Starting with Blockchain Chaincode using Golang

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Chaincode, or a smart contract is a fragment of code written in supported languages like Java or Go that is deployed onto a network of HyperLedger Fabric peer nodes. In this blog, we will learn how to develop Chaincode using Golang for a blockchain network based on Hyperledger Fabric v0.6.

Chaincodes run network transactions which are validated and then appended to the shared ledger. In simple terms, they are encapsulation of business network transactions in a code.

Development environment required for Chaincode development:

Go Go 1.6 install
Hyperledger Fabric
- Use the following command to install Hyperledger fabric 0.6:

git clone -b v0.6 http://gerrit.hyperledger.org/r/fabric

Implementing the chaincode interface

Following are the main functions required to implement the chaincode shim interface in your Go code.

Init() :

Init() is called when you first deploy your chaincode.

An example of using this function with Go can be,

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	func (t *SampleChaincode) Init(stub shim.ChaincodeStubInterface, function string, args []string) ([]byte, error) {

	if len(args) != 1 {

	return nil, errors.New("Incorrect number of arguments. Expecting 1")

	}

	err := stub.PutState("emp_name", []byte(args[0]))

	if err != nil {

	return nil, err

	}

	return nil, nil

	}

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Init()

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As the name suggests, Init() is used to do any initialization your chaincode needs. In this example, stub.PutState() stores the first value in the args argument in the key “emp_name”.

Invoke()

It is called when we expect some “real work” to be done by the chaincode.

Example: Every time there’s a transaction, the ledger will be updated by invoking the chaincode.

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	func (t *SimpleChaincode) Invoke(stub shim.ChaincodeStubInterface, function string, args []string) ([]byte, error) {
	fmt.Println("invoke is running " + function)

	// Handle different functions
	if function == "init" {
	return t.Init(stub, "init", args)
	} else if function == "write" {
	return t.write(stub, args)
	}
	fmt.Println("invoke did not find func: " + function)

	return nil, errors.New("Received unknown function invocation")
	}

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Invoke()

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Here, the Invoke() function calls a generic write function which will help you store any key value pair in the ledger.

Query()

This function is called to get the state of chaincode.
Query is used to only read the value of your chaincode state’s key/value pairs and it does not add any new blocks to the ledger.

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	func (t *SampleChaincode) Query(stub shim.ChaincodeStubInterface, function string, args []string) ([]byte, error) {

	fmt.Println("query is running " + function)

	// Handle different functions

	if function == "dummy_query" { //read a variable

	fmt.Println("hi there " + function) //error

	return nil, nil;

	}

	fmt.Println("query did not find func: " + function) //error

	return nil, errors.New("Received unknown function query: " + function)

	}

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Query()

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Main()

The starting point for any Go program is the main function, so here it is used for starting the chaincode.
When the peer deploys its instance of the chaincode, the main function gets executed.

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	func main() {
	err := shim.Start(new(SampleChaincode))
	if err != nil {
	fmt.Println("Could not start SampleChaincode")
	} else {
	fmt.Println("SampleChaincode successfully started")
	}

	}

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main()

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In this example, shim.Start(new(SampleChaincode)) line is used to start the chaincode and register it with the peer.
To verify this locally, run the chaincode in your development environment, which should produce the following error: