## 2 Background and Related Work  
### Blockchain Technology  
[PERSON] conceptualized the first blockchain in 2008. He presented the design of using a secure hash function for chaining blocks into the first blockchain platform, Bitcoin [1], which provides the most successful decentralized electronic cash system. Bitcoin is a digital ledger-based system using the proof-of-work (PoW) consensus algorithm and a few other technologies to help verify transactions and create a global peer-to-peer (p2p), decentralized cryptocurrency. In 2014, a new blockchain platform was born called Ethereum. Ethereum is proposed by [PERSON] [9]. [PERSON] suggested the usage of smart contracts for providing programming capability to the blockchain. Moreover, one can execute smart contracts on a blockchain base using simple business logic, and the involved data are immutable and transparent. Finally, the internet-like nature of blockchain means that anyone of interest can join the blockchain network anonymously. Due to its trustworthy and decentralized nature, blockchain technology has been successfully applied to solve social business and social credit issues [10; 11; 12], besides its regular banking industry applications [13; 14; 15].  
The adopted blockchain platform, called Hyperledger Fabric [16], belongs to the category of permissioned blockchains, in which the member's identification is strictly restricted. Fabric delivers high degrees of confidentiality, flexibility, resiliency, and scalability. These characteristics make the solution developed based on Fabric suitable for lots of industrial applications. The membership service in Fabric's Permissioned Model can be integrated with various standard identity management systems. Fabric adopted a novel architectural approach to support this flexibility and improved how the blockchain responds to non-determinism, resource exhaustion, and performance attacks. Fabric can also create channels enabling a group of participants to create separate transaction ledgers. This capability is essential for networks where some participants may not want to have a competitor for each transaction.  
Inspired by the pre-described characteristics of permissioned blockchain and the requirements of a TimeBank system, we realized a blockchain-based TimeBank system based on the Hyperledger Fabric framework [5]. Our TimeBank provided a grading system allowing involved members to give each other a grade to reflect their degrees of satisfaction about the system's services. This grading system incentivized the members to provide a better quality of service and adopt a friendlier attitude for receiving a service, which may positively endorse the development of a worldwide TimeBank system. For simplicity, the system realized in [5] matched up the posted services with a naive matching method, i.e., the matching between services' supply-and-demand tasks was done intuitively and conducted directly through autonomous smart contracts. Once people complete their service exchange, the one who provides the service can execute the token transferring function to get their time credit as a return. Although the flexibility of this naive approach has been proved by the simulation conducted in [5], we believe that to fit the dynamic nature of the supply-and-demand of a practical mutual assistance community, a more effective matching algorithm is a must. Therefore, this work presents an efficient blockchain-based TimeBank realization with a newly proposed dynamic service matching algorithm (DSMA) to achieve the goal mentioned above.