Consensus vs Sybil Control

I seem to have had this conversation a few times recently, so thought it may be useful to throw some thoughts in a post.

People tend to think that Proof of Work (PoW) and Proof of Stake (PoS) are consensus protocols, which is incorrect.

PoW and PoS are sybil control mechanisms, which do not achieve consensus on their own, but are rather cryptographic approaches to limit access to a resource, almost as a spam filter. In blockchains and other distributed systems, they are coupled with different consensus algorithms, for example in the bitcoin network the consensus protocol is based on the longest chain consensus rule.

Sybil control mechanisms have been around since before blockchains and crypto, Proof of Work was first mentioned in a paper in in 1993 titled: Pricing via Processing, Or, Combatting Junk Mail, Advances in Cryptology
TLDR:
If a system is required to perform some computations that take 0.5s before it is able to send an electronic message, it will not affect regular users, but the time and resources required to send 2m spam messages will be prohibitive for spammers as it would take 277 hours to disburse these messages.

There are a number of consensus algorithms adopted by various blockchains, such as PBFT (used by Stellar, Ripple and Hyperledger), GHOST (a modified version of the bitcoin network protocol, a simplified version of which is currently used by Ethereum), Cosmos and more recently Avalanche.

Hope this was helpful.