[NetBehaviour] Forks in the blockchain

[Rob Myers]

On 08/12/17 01:31 PM, Edward Picot via NetBehaviour wrote:
> A question for Rob - or anyone else who happens to know - when there's a
> fork in the Blockchain, what happens to the old, discredited side of the
> fork? What HTML coders used to call the 'deprecated' bit? Does it
> actually get disabled, disconnected or stopped in some way, or does it
> just stay out there on the Blockchain doing its thing, and you hope that
> all the users have moved on to the new improved version, but maybe quite
> a lot of them haven't and never will
Yes that's what happens.

Some blockchain forks are unintentional and temporary. These happen when
different miners running the same software find new blocks at almost the
same time and start continuing the chain on top of them until eventually
one loses out to the other. This occurs surprisingly often and is why
people should wait several blocks (usually around 6, an hour on the
Bitcoin network) before trusting that they have really, finally,
received their coins and that they are not on a transient fork.

Some blockchain forks are intentional, permanent forks. These are tied
to a software fork that changes the rules for creating blocks in the
blockchain. There are actually two kinds of intentional forks, soft and
hard forks. Soft forks don't concern us here as they do not split the
network. But hard forks do.

Hard forks change the rules for finding blocks and building the
blockchain in a way that is incompatible with the old version of the
software. Some miners will run this software, some won't. So some miners
continue building on the old chain, some start building on the new one.
This is where the fork happens.

Over time either everyone will move to the new chain, everyone will
abandon the new chain and return to the old one, or both chains will
continue with a greater or lesser number of the original miners. There
is little economic incentive to store or mine an abandoned chain, but
some people do for whatever reason (there are "block explorer" web sites
for cryptocurrencies that still show new blocks every few minutes, empty
of transactions...).

The best examples of long-lived hard forks at the moment are Ethereum
Classic (from Ethereum) and Bitcoin Cash (from Bitcoin). Proponents of
both will tell you that these are the *real* versions of each chain.

- Rob.