# Systematic Literature Review of Challenges in Blockchain Scalability
## 7 RQ2: What Vital Root Factors Are Causing Scalability Issue in Blockchain? With the increasing popularity of Blockchain technology and its permissionless application such as cryptocurrency-based applications, scalability issue has become a primary focus in the blockchain research community. Many researchers are attempting to analyze in details on this issue [5; 37]. From our SLR, there seems to be no rule-of-thumb or state-of-the-art research with good matrices to address scalability issue in public blockchains. This section intends to discuss factors causing scalability in more detail. Figure 8 illustrates the factors causing scalability issue in public blockchains. Among these factors, transaction throughput has received the most attention. Out of the 121 selected papers, 39 papers discussed transaction throughput as main factor/concern for low scalability in public blockchains. Twenty papers highlighted the consensus mechanism as the second most discussed factor concerning public blockchain scalability. Consensus mechanism is somewhat related to throughput. There are nine papers talked about the computational power involving scalability in public blockchains. Latency and storage are discussed in six papers. The remaining factors such as block size, cost issue, number of nodes, network load, and overall performance are discussed in 1-3 articles, respectively. There are 23 papers clearly stating scalability as an issue, but reasons causing scalability are somewhat unclear. The Table 4 shows the factors related to scalability issues, along with the relevant reference. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**No.** & **Factor** & **Description** & **Ref. Source** \\ \hline \multirow{3}{*}{1} & Transaction & \multicolumn{2}{c}{This implies the total number of} \\  & Throughput & \multicolumn{2}{c}{transactions that the protocol may handle in} \\  & & \multicolumn{2}{c}{one second.} \\  & & \multicolumn{2}{c}{This applies to the time it takes for a} \\  & & \multicolumn{2}{c}{transaction to be initiated to achieve a} \\
2 & Latency & \multicolumn{2}{c}{consensus on it. It is also regarded as a} \\  & & \multicolumn{2}{c}{finality.} \\
3 & Storage & \multicolumn{2}{c}{It refers to the total space/capacity a} \\  & & \multicolumn{2}{c}{blockchain network can consume.} \\  & & \multicolumn{2}{c}{This is total storage capacity of a block to be} \\
4 & Block Size & \multicolumn{2}{c}{utilized by the transactions. The network} \\  & & \multicolumn{2}{c}{will reject the block if it exceeds the storage capacity.} \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Factors related to blockchain scalability. Figure 8: Identified factors causing scalability issue in public blockchains. It should be mentioned here that public blockchain scalability is not a singular term. It is a combination of various parameters, and these parameters are interdependent. In many papers, every parameter is discussed somehow directly or indirectly to the consensus model deployed. For example, transaction throughput, latency, and computational energy are dependent on the efficiency/performance of the consensus model. Block size and storage are also interdependent. Block size can affect transaction throughput and latency, which in turn can be indirectly linked to consensus model. A larger block can store more transactions, thus directly raising the throughput, but it also causes an increase in block propagation time. In Bitcoin, the block interval is about 10 min with a block size of around 1 MB, to illustrate the limits on the number of transactions that can be stored in each block. The consensus mechanism is a process in which the nodes in the public blockchains network agree with each other about the ledger that they hold. The consensus protocol is thus the most fundamental and indispensable component in Blockchain. It provides the essential process flow for the nodes to verify and validate every single transaction and hence to append a new block to a blockchain.