Hard fork

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Hard fork
If the consensus rules of the blockchain software are changed, and this rule change is not forward compatible, the old node cannot recognize the block generated by the new node, which is a hard fork.

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At this time, the old node will reject the block of the new rule, so the new node and the old node will start to run on different blockchains (mining, transaction, verification, etc.). Since the new and old nodes may exist for a long time, this kind of fork is also It may last for a long time.

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In the above figure, Block 4a and Block 4b are the result of natural bifurcation, and finally reached final consistency in the entire blockchain of Block 5 (see "In-depth Bitcoin Principles (12)" for details)
Suppose that when the blockchain expands to Block 6, a new client version that changes the consensus rules is released, so the nodes running different clients diverge:
Miners running the new version will accept the block Block 7b generated under the new rules;
Miners running the old version will reject Block 7b because they cannot recognize the new rules and mine Block 7a based on the old rules;
At this time, the blockchain has undergone a hard fork. The old and new miners will mine on their respective blockchains, and their stay will determine the life, death and value of the two chains.

Hard fork process

1. The developer releases a new client. The new client has changed the consensus rules of the blockchain and is not compatible with the old client, so the client software has a software fork.
2. Some nodes in the blockchain network start to run new clients, the transactions and blocks generated under the new rules will be rejected by the old nodes, and the old nodes will start to disconnect briefly and send these transactions and blocks rejected by themselves With the connection of new nodes, the entire blockchain network has a network fork.
3. Miners running the new client start mining based on the new rules, and will accept transactions and blocks under the new rules. As a result, miners running different client versions have a mining fork.
4. The miners running the new client started to mine blocks based on the new rules, while the miners of the old client would still mine blocks based on the old rules, so a chain fork appeared in the entire blockchain.

The two sides of a hard fork
Without the consent of almost all participants in the ecosystem, a hard fork is likely to split the entire Bitcoin ecosystem, so this is a highly controversial and dangerous blockchain upgrade technology. The entire Bitcoin community is very cautious about hard forks. So far, there have been two hard forks caused by software upgrades in Bitcoin, but they were all caused by accidents (software bugs). In the end, the miners passed the rollback or all upgrades. A consensus was reached so that the impact of the situation did not expand.
But I think the hard fork will play a huge role at critical moments in the future, and it is the foundation for the long-term development of Bitcoin.
It is foreseeable that when Bitcoin is about to encounter a major risk or even its extinction, the entire Bitcoin ecosystem will easily reach a consensus due to the stakes, and upgrade the Bitcoin client to avoid crisis.
To give a vivid example, if one day humans can emigrate to Mars, someone will definitely stand up and say that humans should not emigrate to Mars, because that would split human society. But if one day the earth's resources are exhausted and human survival cannot be satisfied, I believe that almost everyone will reach a consensus to emigrate.
Nowadays, I often hear various theories about the death of Bitcoin, such as the development of quantum computers, the cracking of Bitcoin cryptographic algorithms, etc. Many experts vowed to declare "If XXX happens, Bitcoin will die, so Bitcoin has no future." .
Now that you understand the existence of hard forks, I believe you can understand that solving these problems is not technically difficult. In a decentralized system, it is difficult to reach consensus. Reaching consensus can be difficult most of the time, but sometimes it can be simple.

IFO (Initial Fork Offerings)
IFO is a new concept created after the ICO. Some communities or groups have forked a new blockchain in order to improve the mechanism of Bitcoin. This is actually a hard fork scenario.
For example, on August 1, 2017, the Bitcoin Cash (BCH) blockchain successfully separated from the main chain at block height 478559. The default block size of this new cryptocurrency is 8MB, and the block capacity can be dynamically adjusted.
Since the old node only recognizes blocks less than 1MB, the blocks generated by running the BCH client node are not forward compatible and will be rejected by the old node. Finally, miners running different clients will run on two different blockchains for a long time. Up (BTC and BCH).

IFO could have been a very good experimental method for Bitcoin transformation, but within a short period of time after BCH, BTG (Bitcoin Gold), BCD (Bitcoin Diamond), SBTC (Super Bitcoin), etc. appeared continuously A forked chain has become a tool for many interest groups to accumulate money.