Why does a permanent chain split occur only in a Hard Fork?

Question

Have read a number of articles on what hard fork and soft fork actually are. But couldn't really get the essence of it. My understanding is Hard Fork brings a change in the core Blockchain code (Accounts, Transactions, Blocks). So the consensus actually breaks cuz the nodes those upgraded and not upgraded have two separate versions of Blockchain.

Whereas in case of Soft Fork, the core Blockchain code remains the same but may be the mining client has some restrictions applied on it. Basically if I want to soft fork say Block Gas Limit from 1.5 mil to 1 mil, instead of changing the core Blockchain code itself, the mining clients and full nodes just apply an external filter to accept only till 1 mil Gas Limit. So no matter if all the nodes upgrade or not, they are governed by the same Blockchain code. Hence any splits shall occur on the same chain and be resolved through GHOST protocol.

Am I clear with my understanding? Someone pls clarify

Answer 1

Your understanding is close but it's not quite right.

A soft fork is a change that makes some block, that was previously legal, illegal.

A hard fork is a change that makes some block, that was previously illegal, legal.

Now imagine the majority of the network has applied the change, and think about this from the point of view of a node that hasn't applied the change.

In the case of a soft fork, all the blocks in the longest chain appear legal. There may occasionally be blocks created by un-upgraded miners that are considered illegal by the rest of the network, but they will soon be orphaned by the majority, so although the un-upgraded node will sometimes have a different opinion about which is the longest chain than the majority, this situation will not persist for long.

In the case of a hard fork, the blocks created in the longest chain appear illegal. So an un-upgraded node will not build on top of them, but will instead build on the longest chain created by its fellow un-upgraded nodes.

So with a soft fork the network will occasionally split but quickly converge, whereas with a hard fork the network will split into two separate chains, which will continue growing in parallel indefinitely.