# Multi-Blockchain Structure for a Crowdsensing-Based Smart Parking System
## 3 Proposed System
### Overview of Proposed System  
The proposed scheme uses two types of blockchain. One is a public blockchain, wherein contributors participate in MCS, and the other is a private blockchain, wherein service providers and users participate in the processing, provision, and use of data. Bridge nodes participate in both blockchains to transfer data between the two blockchains.  
Figure 2 shows the overall schematic structure of the system proposed in this paper. Arrows indicate the direction of the process of data exchange between the objects. 1 is a public blockchain, in which the session management for data provision module and the incentive management module operate, and 2 is a private blockchain, in which the session management for data gathering and the data processing module operate.  
Crowd contributors provide data to service providers using smart contracts placed on the blockchains. The transaction is executed through the deployed smart contract, and the result is stored in a public block. The bridge node executes the corresponding contract in the private blockchain based on the block contents of the public blockchain and stores it in the private block.  
The service provider processes the data delivered by the contributor, for the service user to be able to identify the parking space. When the user confirms the data delivered by the contributor, verifies its validity, and distributes it to the private blockchain network, the bridge node executes a contract for reward payment to the public blockchain and stores it in the public block.  
The service provider checks the validity of the data delivered by the contributor, stores it on local storage, provides it also to the private blockchain, and provides the corresponding parking space information when requested by the service user. The user checks and uses the validity of the provided parking space information, and when the validity information is announced to the private blockchain network, the bridge node executes a contract disclosing incentive information for reward payment in the public blockchain and stores the incentive information in the public block.  
Figure 2: Structure of the proposed system.  
When a user searches through a distributed application (dApp) for a parking lot at a location that the user wants to use, the user finds the service provider of the parking lot searched through a smart contract of the private blockchain, and the smart contract sends a data request to the service provider of the corresponding parking lot. The service provider finds the requested data to provide and record the log information for the provided data in the private blockchain, and provides the data with the data hash to the user.  
A bridge node is one or more nodes connected simultaneously to a public blockchain and a private blockchain. The bridge node monitors both blockchains, and when confirming a transaction to be performed in one blockchain, it transmits the relative information of the transaction to the other blockchain based on the contents of the transaction. What should be checked on both blockchains are the processes of establishing a session to transmit data, and of paying the reward for crowdsensing.