Methods of reaching consensus

In a blockchain, or in any distributed database, there are four common algorithms to reach consensus:

1. Proof-of-work (PoW)
2. Practical Byzantine fault tolerance (PBFT)
3. Proof-of-stake (PoS)
4. Delegated proof-of-stake (DPoS)

A good consensus scheme should solve the following questions:

• Who is allowed to append blocks to the chain?
• When do updates happen on the blockchain?
• What transactions should be added to the block?
• How can you permit secure software changes to the blockchain?
• How can attacks or inconsistencies be solved?