Chaincode for Developers
What is Chaincode?
Chaincode is a program, written in Go, node.js, or Java that implements a prescribed interface. Chaincode runs in a seperate process from the peer and initializes and manages the ledger state through transactions submitted by applications.
A chaincode typically handles business logic agreed to by members of the network, so it similar to a “smart contract”. A chaincode can be invoked to update or query the ledger in a proposal transaction. Given the appropriate permission, a chaincode may invoke another chaincode, either in the same channel or in different channels, to access its state. Note that, if the called chaincode is on a different channel from the calling chaincode, only read query is allowed. That is, the called chaincode on a different channel is only a Query, which does not participate in state validation checks in subsequent commit phase.
In the following sections, we will explore chaincode through the eyes of an application developer. We’ll present a simple chaincode sample application and walk through the purpose of each method in the Chaincode Shim API. If you are a network operator who is deploying a chaincode to running network, visit the Deploying a smart contract to a channel tutorial and the Fabric chaincode lifecycle concept topic.
Chaincode API
Every chaincode program must implement the Chaincode interface whose methods are called in response to received transactions. You can find the reference documentation of the Chaincode Shim API for different languages below:
In each language, the Invoke method is called by clients to submit transaction proposals. This method allows you to use the chaincode to read and write data on the channel ledger.
You also need to include an Init method that will serve as the initialization function for your chaincode. This method will be called in order to initialize the chaincode when it is started or upgraded. By default, this function is never executed. However, you can use the chaincode definition to request that the Init function be executed. If execution of Init is requested, fabric will ensure that Init is invoked before any other function and is only invoked once. This option provides you additional control over which users can initialize the chaincode and the ability to add initial data to the ledger. If you are using the peer CLI to approve the chaincode definition, use the --init-required flag to request the execution of the Init function. Then call the Init function by using the peer chaincode invoke command and passing the --isInit flag. For more information, see Fabric chaincode lifecycle.
The other interface in the chaincode “shim” APIs is the ChaincodeStubInterface:
which is used to access and modify the ledger, and to make invocations between chaincodes.
In this tutorial using Go chaincode, we will demonstrate the use of these APIs by implementing a simple chaincode application that manages simple “assets”.
Simple Asset Chaincode
Our application is a basic sample chaincode to create assets (key-value pairs) on the ledger.
Choosing a Location for the Code
If you haven’t been doing programming in Go, you may want to make sure that you have Go Programming Language installed and your system properly configured.
Now, you will want to create a directory for your chaincode application as a child directory of $GOPATH/src/.
To keep things simple, let’s use the following command:
mkdir -p $GOPATH/src/sacc && cd $GOPATH/src/sacc
Now, let’s create the source file that we’ll fill in with code:
touch sacc.go
Housekeeping
First, let’s start with some housekeeping. As with every chaincode, it implements the Chaincode interface in particular, Init and Invoke functions. So, let’s add the Go import statements for the necessary dependencies for our chaincode. We’ll import the chaincode shim package and the peer protobuf package. Next, let’s add a struct SimpleAsset as a receiver for Chaincode shim functions.
package main
import (
"fmt"
"github.com/hyperledger/fabric-chaincode-go/shim"
"github.com/hyperledger/fabric-protos-go/peer"
)
// SimpleAsset implements a simple chaincode to manage an asset
type SimpleAsset struct {
}
Initializing the Chaincode
Next, we’ll implement the Init function.
// Init is called during chaincode instantiation to initialize any data.
func (t *SimpleAsset) Init(stub shim.ChaincodeStubInterface) peer.Response {
}
Note
Note that chaincode upgrade also calls this function. When writing a chaincode that will upgrade an existing one, make sure to modify the Init function appropriately. In particular, provide an empty “Init” method if there’s no “migration” or nothing to be initialized as part of the upgrade.
Next, we’ll retrieve the arguments to the Init call using the ChaincodeStubInterface.GetStringArgs function and check for validity. In our case, we are expecting a key-value pair.
// Init is called during chaincode instantiation to initialize any
// data. Note that chaincode upgrade also calls this function to reset
// or to migrate data, so be careful to avoid a scenario where you
// inadvertently clobber your ledger's data!
func (t *SimpleAsset) Init(stub shim.ChaincodeStubInterface) peer.Response {
// Get the args from the transaction proposal
args := stub.GetStringArgs()
if len(args) != 2 {
return shim.Error("Incorrect arguments. Expecting a key and a value")
}
}
Next, now that we have established that the call is valid, we’ll store the initial state in the ledger. To do this, we will call ChaincodeStubInterface.PutState with the key and value passed in as the arguments. Assuming all went well, return a peer.Response object that indicates the initialization was a success.
// Init is called during chaincode instantiation to initialize any
// data. Note that chaincode upgrade also calls this function to reset
// or to migrate data, so be careful to avoid a scenario where you
// inadvertently clobber your ledger's data!
func (t *SimpleAsset) Init(stub shim.ChaincodeStubInterface) peer.Response {
// Get the args from the transaction proposal
args := stub.GetStringArgs()
if len(args) != 2 {
return shim.Error("Incorrect arguments. Expecting a key and a value")
}
// Set up any variables or assets here by calling stub.PutState()
// We store the key and the value on the ledger
err := stub.PutState(args[0], []byte(args[1]))
if err != nil {
return shim.Error(fmt.Sprintf("Failed to create asset: %s", args[0]))
}
return shim.Success(nil)
}
Invoking the Chaincode
First, let’s add the Invoke function’s signature.
// Invoke is called per transaction on the chaincode. Each transaction is
// either a 'get' or a 'set' on the asset created by Init function. The 'set'
// method may create a new asset by specifying a new key-value pair.
func (t *SimpleAsset) Invoke(stub shim.ChaincodeStubInterface) peer.Response {
}
As with the Init function above, we need to extract the arguments from the ChaincodeStubInterface. The Invoke function’s arguments will be the name of the chaincode application function to invoke. In our case, our application will simply have two functions: set and get, that allow the value of an asset to be set or its current state to be retrieved. We first call ChaincodeStubInterface.GetFunctionAndParameters to extract the function name and the parameters to that chaincode application function.
// Invoke is called per transaction on the chaincode. Each transaction is
// either a 'get' or a 'set' on the asset created by Init function. The Set
// method may create a new asset by specifying a new key-value pair.
func (t *SimpleAsset) Invoke(stub shim.ChaincodeStubInterface) peer.Response {
// Extract the function and args from the transaction proposal
fn, args := stub.GetFunctionAndParameters()
}
Next, we’ll validate the function name as being either set or get, and invoke those chaincode application functions, returning an appropriate response via the shim.Success or shim.Error functions that will serialize the response into a gRPC protobuf message.
// Invoke is called per transaction on the chaincode. Each transaction is
// either a 'get' or a 'set' on the asset created by Init function. The Set
// method may create a new asset by specifying a new key-value pair.
func (t *SimpleAsset) Invoke(stub shim.ChaincodeStubInterface) peer.Response {
// Extract the function and args from the transaction proposal
fn, args := stub.GetFunctionAndParameters()
var result string
var err error
if fn == "set" {
result, err = set(stub, args)
} else {
result, err = get(stub, args)
}
if err != nil {
return shim.Error(err.Error())
}
// Return the result as success payload
return shim.Success([]byte(result))
}
Implementing the Chaincode Application
As noted, our chaincode application implements two functions that can be invoked via the Invoke function. Let’s implement those functions now. Note that as we mentioned above, to access the ledger’s state, we will leverage the ChaincodeStubInterface.PutState and ChaincodeStubInterface.GetState functions of the chaincode shim API.
// Set stores the asset (both key and value) on the ledger. If the key exists,
// it will override the value with the new one
func set(stub shim.ChaincodeStubInterface, args []string) (string, error) {
if len(args) != 2 {
return "", fmt.Errorf("Incorrect arguments. Expecting a key and a value")
}
err := stub.PutState(args[0], []byte(args[1]))
if err != nil {
return "", fmt.Errorf("Failed to set asset: %s", args[0])
}
return args[1], nil
}
// Get returns the value of the specified asset key
func get(stub shim.ChaincodeStubInterface, args []string) (string, error) {
if len(args) != 1 {
return "", fmt.Errorf("Incorrect arguments. Expecting a key")
}
value, err := stub.GetState(args[0])
if err != nil {
return "", fmt.Errorf("Failed to get asset: %s with error: %s", args[0], err)
}
if value == nil {
return "", fmt.Errorf("Asset not found: %s", args[0])
}
return string(value), nil
}
Pulling it All Together
Finally, we need to add the main function, which will call the shim.Start function. Here’s the whole chaincode program source.
package main
import (
"fmt"
"github.com/hyperledger/fabric-chaincode-go/shim"
"github.com/hyperledger/fabric-protos-go/peer"
)
// SimpleAsset implements a simple chaincode to manage an asset
type SimpleAsset struct {
}
// Init is called during chaincode instantiation to initialize any
// data. Note that chaincode upgrade also calls this function to reset
// or to migrate data.
func (t *SimpleAsset) Init(stub shim.ChaincodeStubInterface) peer.Response {
// Get the args from the transaction proposal
args := stub.GetStringArgs()
if len(args) != 2 {
return shim.Error("Incorrect arguments. Expecting a key and a value")
}
// Set up any variables or assets here by calling stub.PutState()
// We store the key and the value on the ledger
err := stub.PutState(args[0], []byte(args[1]))
if err != nil {
return shim.Error(fmt.Sprintf("Failed to create asset: %s", args[0]))
}
return shim.Success(nil)
}
// Invoke is called per transaction on the chaincode. Each transaction is
// either a 'get' or a 'set' on the asset created by Init function. The Set
// method may create a new asset by specifying a new key-value pair.
func (t *SimpleAsset) Invoke(stub shim.ChaincodeStubInterface) peer.Response {
// Extract the function and args from the transaction proposal
fn, args := stub.GetFunctionAndParameters()
var result string
var err error
if fn == "set" {
result, err = set(stub, args)
} else { // assume 'get' even if fn is nil
result, err = get(stub, args)
}
if err != nil {
return shim.Error(err.Error())
}
// Return the result as success payload
return shim.Success([]byte(result))
}
// Set stores the asset (both key and value) on the ledger. If the key exists,
// it will override the value with the new one
func set(stub shim.ChaincodeStubInterface, args []string) (string, error) {
if len(args) != 2 {
return "", fmt.Errorf("Incorrect arguments. Expecting a key and a value")
}
err := stub.PutState(args[0], []byte(args[1]))
if err != nil {
return "", fmt.Errorf("Failed to set asset: %s", args[0])
}
return args[1], nil
}
// Get returns the value of the specified asset key
func get(stub shim.ChaincodeStubInterface, args []string) (string, error) {
if len(args) != 1 {
return "", fmt.Errorf("Incorrect arguments. Expecting a key")
}
value, err := stub.GetState(args[0])
if err != nil {
return "", fmt.Errorf("Failed to get asset: %s with error: %s", args[0], err)
}
if value == nil {
return "", fmt.Errorf("Asset not found: %s", args[0])
}
return string(value), nil
}
// main function starts up the chaincode in the container during instantiate
func main() {
if err := shim.Start(new(SimpleAsset)); err != nil {
fmt.Printf("Error starting SimpleAsset chaincode: %s", err)
}
}
Chaincode access control
Chaincode can utilize the client (submitter) certificate for access control decisions by calling the GetCreator() function. Additionally the Go shim provides extension APIs that extract client identity from the submitter’s certificate that can be used for access control decisions, whether that is based on client identity itself, or the org identity, or on a client identity attribute.
For example an asset that is represented as a key/value may include the client’s identity as part of the value (for example as a JSON attribute indicating that asset owner), and only this client may be authorized to make updates to the key/value in the future. The client identity library extension APIs can be used within chaincode to retrieve this submitter information to make such access control decisions.
See the client identity (CID) library documentation for more details.
To add the client identity shim extension to your chaincode as a dependency, see Managing external dependencies for chaincode written in Go.
Managing external dependencies for chaincode written in Go
Your Go chaincode requires packages (like the chaincode shim) that are not part of the Go standard library. These packages must be included in your chaincode package.
There are many tools available for managing (or “vendoring”) these dependencies. The following demonstrates how to use govendor:
govendor init govendor add +external // Add all external package, or govendor add github.com/external/pkg // Add specific external package
This imports the external dependencies into a local vendor directory. If you are vendoring the Fabric shim or shim extensions, clone the Fabric repository to your $GOPATH/src/github.com/hyperledger directory, before executing the govendor commands.
Once dependencies are vendored in your chaincode directory, peer chaincode package and peer chaincode install operations will then include code associated with the dependencies into the chaincode package.
This article has been published from the source link without modifications to the text. Only the headline has been changed.
