Blockchain Technology - Public VS Private Blockchain

Blockchain is collection of technologies, like LEGO Toys :) Like LEGO Bag, you can take different bricks/technologies and build and get different results.

Public vs private blockchains

There is a big difference in what technologies you need, depending on whether you allow anyone to write to your blockchain, or known, vetted participants.   Bitcoin allows anyone to write to its ledger.

Public blockchains.  Ledgers can be ‘public’ in two senses:

1. Anyone, without permission granted by another authority, can write data
2. Anyone, without permission granted by another authority, can read data

Usually, when people talk about public blockchains, they mean anyone-can-write.
Because bitcoin is designed as a ‘anyone-can-write’ blockchain, where participants aren’t vetted and can add to the ledger without needing approval, it needs ways of arbitrating discrepancies (there is no ‘boss’ to decide), and defence mechanisms against attacks (anyone can misbehave with relative impunity, if there is a financial incentive to do so).  These create cost and complexity to running this blockchain.

Private blockchains.  Conversely, a ‘private’ blockchain network is where the participants are known and trusted: for example, an industry group, or a group of companies owned by an umbrella company.  Many of the mechanisms aren’t needed – or rather they are replaced with legal contracts – “You’ll behave because you’ve signed this piece of paper.”.  This changes the technical decisions as to which bricks are used to build the solution.