This project focuses on db and private chain. When user use create/update/delete, it will record hash into private chain automatically for improving trustness.
It means user can request private chain for ensure the data in db isn't modified.

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Smart Contract
1. Private chain

2. Public chain

Note:
	1. The feature to use Reverse hash to find key is removed, because the value will the same in different key. It can be solved by record all keys into an array. However, skip it to implement backend first.
	2. Private chain can be different blockchain implementation, such as Ethereum, NEO or EOS. Right now, I just use ethereum as infrastructure.
	3. Because it is private chain, the gas comsumption isn't the most important concern here.

Backend Implementation
1. Library for db operation.
	The library is provided for implementation. Developers don't consider any lower
	level implementation in smart contract.
2. Node for private chain.

3. Node for public chain
	Use gevent to implement as async daemon.

Note:
	1. Due to tracing pyethereum and related python project in ethereum, I decided to use gevent as
		asynchorize library.
	2. Now json file is used instead traditional database due to the POC. It will be easy to change other traditional database implementation.