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commentIssueId: 36

There's an

[interesting post on scaling](https://blog.ethereum.org/2014/10/21/scalability-part-2-hypercubes/)

by Vitalik Buterin over at the Ethereum blog. I always read these posts with

excitement and a little trepidation, as I wonder if they'll come up with

some scaling trick I missed for pettycoin...

After several false starts, I rejected the idea of having multiple

blockchains and special inter-shard messages, and this post provides a

nice overview of the problems. In particular, the problem where a

miner on one shard refuses to release the details of their block;

others watching that block can complain, but by definition they're in

the minority. Vitalik suggests randomly querying your peers, but that

leaves you open to a Sybil attack, where the bad miner creates fake

peers who talk to everyone, and then discloses enough of the unknown

transactions to placate the majority.

The key weakness of the sketch, however, is that Vitalik asserts that

most transactions will be shard-local. That might be true of

Ethereum, but it's not true of a bitcoin-like payment network. Even

if we allow that those dealing with large numbers of transactions will

sit on numerous shards, you still have most transactions crossing

multiple boundaries. That means everyone wants to be neighbors, and

if you're successful in discouraging them all you've done is increase

messages until the point where scalability gains from sharding are

defeated by the increase in transactions simply relaying messages for

other nodes.

In the end, pettycoin regretfully returned to a "miners must know all"

model, but nodes can prove any mistakes they find. In particular, to

address the data unavailabilty attack, miners must "prove" they know

all the transactions in various previous blocks by hashing them with

their own reward address. This is a slightly-ungainly[1] compromise,

since we have 1 byte of hash for each shard in the previous

(power-of-two-spaced) 10 blocks, and if a miner gets it wrong you need

to send all the transactions in the incorrect shard to produce a

proof.

[1] ungainly: an irregular noun. My solution was "ungainly", your

solution was "ugly", their solution was "a horrible hack".