A consensus mechanism is a fault-tolerant mechanism that is used in computer and blockchain systems to achieve the necessary agreement on a single data value or a single state of the network among distributed processes or multi-agent systems, such as with cryptocurrencies. It is useful in record-keeping, among other things.
On the Bitcoin blockchain, for instance, the consensus mechanism is known as Proof of Work (PoW), which requires the exertion of computational power in order to solve a difficult but arbitrary puzzle in order to keep all nodes in the network honest.
In any centralized system, like a database holding key information about driving licenses in a country, a central administrator has the authority to maintain and update the database. The task of making any updates — like adding/deleting/updating names of people who qualified for certain licenses — is performed by a central authority that remains solely responsible for maintaining genuine records.
Public blockchains that operate as decentralized, self-regulating systems work on a global scale without any single authority. They involve contributions from hundreds of thousands of participants who work on verification and authentication of transactions occurring on the blockchain as well as block mining activities.
In such a dynamically changing status of the blockchain, these publicly shared ledgers need an efficient, fair, real-time, functional, reliable and secure mechanism to ensure that all the transactions occurring on the network are genuine and all participants agree on a consensus on the status of the ledger. This all-important task is performed by the consensus mechanism, which is a set of rules that decides on the legitimacy of contributions made by the various participants (i.e., nodes or transactors) of the blockchain.
There are different kinds of consensus mechanism algorithms, each of which works on different principles.
Proof of Work (PoW) is a common consensus algorithm used by the most popular cryptocurrency networks like Bitcoin and Litecoin. It requires a participant node to prove that the work that they complete and submit qualifies them to receive the right to add new transactions to the blockchain. However, this whole mining mechanism of bitcoin needs high energy consumption and a longer processing time.
Proof of Stake (PoS) is another common consensus algorithm that evolved as a low-cost, low-energy consuming alternative to the PoW algorithm. It allocates responsibility in maintaining the public ledger to a participant node in proportion to the number of virtual currency tokens held by it. However, this comes with the drawback that it incentivizes hoarding instead of spending.
While PoW and PoS are by far the most prevalent in the blockchain space, there are other consensus algorithms.
Proof of Capacity (PoC) allows the sharing of memory space of the contributing nodes on the blockchain network. The more memory or hard disk space a node has, the more rights it is granted for maintaining the public ledger.
Another, called Proof of History (PoH), developed by the Solana Project and similar to Proof of Elapsed Time (PoET), encodes the passage of time itself cryptographically to achieve consensus without expending many resources.