[DennisP]

There's a fundamental difference: a rollback reverses everybody's transactions, the DAO fork left unrelated transactions alone.

Bitcoin had an actual five-hour rollback in its early days, when someone figured out an exploit and awarded themselves over a billion coins.

[codehalo]

That’s incorrect. There was never a rollback in Bitcoin. You may be referring to the leveldb bug that caused a fork, or Gavin and Satoshi’s discovery of the billion coins bug, which was fixed by a software update, but none of these involved a rollback.

[DennisP]

I was thinking of the billion coins bug. I did some searching and it appears you're mostly correct; the errant transaction was removed, along with subsequent transactions that descended from it, but unrelated transactions were left in place.

https://bitcointalk.org/index.php?topic=823.msg9557

[mannykannot]

A rollback followed by a replay of desired transactions is still a rollback. This is how you can tell if an attempt to refute-by-redefinition is denial: first determine why it would be unfortunate if the predicate being refuted were true, and then ask if the redefinition actually fixes the problem.

[DennisP]

Ethereum didn't have to replay transactions, because there was no rollback at all; it was an isolated change to the data in one contract. Ethereum doesn't work the way Bitcoin does.

[mannykannot]

Here we have another distinction without a relevant difference. A rollback is defined by its consequences, not the details of its process.

[DennisP]

If you want to redefine "rollback" as "any fork that changes a blockchain's data" then ok, but that seems like a misleading definition to me. Normally a rollback means "all transactions are rolled back," which has entirely different consequences. In Ethereum's case, the data afterwards is different even from a rollback/reapply, since the theft is still in the transaction history.

In any case it seems fairly useless to debate what the meaning of "rollback" should be.

[mannykannot]

The meaning of rollback transcends and precedes blockchains, and it use here is in that sense.

[DennisP]

If you're using the original definition from SQL, the DAO event still doesn't qualify. A SQL transaction rollback removes the transaction from the log, while the DAO theft transaction is still on chain. What Ethereum did was an additional update transaction.