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In the blockchain setting, certainty ensures that all valid blocks will not be revoked once they are submitted to the blockchain. When users make a transaction, they also want to ensure that the transfer operation can not be changed or cancelled at will after the transfer is completed. Therefore, certainty becomes very important when designing blockchain consensus protocols. In the current system based on the Nakamoto Satoshi consensus, 51% of the mining activities and selfish mining activities threaten the integrity of the system only by allowing the possibility of revocation of blocks (for example, if the wrongdoer has accumulated 51% of the mining capacity, they can have two flowers). This protocol provides probabilistic determination, while some other protocols achieve absolute certainty.

The type of certainty

Probabilistic determination (Probabilistic Finality) is a deterministic type of protocol based on blockchain (for example, the Satoshi Nakamoto consensus of Bitcoin). In probabilistic determination, the deeper the block containing the transaction is buried in the chain, the less likely it is that the transaction will be cancelled. Because the more blocks behind a block, the more likely the (bifurcated) chain containing that block is the longest chain. This is why it is recommended that you wait until the block containing the transaction is six blocks deep in the Bitcoin blockchain to confirm that the transaction is completed (about an hour), because the possibility of canceling the transaction is very low.

Absolute determination (Absolute Finality) is a deterministic type based on Byzantine fault-tolerant (PBFT) protocols such as Tendermint. In absolute determination, once the transaction is included in the block and added to the block chain, the transaction is immediately considered final. In this case, a verifier will first propose a block, which must be approved by enough verifiers on the committee before it can be submitted to the block chain.

Economic certainty (Economic Finality), that is, the cost of capital required to withdraw the block is very high. In the basic system of proof of interest using the forfeiture mechanism (such as Casper FFG,Tendermint), if the equity holder signs on two blocks of the same height, then all their rights and interests will be forfeited, which is a high price for compromising certainty. For example, if a network of 100 equity holders each holds equity worth $1 million, the entire network has a total equity value of $100m. If two blocks appear at the same height of the blockchain, named B and B, and B gets 66% of the votes of equity holders ($66 million) and B' gets 66% of the votes ($66 million), then the intersection of B and B'(at least 33% of malicious equity holders) will lose all their rights and interests (at least $33 million).

CAP Theorem and certainty

It seems that absolute determination is more feasible than probabilistic determination, but there are still some basic tradeoffs that suggest that it is better to choose a block chain that supports probabilistic determination. Eric Brewer's CAP theorem plays a role when considering how to strike a proper balance between probabilistic determination and Byzantine fault-tolerant certainty. CAP theorem points out that in the case of network partition, distributed systems can only satisfy consistency or availability. Systems that meet consistency will stop running and do not allow wrong transactions to pass. Systems that meet availability will continue to run even if the wrong transaction is allowed to pass. The consistent system has Byzantine fault tolerance certainty, while the availability system has probabilistic determination.

In payment scenarios, users usually choose the availability provided by probabilistic blockchain (which is why many DAG-based protocols focus on supporting payment because these protocols support usability rather than consistency). However, many blockchain platforms provide not only payment, but also decentralized applications (DApp) based on smart contracts. Different DApp may have different preferences in terms of certainty: those DApp that need feasibility, even if the transaction information is inaccurate, will always let the transaction through, preferring the probabilistic determination chain, while the consistent DApp will stop the whole application from running to prevent incorrect transactions from passing, preferring the absolute determination chain. Therefore, certainty fundamentally affects the user experience.

Rights and interests prove certainty in consensus

In the meta-analysis of alternative consensus protocols, we consider the certainty guarantees of some major PoS (proof of interest) platforms, including Tendermint, Thunderella, Algorand, Dfinity, Ouroboros Genesis, Casper FFG and Casper CBC. Here, we will give a brief overview of how these platforms achieve certainty, but it is more important to decide which protocol to adopt as a whole, rather than just considering a parameter (here, the guarantee of certainty). Wuxi Gynecology Hospital × × http://wapyyk.39.net/wx/zonghe/fc96e.html

Tendermint:Tendermint realizes absolute determination. Any blocks that receive pre-voting and pre-submission of ⅔ or above will be finalized and the process will continue indefinitely. Unless the verifier of ⅓ or above does not respond, causing the network to stop running. Therefore, Tendermint prefers consistency to usability. In addition, when the penalty rule of interest proof is applied to Tendermint, Tendermint protocol can also achieve economic certainty.

Thunderella:Thunderella 's fast path provides absolute certainty. Any notarized maximum transaction sequence is considered to be a fully confirmed output. If the Fast track Committee of 3Accord 4 is honest and online, and the proponent is honest, then valid transactions can be confirmed immediately. However, fast path confirmation is different from general certainty in that it is optimistic. Once the transaction is recorded on the underlying block chain, the transaction is fully determined, which can be either chain-based or Byzantine fault tolerance. However, when there is a problem with the fast path, Thunderella falls back to the underlying blockchain, so Thunderella is a priority for availability.

Algorand:Algorand implements probabilistic determination. As long as the value of the agreement currency controlled by the party is less than the total value of 1x3je Algorand, the bifurcation is guaranteed to be almost impossible, allowing the protocol to operate in a strongly synchronous manner, so that each block is ultimately consistent. In weak synchronization, Algorand may bifurcate, but BA* is used to determine which bifurcation chain to choose. Therefore, when the protocol resumes strong synchronization, the transaction in Algorand can be finally determined. Algorand prioritizes consistency over availability because it prefers to generate blank chunks rather than sacrifice consistency.

Dfinity:Dfinity realizes probabilistic determination, and its deterministic probability increases with the increase of block weight on the chain. Suppose we refuse to accept more notarized blocks in each r cycle. During this cycle, we can finalize the r round because we know that the notarized block of the r round contains all the transactions on the chain before the r round. In round r, near-instant certainty can be guaranteed as long as the operation is correct, and after two confirmations plus network transmission delays, any transaction included in the block in round r is final for the observer. Dfinity gives priority to consistency, and if the network partition forms two halves of almost the same size, it automatically suspends the random beacon (random beacon) and does not allow any half of the network to continue to operate.

Ouroboros Genesis:Genesis protocol can realize probabilistic determination according to how to select the rules of block chain. The specific rule is that for short distance * * (up to k blocks, where k is the security parameter), the longest chain principle is adopted, while for long range * (more than k blocks), the plenitude rule rule is adopted, that is, the time period is checked immediately after the current chain bifurcates, and then select the chain with higher density.

The goal of Casper FFG:Casper FFG is to provide absolute / economic certainty for chain-based systems, which can be achieved by signing a block after the ⅔ majority vote is weighted by the committee. In this way of building Casper FFG, even if the author controls the proposal mechanism of the underlying blockchain, the checkpoint where the conflict occurs can never be finally determined. However, FFG provides security and the proposal mechanism provides activity, so people can delay reaching a consensus to prevent Casper from determining future checkpoints. FFG gives priority to consistency because it does not allow the checkpoint to be finalized without the consent of the ⅔ verifier, otherwise the determination is invalid. In addition, FFG can also achieve economic certainty through forfeiture mechanism.

Casper TFG:TFG achieves absolute determination through verifiers with different fault-tolerant thresholds. In other words, the protocol is asynchronous secure and Byzantine fault-tolerant, allowing verifiers to have different fault-tolerant thresholds.

Revocation of blocks can result in millions of dollars in losses or affect the basic operation of decentralized applications. Therefore, determinism plays a vital role in building a robust blockchain platform and how to choose the platform on which to develop applications.

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