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Can someone explain, technically, what would happen if a majority of the miners adopted a soft fork (forward-compatible) making some tx invalid and then a miner who didn't fork happened to mine a block containing an invalid tx (but still valid on his fork)?

Basically what I don't understand is that there's a proposal where, apparently, miners can choose which addresses they want to block.

So what if, say, there are 4 groups of miners:

  1. group 1 considers invalid tx to/from address A (17% of miners)
  2. group 2 considers invalid tx to/from address B (23% of miners)
  3. group 3 considers invalid tx to/from address A & B (20% of miners)
  4. group 4 doesn't want to soft fork or doesn't bother (40% of miners)

Isn't group 4 guaranteed to "win", because group 1, 2 and 3 will each be on their own fork, resulting in four forks? Although should have groups 1,2 and 3 all agreed on the same rules for what was invalid, they'd have 60% of the mining power and hence "won"?

Or is the upcoming proposal going to be the same for everyone, and everybody who wants to fork is going to consider the exact same blocks as invalids?

And the second thing I don't understand: there has to be a block at which the new rule kick in and 50%+1 of the miners have to have updated to the soft fork by then for the (forward-compatible) fork to be succesful right? Yet I don't see any block number at which the new rules would kick in the current soft fork proposal code.

Any technical answer is very welcome (*).

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  • (*) I don't have any financial interest in the actual outcome of the upcoming fork. I'm after the technical details to better understand how blockchains (and Ethereum in particular) softforks do work. – Cedric Martin Jun 18 '16 at 19:59
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No need of technical answer here. Soft fork is a fork. As soon as you don't run the consensual software you are on a fork . So a miner that validates a transaction that other won't because he doesn't have the same forked software as the majority, it means that this miner is not in the fork. So he could mine whatever he wants, this would remain on the old, now so called alt chain.

So yes, there would be 4 forks in your case and only the one where majority mines would be official one.

Miners have to agree on all TX to be considered playing in the same chain version.

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  • Yes it would be. But how is the largest chain built? Choosing on which chain to to mine and continue enlarging it is tied to how miner software version code is written. So if all miners use the same software, in the end they will choose the same chain. If it's written not to select the hacked chain in the fork code and a majority of miner runs it, the longest chain will be the one that erases the hack in the end. – Nicolas Massart Jun 18 '16 at 23:55
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    how can you determine which chain the majority of miners are on? – arboles Jun 19 '16 at 2:22
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    You can't, the mining software can. Just run the same software as the majority and you're done. – Nicolas Massart Jun 19 '16 at 8:40

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