# Systematic Literature Review of Challenges in Blockchain Scalability
## 1 Introduction  
Many online transactions between individuals or organizations are based on a centralized controlled system (or controlled by a third-party organization). For example, a bank or a credit card vendor is acting as a third-party entity in executing a digital payment or money transfer process between two organizations (or individuals). The third-party vendor takes a fee for every successful transaction. In this centralized mechanism, the third-party controls and manages almost all the information of the stakeholders that are involved in the online transaction. This approach requires the third party to uphold the transaction's security. In contrast, blockchain is an immutable distributed ledger of cryptographicallysigned transactions maintained by a peer-to-peer network, where no third party is required to manage the information, and trust is no longer an issue among the network participants. Blockchain technology is one of the most hyped decentralized innovations, with an enlightening future. Blockchain was introduced by [PERSON] and [PERSON] [1] and later gained intense attention because of the Bitcoin principle by [PERSON] in 2008 [2]. Bitcoin is highly successful in the cryptocurrency arena. Many similar currencies have were launched following Bitcoin. There were 2017 crypto currencies available on the internet by 2019 [3], with different business models. Besides global crypto currency hype, Bitcoin holds the highest market capitalization up to 53%. Blockchain is serving as the fundamental technology behind Bitcoin. The survey conducted by World Economic Forum [4] showed that blockchain will be soaring to 10% of global GDP by 2027. It has been two decades since the launch of Bitcoin as the first public blockchain application. Till then, blockchain technology had been restricted to cryptocurrency (Bitcoin and Ethereum) public blockchains settings. It has hardly been accepted in other industries since. Among the many hindrances, scalability is found to be the key hurdle in implementing public blockchains in many real business environments. In general, scalability has not been well-defined in the literature. Basically, the scalability issue arises with the increasing number of nodes and transactions in blockchain. This issue is indeed present in major public blockchain applications (e.g., Bitcoin and Ethereum) because every node needs to store and execute a computational task to validate every transaction. The public blockchains are therefore always demanding a huge amount of computational power, a high bandwidth internet connection, and massive storage space. Transaction throughput and transaction latency are the two most discussed performance metrics in blockchain, and both have not reached a satisfactory QoS level in many recent popular public blockchain systems. For instance, Bitcoin and Ethereum are able to process 7 [5,6] to 20 [7] transactions-per-second (TPS), but they also face high consensus processing time delays (the average time required to mine a block) at a magnitude of up to 10 min. Besides efficiency, the current size of Bitcoin, Ethereum, and Litcoin are, respectively, 305.23 GB, 667.10 GB, and 28.45 GB [8], causing great demand on storage spaces. The time needed to download the whole blockchain is considerable. Several studies deliberated the concept of scalability trilemma [5,8]. Initially, it was described by [PERSON], the co-founder of Ethereum [9]. [PERSON] stated that trade-offs are inevitable between three important blockchain properties: decentralization, scalability, and security (see Figure 1). Decentralization is the core and the nature of blockchain. Security is an essential propriety, whereas scalability is the main challenge. In other words, the scalability trilemma states that trade-offs are almost inevitable among these characteristics of blockchain [10,11]. Figure 1: Scalability trilemma.