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.
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.
This changes the technical decisions as to which bricks are used to build the solution.
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.