bug-for-bug compatibility, it would =
have returned an exception rather than =E2=80=9Cinvalid.=E2=80=9D  At =
that point, the software would have checked what decision the rest of =
the network came to about the block in question.   My node would have =
forked to the longest chain, automatically assuming that the =
questionable block was valid; your node may have made a different =
decision (it=E2=80=99s a question of ideology at that point).     =20

A group of us have been exploring this =E2=80=9Cmeta-cognition=E2=80=9D =
idea with Bitcoin Unlimited.  For example, Bitcoin Unlimited can be =
(optionally) made to automatically fork to the longest chain if it =
=E2=80=9Cgets stuck=E2=80=9D and can neither prove that a block is valid =
nor that the block is invalid.  Similarly, Bitcoin Unlimited can be =
(optionally) made to automatically fork to a chain that contains a block =
marginally bigger than its block size limit=E2=80=94once that block is =
buried sufficiently in what has emerged as the longest persistent chain.=20=


Thinking from this perspective might go along way towards decentralizing =
development, the emergence of multiple competing implementations of the =
protocol, and true =E2=80=9Cbottom up=E2=80=9D governance. =20

Best regards,
Peter


> On Oct 29, 2015, at 9:28 PM, Gregory Maxwell <gmaxwell@gmail.com> =
wrote:
>=20
> On Fri, Oct 30, 2015 at 4:04 AM, Peter R <peter_r@gmx.com> wrote:
>> Can you give a specific example of how nodes that used different =
database technologies might determine different answers to whether a =
given transaction is valid or invalid?  I=E2=80=99m not a database =
expert, but to me it would seem that if all the unspent outputs can be =
found in the database, and if the relevant information about each output =
can be retrieved without corruption, then that=E2=80=99s all that really =
matters as far as the database is concerned.
>=20
> If you add to those set of assumptions the handling of write ordering
> is the same (e.g. multiple updates in an change end up with the same
> entry surviving) and read/write interleave returning the same results
> then it wouldn't.
>=20
> But databases sometimes have errors which cause them to fail to return
> records, or to return stale data. And if those exist consistency must
> be maintained; and "fixing" the bug can cause a divergence in
> consensus state that could open users up to theft.
>=20
> Case in point, prior to leveldb's use in Bitcoin Core it had a bug
> that, under rare conditions, could cause it to consistently return not
> found on records that were really there (I'm running from memory so I
> don't recall the specific cause).  Leveldb fixed this serious bug in a
> minor update.  But deploying a fix like this in an uncontrolled manner
> in the bitcoin network would potentially cause a fork in the consensus
> state; so any such fix would need to be rolled out in an orderly
> manner.
>=20
>> I=E2=80=99d like a concrete example to help me understand why more =
than one implementation of something like the UTXO database would be =
unreasonable.
>=20
> It's not unreasonable, but great care is required around the =
specifics.
>=20
> Bitcoin consensus implements a mathematical function that defines the
> operation of the system and above all else all systems must agree (or
> else the state can diverge and permit double-spends);  if you could
> prove that a component behaves identically under all inputs to another
> function then it can be replaced without concern but this is something
> that cannot be done generally for all software, and proving
> equivalence even in special cases it is an open area of research.  The
> case where the software itself is identical or nearly so is much
> easier to gain confidence in the equivalence of a change through
> testing and review.
>=20
> With that cost in mind one must then consider the other side of the
> equation-- utxo database is an opaque compressed representation,
> several of the posts here have been about desirability of blockchain
> analysis interfaces, and I agree they're sometimes desirable but
> access to the consensus utxo database is not helpful for that.
> Similarly, other things suggested are so phenomenally slow that it's
> unlikely that a node would catch up and stay synced even on powerful
> hardware.  Regardless, in Bitcoin core the storage engine for this is
> fully internally abstracted and so it is relatively straight forward
> for someone to drop something else in to experiment with; whatever the
> motivation.
>=20
> I think people are falling into a trap of thinking "It's a <database>,
> I know a <black box> for that!"; but the application and needs are
> very specialized here; no less than, say-- the table of pre-computed
> EC points used for signing in the ECDSA application. It just so
> happens that on the back of the very bitcoin specific cryptographic
> consensus algorithim there was a slot where a pre-existing high
> performance key-value store fit; and so we're using one and saving
> ourselves some effort.  If, in the future, Bitcoin Core adopts a
> merkelized commitment for the UTXO it would probably need to stop
> using any off-the-shelf key value store entirely, in order to avoid a
> 20+ fold write inflation from updating hash tree paths (And Bram Cohen
> has been working on just such a thing, in fact).