Subject: Re: [bitcoin-dev] Why not archive the backend of Bitcoin blockchain?

From: Brian Lockhart <[email-masked]>
Date: Wed, 13 Jun 2018 08:32:11 -0700

Somewhat related question -

In the interest of avoiding running multiple bitcoind full nodes - is there
a method to allow a Lightning node to point to / access a separate
already-existing node, vs. requiring it to have its own dedicated local
instance of bitcoind running?

I.e. if I already have a full bitcoin node running, could I use RPC calls
or something to tell my Lightning node to use that node, instead of
spinning up *another* full node? I’m currently minimizing the network
thrashing by whitelisting my LN bitcoind node to only point to my existing
full node for updates, but if I could just point my whole LN node at it,
that’s save on disk storage etc. etc. etc.

Apologies if this is already in there (or has been added) and I missed it
because I haven’t kept up with release notes…

On June 13, 2018 at 6:35:49 AM, Christian Decker via bitcoin-dev (
[email-masked]) wrote:

Kulpreet Singh via bitcoin-dev <[email-masked]>
writes:
> But if I understand correctly, lightning nodes need to check if a
> counterparty is broadcasting an old channel state and in response
> broadcast a penalty/justice transaction. Does that mean lightning
> nodes only need to watch for transactions that come after the funding
> transaction? Is that the only reason lightning needs to run bitcoind
> with txindex?

Yes, Lightning nodes need to monitor the network for transactions that
they need to react to. This is basically tailing the blockchain and
looking for anything suspicious. The `bitcoind` sitting next to the
lightning node however does not need to keep an index of the
transactions, at least for c-lightning, because we just ask for the full
block that then gets scanned for transactions of interest and then we
discard the rest of the block. We never ask for a specific transaction
from `bitcoind` and therefore we don't need to run with `-txindex`.

> If that is the case, and a lightning node only needs to query
> transactions broadcast after the funding transaction, then a pruned
> bitcoind instance with txindex might be a bit handy.

Pruned nodes should work, as long as the current blockchain head that
the lightning node has seen does not fall into the pruned range, since
in that case it won't be able to fetch and process the blocks anymore.

> Also from [1] it seems that indexing pruned nodes is not supported
> because it doesn't make sense, not that it was infeasible. Now with
> the lightning requirements, does an indexed pruned node start to make
> sense?

I don't think we should ever require `-txindex` to run a lightning node
(I know some implementations did in the past), since that'd be a very
onerous requirement to run a lightning node. Tailing the blockchain is
more than sufficient to get the necessary data, and hopefully we can get
our reliance on `bitcoind` down to a minimum in the future.

> Once again, please forgive my naive understanding of some of the issues
> involved and thanks for your patience.

Absolutely no problem, it is a common misconception that `-txindex` is
required to run a lightning node in all cases :-)

Cheers,
Christian
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