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Date: Wed, 9 Dec 2015 06:29:53 +0000
Message-ID: <CAAS2fgS-jjEVeHf_LErppTadtAaSeBum+KiGHpoo=Jz5BZArsQ@mail.gmail.com>
From: Gregory Maxwell <greg@xiph.org>
To: Ryan Butler <rryananizer@gmail.com>
Content-Type: text/plain; charset=UTF-8
Cc: Bitcoin Dev <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Capacity increases for the Bitcoin system.
Precedence: list

On Wed, Dec 9, 2015 at 4:44 AM, Ryan Butler <rryananizer@gmail.com> wrote:
>>I agree, but nothing I have advocated creates significant technical
>>debt. It is also a bad engineering practice to combine functional
>>changes (especially ones with poorly understood system wide
>>consequences and low user autonomy) with structural tidying.
>
> I don't think I would classify placing things in consensus critical code
> when it doesn't need to be as "structural tidying".  Gavin said "pile on"
> which you took as implying "a lot", he can correct me, but I believe he
> meant "add to".

Nothing being discussed would move something from consensus critical
code to not consensus critical.

What was being discussed was the location of the witness commitment;
which is consensus critical regardless of where it is placed. Should
it be placed in an available location which is compatible with the
existing network, or should the block hashing data structure
immediately be changed in an incompatible way to accommodate it in
order to satisfy an ascetic sense of purity and to make fraud proofs
somewhat smaller?

I argue that the size difference in the fraud proofs is not
interesting, the disruption to the network in an incompatible upgrade
is interesting; and that if it really were desirable reorganization to
move the commitment point could be done as part of a separate change
that changes only the location of things (and/or other trivial
adjustments); and that proceeding int this fashion would minimize
disruption and risk... by making the incompatible changes that will
force network wide software updates be as small and as simple as
possible.

>> (especially ones with poorly understood system wide consequences and low
>> user autonomy)
>
> This implies there you have no confidence in the unit tests and functional
> testing around Bitcoin and should not be a reason to avoid refactoring.
> It's more a reason to increase testing so that you will have confidence when
> you refactor.

I am speaking from our engineering experience in a  public,
world-wide, multi-vendor, multi-version, inter-operable, distributed
system which is constantly changing and in production contains private
code, unknown and assorted hardware, mixtures of versions, unreliable
networks, undisclosed usage patterns, and more sources of complex
behavior than can be counted-- including complex economic incentives
and malicious participants.

Even if we knew the complete spectrum of possible states for the
system the combinatioric explosion makes complete testing infeasible.

Though testing is essential one cannot "unit test" away all the risks
related to deploying a new behavior in the network.