Chaincode for Developers

What is Chaincode?

Chaincode is a program, written in Go that implements a prescribed interface. Eventually, other programming languages such as Java, will be supported. Chaincode runs in a secured Docker container isolated from the endorsing peer process. Chaincode 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”. Ledger state created by a chaincode is scoped exclusively to that chaincode and can’t be accessed directly by another chaincode. Given the appropriate permission, a chaincode may invoke another chaincode to access its state within the same network.

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.

Chaincode API

Every chaincode program must implement the Chaincode interface whose methods are called in response to received transactions. In particular the Init method is called when a chaincode receives an instantiate or upgrade transaction so that the chaincode may perform any necessary initialization, including initialization of application state. The Invoke method is called in response to receiving an invoke transaction to process transaction proposals.

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, 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.

package main

import (
    "fmt"

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

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

Building Chaincode

Now let’s compile your chaincode.

go get -u --tags nopkcs11 github.com/hyperledger/fabric/core/chaincode/shim
go build --tags nopkcs11

Assuming there are no errors, now we can proceed to the next step, testing your chaincode.

Testing Using dev mode

Normally chaincodes are started and maintained by peer. However in “dev mode”, chaincode is built and started by the user. This mode is useful during chaincode development phase for rapid code/build/run/debug cycle turnaround.

We start “dev mode” by leveraging pre-generated orderer and channel artifacts for a sample dev network. As such, the user can immediately jump into the process of compiling chaincode and driving calls.

Install Hyperledger Fabric Samples

If you haven’t already done so, please install the Hyperledger Fabric Samples.

Navigate to the chaincode-docker-devmode directory of the fabric-samples clone:

cd chaincode-docker-devmode

Download Docker images

We need four Docker images in order for “dev mode” to run against the supplied docker compose script. If you installed the fabric-samples repo clone and followed the instructions to download-platform-specific-binaries, then you should have the necessary Docker images installed locally.

Note

If you choose to manually pull the images then you must retag them as latest.

Issue a docker images command to reveal your local Docker Registry. You should see something similar to following:

docker images
REPOSITORY                     TAG                                  IMAGE ID            CREATED             SIZE
hyperledger/fabric-tools       latest                               e09f38f8928d        4 hours ago         1.32 GB
hyperledger/fabric-tools       x86_64-1.0.0                         e09f38f8928d        4 hours ago         1.32 GB
hyperledger/fabric-orderer     latest                               0df93ba35a25        4 hours ago         179 MB
hyperledger/fabric-orderer     x86_64-1.0.0                         0df93ba35a25        4 hours ago         179 MB
hyperledger/fabric-peer        latest                               533aec3f5a01        4 hours ago         182 MB
hyperledger/fabric-peer        x86_64-1.0.0                         533aec3f5a01        4 hours ago         182 MB
hyperledger/fabric-ccenv       latest                               4b70698a71d3        4 hours ago         1.29 GB
hyperledger/fabric-ccenv       x86_64-1.0.0                         4b70698a71d3        4 hours ago         1.29 GB

Note

If you retrieved the images through the download-platform-specific-binaries, then you will see additional images listed. However, we are only concerned with these four.

Now open three terminals and navigate to your chaincode-docker-devmode directory in each.

Terminal 1 - Start the network

docker-compose -f docker-compose-simple.yaml up

The above starts the network with the SingleSampleMSPSolo orderer profile and launches the peer in “dev mode”. It also launches two additional containers - one for the chaincode environment and a CLI to interact with the chaincode. The commands for create and join channel are embedded in the CLI container, so we can jump immediately to the chaincode calls.

Terminal 2 - Build & start the chaincode

docker exec -it chaincode bash

You should see the following:

root@d2629980e76b:/opt/gopath/src/chaincode#

Now, compile your chaincode:

cd sacc
go build

Now run the chaincode:

CORE_PEER_ADDRESS=peer:7051 CORE_CHAINCODE_ID_NAME=mycc:0 ./sacc

The chaincode is started with peer and chaincode logs indicating successful registration with the peer. Note that at this stage the chaincode is not associated with any channel. This is done in subsequent steps using the instantiate command.

Terminal 3 - Use the chaincode

Even though you are in --peer-chaincodedev mode, you still have to install the chaincode so the life-cycle system chaincode can go through its checks normally. This requirement may be removed in future when in --peer-chaincodedev mode.

We’ll leverage the CLI container to drive these calls.

docker exec -it cli bash
peer chaincode install -p chaincodedev/chaincode/sacc -n mycc -v 0
peer chaincode instantiate -n mycc -v 0 -c '{"Args":["a","10"]}' -C myc

Now issue an invoke to change the value of “a” to “20”.

peer chaincode invoke -n mycc -c '{"Args":["set", "a", "20"]}' -C myc

Finally, query a. We should see a value of 20.

peer chaincode query -n mycc -c '{"Args":["query","a"]}' -C myc

Testing new chaincode

By default, we mount only sacc. However, you can easily test different chaincodes by adding them to the chaincode subdirectory and relaunching your network. At this point they will be accessible in your chaincode container.