Blockchain Academy
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Hard Fork

Just as a cutlery 'fork' meaning diverging 'paths', this is a term for developers to develop independent software by copying one piece of software code. It says the 'the fork occurred' when two or more blocks have the same number of blocks on the same block height which indicates ‘branching’ chains.

Fork is classified as 'Hard Fork' and 'Soft Fork'. Hard fork happens with a change in blockchain protocol or a switch of the main chain to the old version by the catastrophic bugs or hacking. Therefore all of the groups of node in the network must update the system to the new version in order to use the corresponding blockchain.

(Image source : https://www.investopedia.com/terms/h/hard-fork.asp)

The permanent fork remains with two chains after the hard fork occurs in the blockchain. The users continue to use the chain with the new software, which becomes the priority and maintains the main chain, and another chain with the old software becomes invalid. In 2016, DAO, a digital decentralized autonomous organization and an Ethereum community, had been attacked by hackers, who exploited a vulnerability in its code, and lost about $50million. Ethereum developers caught suspicious movements, and there was a holding period in DAO contract before withdrawing the funds. So they had some time to debate whether to consider a hard fork or a soft fork to solve this problem. Finally a hard fork was chosen. After this incident, the users who preferred to use the old version before the update continued on using the old blockchain, which is now Ethereum Classic. Though there are many compatible parts between Ethereum Classic and Ethereum until now. But as time goes by, they adopt new functions separately and become more and more independent chains apart from each other.