# Systematic Literature Review of Challenges in Blockchain Scalability
## 8 RQ3: How Researchers Address the Scalability Issue in Blockchain?
### On-Chain Solution
#### 8.1.4 Consensus-Protocol-Related Approaches  
In this section, some major consensus techniques are discussed. These techniques have been implemented and applied in different applications for the sake of improving scalability in public blockchains. Based on this SLR findings, the consensus protocol is found to be the second-most-discussed factor. The inefficiency in the consensus protocol is the main cause of scalability issue in public blockchains. The research community has therefore tried to address the scalability issue with different innovative consensus approaches. 1. Proof of work  
In 2008, [PERSON] (an individual or a group) proposed the initial idea of Proof-of-Work (PoW) model with Bitcoin [2]. Since then, it has been widely used in public blockchains, especially in cryptocurrencies like the Bitcoin. In this model, the miners (nodes) on every transaction compete to solve an intensive mathematical puzzle (hashing), to win a chance to add the next Block to the chain and to earn a reward in the form of Bitcoin for their energy consumed and work done in the mining process [24; 124]. For getting a chance to append a new Block, every miner (node) must justify that it has accomplished sufficient work done. Therefore, it is referred to as proof-of-work consensus protocol. It is important to mention here that the Bitcoin dynamically controls the difficulty level of the cryptographic puzzle [125]. 2. Proof of stake  
Proof-of-stake (PoS) [126] consensus model is considered an energy-efficient version of PoW because, comparatively, it saves more energy than PoW. In this model, miners (nodes) are supposed to affirm the ownership on the currency they have. Therefore, the miner (node) possessing the highest number of currencies would be given a chance for adding next block to the chains. It is believed that the miners (nodes) possessing more currency would not attack the blockchain network or it would be less likely they would attack. Furthermore, the node associated with higher monetary reward will dominantly control the network [124] because it will always secure chance to publish the block. This is not fair to newer nodes with less currency. There are various PoS variants available with the deployment of the appropriate stake size for selecting a node to append the next block in public blockchains, e.g., peercoin and Blackcoin. 3. Delegated Proof-of-Stake consensus  
The Delegated PoS (DPoS) [18] is considered a variant of the proof-of-stake. It does not constitute a significant improvement, but the discrepancy between the proof-of-stake (PoS) and the DPoS is mainly based on direct democracy, while the other is based on a democratic representative [107]. In DPoS model, the miners (nodes) are given the right to find their representative, which is called delegate. The delegate is required to perform three tasks, including creating, validating, and verifying the block. The process of validating would be much faster if a limited miner (nodes) performed the validation process instead of the whole network. Therefore, it would directly affect the transaction throughput. Furthermore, the delegates are accountable for controlling and managing the block size. The dishonest delegate should not be a concern because every node has the right to vote for delegate of their choice. Bitshares is an example of DPoS implementation. 4. Practical byzantine fault tolerance PBFT consensus protocol is typically used to accept Byzantine faults [18] and is often used by permissioned blockchain, i.e., Hyperledger, because it is able handle up to 1/3 malicious byzantine replica. This model works in rounds process. There are predefined steps to follow in every round in selecting a primary node. The processing of PBFT consensus protocol is divided into three stages: pre-prepared, prepared, and commit. The node should get the majority of 2/3 votes from all the nodes to change the state of the Blockchain. This will confirm that majority of the nodes are identifiable and known to every node.