Consensus & NPOS

What is Consensus in Blockchain?

Blockchain is a secure and decentralized network that relies on a consensus protocol to ensure the accuracy and reliability of its transactions. This protocol enables all nodes in the network to agree on the current state of the ledger, thereby establishing trust between unknown peers in a distributed computing environment. The consensus algorithm helps achieve reliability, collaboration, co-operation, equal rights for every node, and mandatory participation from all nodes in the consensus process. This creates a single version of the truth that is agreed upon by all nodes in the network, making it a win for everyone involved.

Why do we need Consensus?

Consensus is a crucial method for achieving agreement and synchronization among nodes in a decentralized blockchain network. It ensures that all nodes agree on the shared state and allows for the building and progression of the blockchain. Consensus aims to provide an objective view of the state by reconciling the subjective views of the participants. This process enables the nodes to communicate, reach agreement, and build new blocks.

Widely known consensus

Proof of Work

This consensus algorithm used by Bitcoin to select a miner for the next block generation. The algorithm involves solving a complex mathematical puzzle that requires significant computational power. The node that solves the puzzle first is rewarded with the opportunity to mine the next block.

Proof of Stake

This is the most common alternative to PoW. Ethereum has shifted from PoW to PoS consensus. In this type of consensus algorithm, instead of investing in expensive hardware to solve a complex puzzle, validators invest in the coins of the system by locking up some of their coins as stake. After that, all the validators will start validating the blocks. Validators will validate blocks by placing a bet on it if they discover a block which they think can be added to the chain. Based on the actual blocks added in the Blockchain, all the validators get a reward proportionate to their bets and their stake increase accordingly. In the end, a validator is chosen to generate a new block based on their economic stake in the network. Thus, PoS encourages validators through an incentive mechanism to reach to an agreement.

Ternoa's Consensus

Nominated Proof of Stake

In traditional PoS systems, block production participation is dependent on token holdings as opposed to computational power. While PoS developers usually have a proponent for equitable participation in a decentralized manner, most projects end up proposing some level of centralized operation, where the number of validators with full participation rights is limited. These validators are often seen to be the most wealthy, and, as a result, influence the PoS network as they are the most staked. Usually, the number of candidates to maintain the network with the necessary knowledge (and equipment) is limited; this can directly increase operational costs as well. Systems with a large number of validators tend to form pools to decrease the variance of their revenue and profit from economies of scale. These pools are often off-chain.

A way to alleviate this is to implement pool formation on-chain and allow token holders to vote [with their stake] for validators to represent them.

Ternoa uses NPoS (Nominated Proof-of-Stake) as its mechanism for selecting the validator set. It is designed with the roles of validators and nominators, to maximize chain security. Actors who are interested in maintaining the network can run a validator node.

Validators assume the role of producing new blocks in BABE, validating blocks, and guaranteeing finality. Nominators can choose to back validators with their stake. Nominators can approve candidates that they trust and back them with their tokens.

Probabilistic vs. Provable Finality

A pure Nakamoto consensus blockchain that runs PoW is only able to achieve the notion of probabilistic finality and reach eventual consensus. Probabilistic finality means that under some assumptions about the network and participants if we see a few blocks building on a given block, we can estimate the probability that it is final. Eventual consensus means that at some point in the future, all nodes will agree on the truthfulness of one set of data. This eventual consensus may take a long time and will not be able to determine how long it will take ahead of time. However, finality gadgets such as GRANDPA (GHOST-based Recursive ANcestor Deriving Prefix Agreement) or Ethereum's Casper FFG (the Friendly Finality Gadget) are designed to give stronger and quicker guarantees on the finality of blocks - specifically, that they can never be reverted after some process of Byzantine agreements has taken place. The notion of irreversible consensus is known as provable finality.

In the GRANDPA paper, it is phrased in this way:

We say an oracle A in a protocol is eventually consistent if it returns the same value to all participants after some unspecified time.

Hybrid Consensus

There are two protocols we use when we talk about the consensus protocol of Ternoa, GRANDPA and BABE (Blind Assignment for Blockchain Extension). We talk about both of these because Ternoa uses what is known as hybrid consensus. Hybrid consensus splits up the finality gadget from the block production mechanism.

This is a way of getting the benefits of probabilistic finality (the ability to always produce new blocks) and provable finality (having a universal agreement on the canonical chain with no chance for reversion) in Ternoa. It also avoids the corresponding drawbacks of each mechanism (the chance of unknowingly following the wrong fork in probabilistic finality, and a chance for "stalling" - not being able to produce new blocks - in provable finality). By combining these two mechanisms, Ternoa allows for blocks to be rapidly produced, and the slower finality mechanism to run in a separate process to finalize blocks without risking slower transaction processing or stalling.

Hybrid consensus has been proposed in the past. Notably, it was proposed (now defunct) as a step in Ethereum's transition to proof of stake in EIP 1011, which specified Casper FFG.

Block Production

BABE (Blind Assignment for Blockchain Extension) is the block production mechanism that runs between the validator nodes and determines the authors of new blocks. BABE is comparable as an algorithm to Ouroboros Praos, with some key differences in chain selection rule and slot time adjustments. BABE assigns block production slots to validators according to stake and using the Ternoa randomness cycle. The chain's runtime is required to provide the BABE authority list and randomness to the host via a consensus message in the header of the first block of each epoch.

BABE execution happens in sequential non-overlapping phases known as epochs. Each epoch is divided into a predefined number of slots. All slots in each epoch are sequentially indexed starting from 0 (slot number). At the beginning of each epoch, the BABE node needs to run the Block-Production-Lottery algorithm to find out in which slots it should produce a block and gossip to the other block producers.

Validators participate in a lottery for every slot, which will inform whether or not they are the block producer candidate for that slot. Slots are discrete units of time of approximately 6 seconds in length. Because the mechanism of allocating slots to validators is based on a randomized design, multiple validators could be candidates for the same slot. Other times, a slot could be empty, resulting in inconsistent block time.

Multiple Validators per Slot

When multiple validators are block producer candidates in a given slot, all will produce a block and broadcast it to the network. At that point, it's a race. The validator whose block reaches most of the network first wins. Depending on network topology and latency, both chains will continue to build in some capacity, until finalization kicks in and amputates a fork.

No Validators in Slot

When no validators have rolled low enough in the randomness lottery to qualify for block production, a slot can remain seemingly blockless. We avoid this by running a secondary, round-robin style validator selection algorithm in the background. The validators selected to produce blocks through this algorithm always produce blocks, but these secondary blocks are ignored if the same slot also produces a primary block from a VRF-selected validator. Thus, a slot can have either a primary or a secondary block, and no slots are ever skipped.

Finality Gadget

GRANDPA (GHOST-based Recursive ANcestor Deriving Prefix Agreement) is the finality gadget that is implemented for the Ternoa Chain.

The Ternoa uses the GRANDPA Finality protocol to finalize blocks. Finality is obtained by consecutive rounds of voting by the validator nodes. Validators execute the GRANDPA finality process in parallel to Block Production as an independent service.

It works in a partially synchronous network model as long as 2/3 of nodes are honest and can cope with 1/5 Byzantine nodes in an asynchronous setting.

A notable distinction is that GRANDPA reaches agreements on chains rather than blocks, greatly speeding up the finalization process, even after long-term network partitioning or other networking failures.

In other words, as soon as more than 2/3 of validators attest to a chain containing a certain block, all blocks leading up to that one are finalized at once.

Resources

• BABE paper - The academic description of the BABE protocol.

• GRANDPA paper - The academic description of the GRANDPA finality gadget. Contains formal proofs of the algorithm.

• Rust implementation - The reference implementation and the accompanying Substrate pallet.