[bitcoin-dev] Libconsensus phase 2

Eric Voskuil eric at voskuil.org
Wed Jan 13 22:47:44 UTC 2016

On 01/13/2016 12:37 AM, Jorge Timón wrote:
> On Tue, Jan 12, 2016 at 8:17 PM, Eric Voskuil <eric at voskuil.org> wrote:
>> Jorge, first, thanks again for your work on this.
>> Without creating and using a public blockchain interface in phase 2, how
>> will you isolate the database dependency from consensus critical code?
>> Is it that the interface will exist but you will recommend against its use?
> The interface will exist but it will be a C++ interface that fits
> better with Bitcoin Core internals.
> See an initial draft of what could be the storage interface:
> https://github.com/jtimon/bitcoin/blob/1717db89c6db17ea65ddbd5eb19034315db0b059/src/consensus/storage_interfaces_cpp.h
> Phase 3 will consist on discussing and refining that interface to also
> define the C interfaces using structs of function pointers instead of
> classes (see https://github.com/jtimon/bitcoin/blob/2bcc07c014e5dd000030e666be047dfa11f54c10/src/consensus/interfaces.h
> for an early draft) that is needed for the "final" C API.
> But since I think there will be more discussion and work defining
> those interfaces, I would rather start with ANY interface that allows
> us to decouple the consensus code from chain.o and coins.o, which we
> don't want to be built as part of the consensus building package
> (which is used for building both libbitcoinconsensus and Bitcoin
> Core).


> Future potential users are more than welcomed to draft their own C
> APIs and that experience should be useful for phase 3.
> I was expecting you, for example, to include the whole consensus code
> (even if it lacks a C API) in
> https://github.com/libbitcoin/libbitcoin-consensus for better testing
> of the equivalent code in libbitcoin. You are kind of taking the C API
> part out already, so this time you will just have less things to
> delete/ignore.

Generalization of the store interface may be more challenging than you
anticipate, but the objective makes sense.

>> This work presumes that the users of the library reject the argument
>> that the database implementation is consensus critical code. Faithful
>> reproduction of stored data is a prerequisite for a validity. But a
>> common store implementation is only slightly more reasonable for this
>> library than a common RAM implementation.
> This is a concern that has been risen repeatedly.
> I am aware that faithful reproduction of the stored data is a
> prerequisite for consensus validity. On the other hand, my presumption
> is that a libbitcoinconsensus that forces its users to a given unifrom
> storage will likely had much less users and any alternative
> implementation that wants to implement its own custom storage would
> have to necessarily reimplement the consensus validation code.
> Doing it this way is more flexible. We can relatively easily implement
> another library (if I remember correctly, last time we talked about it
> we reffered to it as "libconsensus plus", but there's probably better
> names) also takes care of storage for the users that don't want to
> take the risks of reimplementing the storage (probably just using
> Bitcoin Core's structures).
> Unlike me, Luke Dashjr, for example, advocated for the
> storage-dependent version, but I believe that implementing both
> versions was an acceptable solution to him.
> It is certainly an acceptable solution for me. I don't want to force
> anyone that doesn't want or need to take the risks reimplementing the
> consensus storage part to do so. But at the same time I really believe
> that it would be a mistake to not allow it optionally.
> Does that make sense?

We would not offer libbitcoinconsensus integration if it required us to
incorporate the store. These are distinct logical components, as are p2p
networking and client-server networking (e.g. RPC), for example. I would
not think of these as multiple versions of libbitcoinconsensus but
instead as distinct components of a bitcoin node. It doesn't make sense
to me that you would ship this as two consensus variants. I would work
toward shipping independent component libraries (i.e. consensus and store).


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