Having a commitment to a "balance" of an "address" (I assume you mean
P2SH/P2PKH script) is extremely expensive to create and validate, does
not scale and is not a particularly useful thing for a client to use.
Validating clients should never expect their UTXO to be
inconsistent with that of the network, if something has happened to
cause this loosely committing to the UTXO is of no value.
On 2017-03-08 08:28, praxeology_guy via bitcoin-dev wrote:
A Commitment-suitable UTXO set "Balances" file data structure
- Allows pruned nodes to satisfy SPV nodes
- Allows pruned nodes to trustlessly start synchronizing at a Balances
file's block height instead of the genesis block
- Allows all nodes in the network to verify their UTXO set's data
integrity
For this to work, Bitcoin would need a new policy:
- A UTXO commitment is made every "Balances/UTXO Commitment Period"
(BCP) blocks. The UTXO commitment is made on the state of the UTXO at
BCP blocks ago. For example, if BCP is 5000, and we are creating
block 20,000, then block 20,000 would contain a commitment on what the
state of the UTXO was at block 15,000, right before any changes due to
block 15,001.
- The commitment is made on the state of the UTXO "BCP blocks ago"
instead of the UTXO state at the tip because: 1. Such a commitment can
be made in a background thread and not delay mining/synchronizing node
operations; 2. The work of creating the commitment doesn't have to be
redone in the case of a fork.
- The file/commitment is made in a background thread, starting at
least BCP/2 blocks after the last block containing a utxo commitment.
Balances file summary:
{
Header: 48 bytes
{
File Type: 4 bytes
File version: 4 bytes
size of balances: 8 bytes
root hash: 32 bytes
}
balances: "size of balances" bytes
balance index: "piece count" * (N + 4) bytes, N=4 proposed
merkle tree hashes: ~ 2 * "piece count" * 32 bytes
}
balances: is a list of balances sorted by txid:
{
length: 4 bytes
txid: 32 bytes
CCoins: variable length, depends on UTXO size
}
A "piece" is like in bittorrent's piece. I propose piece size =
32*1024 bytes. 2GB of balance data would then be divided into 65536
pieces.
transaction index is an array (with "piece count" elements) of:
{
txix: the first N bytes of a txid. I'm proposing N = 4
piece offset: 4 bytes, location of the first balance in the piece.
}
merkle tree hashes:
- array of "piece count" leaf hashes, hashing the balance pieces
- array of "(child layer count + 1)/2" node hashes, hashing pairs of
child hashes, or copying up if only one child
- repeat ^ until the root hash is written
... except reverse the layer order. In other words, it should be
breadth first.
==========
Data structure design notes:
- Most of the file's space is used by the balances. For example,
given a 32kB piece size and 2GB balances, the non-balances data only
consumes about 4.5MB. If N was increased to 32, ~6.5MB.
- piece size should be small enough that not that much effort is
wasted when invalid pieces are received.
- piece size should also be small in the case that this data structure
is used instead of block history for SPV proof. Then pruned nodes can
satisfy SPV clients.
- The child count = 2 merkle tree structure is only necessary for if
this data structure is to be used to satisfy SPV clients. If not used
for such a purpose, then technically the root hash could have the leaf
hashes as it's direct children. But practically this doesn't make a
difference: merkle tree size is nothing compared to sizeof(balances).
- The only purpose of the balance index is to support SPV clients
- txix is a truncation of txid to reduce memory usage for a fully
in-memory index to support SPV nodes. Maybe this truncation isn't
worthwhile.
Other notes:
- We could make BCP 4383 blocks, which would be 12 times per year,
given block period was exactly 10 minutes. But since block period is
not exactly 10 minutes, and file names generated with period 4283 are
much less comprehensible than file names generated with period 5000...
I propose 5000.
- Having a shorter BCP period would result in more frequent checks on
UTXO set integrity, and permit new pruning nodes to start synching
closer to the tip. But it may require nodes to keep more copies of
the balances file to satisfy the same backup period, and require more
background work of creating more balances files.
===========
Suggested design change to the chainstate "CCoinsViewDB" utxo
database:
- As it is designed now, the above proposal would require maintaining
a duplicate but lagging UTXO database.
- I propose changing the "CCoins" data structure so that it can keep
track of spends that shouldn't be included in the commitment. Maybe
call it "vtipspends".
Then the process for updating the CCoinsViewDB would be:
1. Mark a txo as spent by adding the vout_ix to vtipspends.
2. SetNull() and Cleanup() during the background thread that creates
Balances commitments. vtipspends would also need to be cleaned.
- The method for checking whether a txo was spent would need to be
changed to check vtipspends.
At the same time, I know there is currently a lot of code complexity
with handling forks and txo spends. Let me propose something to
handle this better too:
- vtipspends could hold {vout_ix, blockhash } instead of just vout_ix.
- Checking whether a txo is spent will then require a parameter that
specifies the "fork tip hash" or "fork chain"
Then in the case of a fork, no work has to be done to update the utxo
database... it is immediately ready to handle answering spend
questions for a different fork.
Feedback welcome. FYI I have coded up the creation of such a file
already... So I am working on the implementation, not just the spec.
I'd really like to hear what you guys think about my proposed changes
to CCoins.
Cheers,
Praxeology
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