Hyperledger中數據存取的實現

簡介

本文介紹了在Hyperledger中數據存取的實現.

API接口

Hyperledger提供基於key/value的數據存儲，其中key是字符串，value則是二進制字節數組，Hyperledger的Go API提供了三個方法用於數據存取：PutState（key, value）用於向Hyperledger中存儲數據， GetState(key)用於從Hyperledger中提取數據，而DelState(key)則從Hyperledger中刪除數據。

數據存取 Chaincode 示例

以下是一個簡單的數據存取Chaincode， 以及其相應的REST請求。

package main

import (
    "errors"
    "fmt"

    "github.com/hyperledger/fabric/core/chaincode/shim"
)


type SaveState1Chaincode struct {
}

func (t *SaveState1Chaincode) Init(stub shim.ChaincodeStubInterface, function string, args []string) ([]byte, error) {
    fmt.Printf("Init called with function %s!\n", function)

    return nil, nil
}

func (t *SaveState1Chaincode) Invoke(stub shim.ChaincodeStubInterface, function string, args []string) ([]byte, error) {
    fmt.Printf("Invoke called with function %s!\n", function)

    var key, value string
    key = args[0]
    value = args[1]

    var err error
    err = stub.PutState(key, []byte(value))

    if err != nil {
        return nil, err
    } 


    return nil, nil    
}

func (t *SaveState1Chaincode) Query(stub shim.ChaincodeStubInterface, function string, args []string) ([]byte, error) {
    fmt.Printf("Query called with function %s!\n", function)

    var key string
    key = args[0]

    valInBytes, err := stub.GetState(key)

    if err != nil {
        return nil, errors.New("Failed to get state for " + key)
    }

    message := "State for "  + key + " = " + string(valInBytes)

    return []byte(message), nil;
}

func main() {
    err := shim.Start(new(SaveState1Chaincode))
    if err != nil {
        fmt.Printf("Error starting Save State chaincode: %s", err)
    }
}

存儲數據的REST請求

{
   "jsonrpc": "2.0",
   "method": "invoke",
   "params": {
       "type": 1,
       "chaincodeID":{
           "name":"mycc"
       },
       "ctorMsg": {
          "function":"invoke",
          "args":["testKey", "testValue"]
       },
       "secureContext": "jim"
   },
   "id": 3
 }  

獲取數據的REST請求

 {
   "jsonrpc": "2.0",
   "method": "query",
   "params": {
       "type": 1,
       "chaincodeID":{
           "name":"mycc"
       },
       "ctorMsg": {
          "function":"query",
          "args":["testKey"]
       },
       "secureContext": "jim"
   },
   "id": 5
 }

關於Immutability

以上代碼也可以看出Hyperledger和BitCoin和Ethereum等區塊鏈對Immutability的不同理解， 在Hyperledger中，數據提交到區塊鏈後不僅可以改變，還甚至可以被刪除，而在BitCoin和Ethereum中數據一旦提交到區塊鏈後就不能再被改變。

這也體現在R3的Corda區塊鏈中，R3 CTO Richard Gendal Brown在這裡 寫道：

Immutability

The fourth feature in the “Blockchain Bundle” is often, if misleadingly, termed “immutability”: data, once committed, cannot be changed.

This isn’t quite true: if I have a piece of data then of course I can change it. What we actually mean is that: once committed, nobody else will accept a transaction from me if it tries to build on a modified version of some data that has already been accepted by other stakeholders.

Blockchains achieve this by having transactions commit to the outputs of previous transactions and have blocks commit to the content of previous blocks. Each new step can only be valid if it really does build upon an unchangeable body of previous activity.

總結

本文介紹了在Hyperledger中數據存取的實現以及關於Immutability的討論.