I've noticed the hard fork, there's some mechanism in the code to ensure and enforce the node to run the updated version. The mechanism is so called "consensus challenge". What is this indeed? Some math computation? I read that there will be a timeout and forced cutoff if one can't respond with correct challenge message. Can someone explain how it works?

Since in the devcon 2, people suggest Ethereum is at its early stage, which still subjects to some changes in the future, how do we envision the changes?

What is the method used to ensure the nodes are running the same set of rule/protocol, or some mining software? How to enforce an update? Is such thing as the "consensus challenge" commonly happened in other blockchain when they have "new version"?

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There's ultimately no way to force another node to run a given implementation. What happens is that the network splits instead.

Each node executes every block independently. As long as each node follows the same rules, then the same blocks will spread across the network. Once a rule has changed, and a block relying on a new rule is executed, any node with the new rules will accept it, and the old ones will reject it. Eventually miners following the new rules will build blocks on this new block. Miners following the old rules will instead mine on a block valid by the old rules. Thus, there will now be two blockchains.

For example, when the DAO rescue fork transfered all ETH in the attacker's hands to the refund contract, all the nodes that didn't agree with this ended up on another chain (Ethereum Classic). The "challenge" is for nodes to identify each other--if a node is pro-fork, there's no point in it asking for any new blocks from an anti-fork node.

Theoretically, the old chain will have less value, and therefore less miners, and eventually die. Ethereum's past "feature" hardfork, Homestead, happened this way. The DAO rescue fork didn't (although many, including myself, expected it to die). Whether future hardforks result in more blockchains will, most likely, depend on how controversial they are.

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