Abstract: As one of the key technologies of distributed ledgers, blockchain solves the trust problem in open network without relying on any trusted third party. Its decentralized feature makes it potential for a wide range of application scenarios. However, it still faces scalability problems. The bottleneck of blockchain scalability is mainly in two aspects: low efficiency and difficulty in functional extension. For instance, Bitcoin can only deal with 7 transactions per second averagely. Obviously, it cannot meet the requirement of current digital payment scenarios, nor can it be carried in other applications such as distributed storage and credit investigation service. On the other hand, the data or assets within different blockchains are difficult to interact with each other. This restricts the functional extension of blockchain system. In reality, there are a variety of blockchain systems which are specially devised for various functionalities or applications. Therefore, it is crucial to establish interaction channels among different blockchains to make them form the Internet of value. So far the research of blockchain scalability has attracted much attention from both academia and industry due to its importance. This paper introduces and analyzes the blockchain scalability related technologies from the aspects of improving efficiency and extending functionality of blockchain system, respectively. Firstly, we introduce three major schemes for performance enhancement of blockchain, including off-chain payment network, Bitcoin-NG and sharding mechanism; and four typical cross-chain approaches for blockchain functionality extension. Then we analyze the merits and demerits of each technology, based on which we give the challenges and suggestions for further research in blockchain scalability.

Key words: blockchain, scalability, off-chain payment channel, Bitcoin-NG, sharding, cross-chain