gateway

Service-oriented Hyperledger Fabric application development with CCKit

Hyperledger Fabric application development evolution

Early Hyperledger Fabric application implementations leveraged JSON data model, simple chaincode method routing and REST API architecture as the de-facto technology stack.

CCKit, library for Hyperledger Fabric application development, addresses several aspects of Hyperledger Fabric application development:

• Chaincode method routing allows to consistently define rules how an chaincode responds to a client requests

• Using protocol buffers can help to define data model once and then easily write and read structured data to and from chaincode world state

• Testing tools enhances the development experience with extended version of MockStub for chaincode testing.

Next step is to standardize following aspects of blockchain application development using gRPC Interface Definition Language (IDL):

• chaincode interface definition
• chaincode SDK and API's creation with code generation
• chaincode documentation building with code generation

Proposed methodology leverages power of gRPC services and messages definitions. A chaincode app developer may express the interface to their application in a high level interface definition language, and CCKit cc-gateway generator will automatically generate:

• chaincode service interface and helper for embedding service into chaincode router
• chaincode gateway for external access (can be used as SDK or exposed as gRPC or REST service)
• chaincode documentation in markdown format
• chaincode REST API specification (swagger)

After generating all this components blockchain developer only need to implement chaincode business logic, using CCKit features for working with state, access control or data encryption.

Blockchain network services

Blockchain network consists of multiple services, on-chain (chaincodes) and off-chain (API's and other external to blockchain applications, interacts with smart contracts).

For example, official Commercial paper chaincode example includes smart contract implementation and cli tools for interacting with deployed сhaincodes. With external applications, implemented with different technologies and programming languages, it is important to have a standard way to define service interfaces and underlying message interchange formats.

Using gRPC ecosystem for chaincode development

Fundamentals of gRPC

With gRPC, a client application can directly call methods on a server application on a remote machine as if it were a local object. gRPC is based on the foundations of conventional Remote Procedure Call (RPC) technology but implemented on top of the modern technology stacks such as HTTP2, protocol buffers etc. to ensure maximum interoperability.

Like many RPC systems, gRPC is based around the idea of defining a service, specifying the methods that can be called remotely with their parameters and return types. gRPC technology stack natively supports a clean and powerful way to specify service contracts using the Interface Definition Language (IDL):

• messages defines data structures of the input parameters and return types.
• services definition outlines methods signatures that can be invoked remotely

When the client invokes the service, the client-side gRPC library uses the protocol buffer and marshals the remote procedure call, which is then sent over HTTP2. On the server side, the request is un-marshaled and the respective procedure invocation is executed using protocol buffers. The response follows a similar execution flow from the server to the client.

The main advantage of developing services and clients with gRPC is that your service code or client side code doesn’t need to worry about parsing JSON or similar text-based message formats. What comes in the wire is a binary format, which is unmarshalled into an object. Also, having first-class support for defining a service interface via an IDL is a powerful feature when we have to deal with multiple microservices and ensure and maintain interoperability.

gRPC service as RESTful HTTP API

gRPC service can be exposed as REST service using grpc-gateway plugin of the Google protocol buffers compiler protoc. It reads protobuf service definitions and generates a reverse-proxy server which translates a RESTful HTTP API into gRPC.

Chaincode as service

Application, interacting with smart contracts can be defined and implemented as gRPC service. But, what if chaincode itself implement with respect to service definition in gRPC format ?

Chaincode interface

Chaincode interacts with the shared ledger and defines business logic for blockchain network: a set of contracts covering common terms, data, rules, concept definitions, and processes (for example, Commercial paper or ERC20 token functionality), lay out the business model that govern all of the interactions between transacting parties.

Chaincode interface is very simple and contains only 2 methods:

type Chaincode interface {
    // Init is called during Instantiate transaction
    Init(stub ChaincodeStubInterface) pb.Response
    // Invoke is called to update or query the ledger
    Invoke(stub ChaincodeStubInterface) pb.Response
}

Using ChaincodeStubInterface getArgs method chaincode implementation can access input parameters as slice (array) of bytes.

At the moment there is no standard way to describe chaincode interface via some definition language. But chaincode itself can be considered as RPC'like service and defined with gRPC Interface Definition Language (IDL), for example:

service HelloService {
  rpc SayHello (HelloRequest) returns (HelloResponse);
}

message HelloRequest {
  string greeting = 1;
}

message HelloResponse {
  string reply = 1;
}

As this service definition strongly typed (input: string ad output: string) versus relaxed basic chaincode interface (input: []byte and output: []byte) we need mechanisms for converting input []byte to target parameter type, depending on service definition.

Example: Commercial paper chaincode

Let's implement Commercial paper chaincode as service using gRPC service definition and code generation.

1. Define data model

At the first step we create .proto description of the data structure you wish to store and input/output payload. You can read details about chaincode state modelling here.

syntax = "proto3";

package schema;

import "google/protobuf/timestamp.proto";
import "github.com/mwitkow/go-proto-validators/validator.proto";

// Commercial Paper state entry
message CommercialPaper {

    enum State {
        ISSUED = 0;
        TRADING = 1;
        REDEEMED = 2;
    }

    // Issuer and Paper number comprises composite primary key of Commercial paper entry
    string issuer = 1;
    string paper_number = 2;

    string owner = 3;
    google.protobuf.Timestamp issue_date = 4;
    google.protobuf.Timestamp maturity_date = 5;
    int32 face_value = 6;
    State state = 7;

    // Additional unique field for entry
    string external_id = 8;
}

// CommercialPaperId identifier part
message CommercialPaperId {
    string issuer = 1;
    string paper_number = 2;
}

// ExternalId
message ExternalId {
    string id = 1;
}

// Container for returning multiple entities
message CommercialPaperList {
    repeated CommercialPaper items = 1;
}

// IssueCommercialPaper event
message IssueCommercialPaper {
    string issuer = 1 [(validator.field) = {string_not_empty : true}];
    string paper_number = 2 [(validator.field) = {string_not_empty : true}];
    google.protobuf.Timestamp issue_date = 3 [(validator.field) = {msg_exists : true}];
    google.protobuf.Timestamp maturity_date = 4 [(validator.field) = {msg_exists : true}];
    int32 face_value = 5 [(validator.field) = {int_gt : 0}];

    // external_id  - once more uniq id of state entry
    string external_id = 6 [(validator.field) = {string_not_empty : true}];
}

// BuyCommercialPaper event
message BuyCommercialPaper {
    string issuer = 1 [(validator.field) = {string_not_empty : true}];
    string paper_number = 2 [(validator.field) = {string_not_empty : true}];
    string current_owner = 3 [(validator.field) = {string_not_empty : true}];
    string new_owner = 4 [(validator.field) = {string_not_empty : true}];
    int32 price = 5 [(validator.field) = {int_gt : 0}];
    google.protobuf.Timestamp purchase_date = 6 [(validator.field) = {msg_exists : true}];
}

// RedeemCommercialPaper event
message RedeemCommercialPaper {
    string issuer = 1 [(validator.field) = {string_not_empty : true}];
    string paper_number = 2 [(validator.field) = {string_not_empty : true}];
    string redeeming_owner = 3 [(validator.field) = {string_not_empty : true}];
    google.protobuf.Timestamp redeem_date = 4 [(validator.field) = {msg_exists : true}];
}

2. Create service definition

Chaincode interface can be described with gRPC service notation. Using grpc-gateway option we can also define mapping for chaincode REST-API.

The grpc-gateway is a plugin of the Google protocol buffers compiler protoc. It reads protobuf service definitions and generates a reverse-proxy server which translates a RESTful HTTP API into gRPC. This server is generated according to the google.api.http annotations in your service definitions.

syntax = "proto3";

package service;

import "google/api/annotations.proto";
import "google/protobuf/empty.proto";
import "github.com/KompiTech/cckit/examples/cpaper_asservice/schema/schema.proto";

service CPaper {
    // List method returns all registered commercial papers
    rpc List (google.protobuf.Empty) returns (schema.CommercialPaperList) {
        option (google.api.http) = {
            get: "/cpaper"
        };
    }

    // Get method returns commercial paper data by id
    rpc Get (schema.CommercialPaperId) returns (schema.CommercialPaper) {
        option (google.api.http) = {
            get: "/cpaper/{issuer}/{paper_number}"
        };
    }

    // GetByExternalId
    rpc GetByExternalId  (schema.ExternalId) returns (schema.CommercialPaper) {
        option (google.api.http) = {
            get: "/cpaper/extid/{id}"
        };
    }

    // Issue commercial paper
    rpc Issue (schema.IssueCommercialPaper) returns (schema.CommercialPaper) {
        option (google.api.http) = {
            post : "/cpaper/issue"
        };
    }

    // Buy commercial paper
    rpc Buy (schema.BuyCommercialPaper) returns (schema.CommercialPaper) {
        option (google.api.http) = {
            post: "/cpaper/buy"
        };
    }

    // Redeem commercial paper
    rpc Redeem (schema.RedeemCommercialPaper) returns (schema.CommercialPaper) {
        option (google.api.http) = {
            post: "/cpaper/redeem"
        };
    }

    // Delete commercial paper
    rpc Delete (schema.CommercialPaperId) returns (schema.CommercialPaper) {
        option (google.api.http) = {
            delete: "/cpaper/{issuer}/{paper_number}"
        };
    }
}

3. Code generation

Chaincode-as-service gateway generator allows to generate auxiliary components from gRPC service definition:

Install the protoc-gen-cc-gateway generator:

GO111MODULE=on go install github.com/KompiTech/cckit/gateway/protoc-gen-cc-gateway

For documentation generation install protoc-gen-doc:

go get -u github.com/pseudomuto/protoc-gen-doc/cmd/protoc-gen-doc

For generating validation code install ProtoBuf Validator Compiler

go get github.com/mwitkow/go-proto-validators/protoc-gen-govalidators

Command for generating chaincode auxiliary code, swagger specification and documentation can be found in Makefile

.: generate

generate:
	@protoc --version
	@echo "commercial paper schema proto generation"
	@protoc -I=./schema/ \
	-I=../../vendor \
	--go_out=./schema/    \
	--govalidators_out=./schema/ \
    --doc_out=./schema/ --doc_opt=markdown,schema.md \
	./schema/schema.proto

	@echo "commercial paper service proto generation"
	@protoc -I=./service/ \
	-I=../../../../../ \
	-I=../../vendor \
	-I=../../third_party/googleapis \
	--go_out=plugins=grpc:./service/    \
 	--cc-gateway_out=logtostderr=true:./service/ \
	--grpc-gateway_out=logtostderr=true:./service/ \
	--swagger_out=logtostderr=true:./service/ \
	--doc_out=./service/ --doc_opt=markdown,service.md \
	./service/service.proto

• -I flag defines source for data mode source (.schema) or service definition
• go_out flag sets output path for protobuf structures and gRPC service client and server
• govalidators_out flag sets output path for protobuf parameter validators
• grpc-gateway_out flag sets output path for REST-API proxy for gRPC service
• swagger_out flag sets output for REST API swagger specification
• doc_out flag sets output for documentation in markdown format

and finally:

• cc-gateway_out flag sets output path for auxiliary code for building on-chain (chaincode) and off-chain (external applications) blockchain network components:

• Chaincode service to ChaincodeStubInterface mapper

• Chaincode gateway - gRPC service implementation for chaincode external access

Chaincode implementation

Chaincode service implementation must conform to interface, generated from service definition CPaperChaincode :

For simple case, such as Commercial Paper chaincode, service acts as Create-Read-Update-Delete (CRUD) application:

• creates commercial paper entry in the chaincode state (Issue method)
• reads from the chaincode state (List, Get, GetByExternalId methods)
• updates commercial paper entry (Buy, Redeem methods)
• deletes commercial paper entry (Delete method)

Using CCKit state wrapper with entity mapping, implementation should be pretty straightforward:

func (cc *CPaperImpl) List(ctx router.Context, in *empty.Empty) (*schema.CommercialPaperList, error) {
	if res, err := cc.state(ctx).List(&schema.CommercialPaper{}); err != nil {
		return nil, err
	} else {
		return res.(*schema.CommercialPaperList), nil
	}
}

func (cc *CPaperImpl) Get(ctx router.Context, id *schema.CommercialPaperId) (*schema.CommercialPaper, error) {
	if res, err := cc.state(ctx).Get(id, &schema.CommercialPaper{}); err != nil {
		return nil, err
	} else {
		return res.(*schema.CommercialPaper), nil
	}
}

func (cc *CPaperImpl) Issue(ctx router.Context, issue *schema.IssueCommercialPaper) (*schema.CommercialPaper, error) {
	// Validate input message using the rules defined in schema
	if err := issue.Validate(); err != nil {
		return nil, errors.Wrap(err, "payload validation")
	}

	// Create state entry
	cpaper := &schema.CommercialPaper{
		Issuer:       issue.Issuer,
		PaperNumber:  issue.PaperNumber,
		Owner:        issue.Issuer,
		IssueDate:    issue.IssueDate,
		MaturityDate: issue.MaturityDate,
		FaceValue:    issue.FaceValue,
		State:        schema.CommercialPaper_ISSUED, // Initial state
		ExternalId:   issue.ExternalId,
	}

	if err := cc.event(ctx).Set(issue); err != nil {
		return nil, err
	}

	if err := cc.state(ctx).Insert(cpaper); err != nil {
		return nil, err
	}
	return cpaper, nil
}

...

Chaincode implementation also must contain state and event mappings

type CPaperImpl struct {
}

func (cc *CPaperImpl) state(ctx router.Context) m.MappedState {
	return m.WrapState(ctx.State(), m.StateMappings{}.
		//  Create mapping for Commercial Paper entity
		Add(&schema.CommercialPaper{},
			m.PKeySchema(&schema.CommercialPaperId{}), // Key namespace will be <"CommercialPaper", Issuer, PaperNumber>
			m.List(&schema.CommercialPaperList{}),     // Structure of result for List method
			m.UniqKey("ExternalId"),                   // External Id is unique
		))
}

func (cc *CPaperImpl) event(ctx router.Context) state.Event {
	return m.WrapEvent(ctx.Event(), m.EventMappings{}.
		// Event name will be "IssueCommercialPaper", payload - same as issue payload
		Add(&schema.IssueCommercialPaper{}).
		// Event name will be "BuyCommercialPaper"
		Add(&schema.BuyCommercialPaper{}).
		// Event name will be "RedeemCommercialPaper"
		Add(&schema.RedeemCommercialPaper{}))
}

Then, chaincode service implementation can be embedded into chaincode method router with generated RegisterCPaperChaincode function:

func NewCC() (*router.Chaincode, error) {
	r, err := CCRouter(`CommercialPaper`)
	if err != nil {
		return nil, err
	}

	return router.NewChaincode(r), nil
}

func CCRouter(name string) (*router.Group, error) {
	r := router.New(name)
	// Store on the ledger the information about chaincode instantiation
	r.Init(owner.InvokeSetFromCreator)

	if err := service.RegisterCPaperChaincode(r, &CPaperImpl{}); err != nil {
		return nil, err
	}

	return r, nil
}

CCKit components for blockchain network layers

Chaincode service to ChaincodeStubInterface mapper

Generated on top of gRPC service definition chaincode service mapper allows to embed chaincode service implementation into CCKit router, leveraging middleware capabilities for converting input and output data.

For example, Commercial Paper as service generated code contains RegisterCPaperChaincode method which maps chaincode Issue method to chaincode service implementation:

// Code generated by protoc-gen-cc-gateway. DO NOT EDIT.
// source: service.proto

import (
	cckit_router "github.com/KompiTech/cckit/router"
	cckit_defparam "github.com/KompiTech/cckit/router/param/defparam"
)

const (
    ..
	CPaperChaincode_Issue = "Issue"
	...
)	
	
// RegisterCPaperChaincode registers service methods as chaincode router handlers
func RegisterCPaperChaincode(r *cckit_router.Group, cc CPaperChaincode) error {

    ...
	r.Invoke(CPaperChaincode_Issue,
		func(ctx cckit_router.Context) (interface{}, error) {
			return cc.Issue(ctx, ctx.Param().(*schema.IssueCommercialPaper))
		},
		cckit_defparam.Proto(&schema.IssueCommercialPaper{}))
    ...
    
}

Chaincode invocation service

Chaincode invocation service defines gRPC service for interacting with smart contract from external application with 3 methods:

• Query ( ChaincodeInput ) returns ( ProposalResponse )
• Invoke ( ChaincodeInput ) returns ( ProposalResponse )
• Events (ChaincodeLocator ) returns ( ChaincodeEvent )

This service used by Chaincode gateway or can be exposed separately as gRPC or REST API. CCKit contains chaincode service implementation based on https://github.com/s7techlab/hlf-sdk-go and version for testing, based on Mockstub

syntax = "proto3";

package service;

import "github.com/hyperledger/fabric/protos/peer/proposal_response.proto";
import "github.com/hyperledger/fabric/protos/peer/chaincode_event.proto";

message ChaincodeInput  {
    // Chaincode name
    string chaincode = 1;
    // Channel name
    string channel =  2;

    // Input contains the arguments for invocation.
    repeated bytes args = 3;

    // TransientMap contains data (e.g. cryptographic material) that might be used
    // to implement some form of application-level confidentiality. The contents
    // of this field are supposed to always be omitted from the transaction and
    // excluded from the ledger.
    map<string, bytes> transient = 4;
}


message ChaincodeLocator {
    // Chaincode name
    string chaincode = 1;
    // Channel name
    string channel =  2;
}

// Chaincode invocation service
service Chaincode {
    // Query chaincode on home peer. Do NOT send to orderer.
    rpc Query (ChaincodeInput) returns (protos.ProposalResponse);
    // Invoke chaincode on peers, according to endorsement policy and the SEND to orderer
    rpc Invoke (ChaincodeInput) returns (protos.ProposalResponse);
    // Chaincode events stream
    rpc Events (ChaincodeLocator) returns (stream protos.ChaincodeEvent);
}

Chaincode gateway

Chaincode gateway use chaincode service to interact with deployed chaincode. It knows about channel and chaincode name, but don't know about chaincode method signatures.

Chaincode gateway supports options for providing transient data during chaincode invocation, and encrypting/ decrypting data.

Using gRPC service definition we can generate gateway for particular chaincode, for example for Commercial Paper. This gateway can be used as:

• gRPC service
• Chaincode SDK for using in other services
• REST service with grpc-gateway

For example, generated chaincode gateway for Commercial Paper example looks like this:

// Code generated by protoc-gen-cc-gateway. DO NOT EDIT.
// source: service.proto

import (
    cckit_ccservice "github.com/KompiTech/cckit/gateway/service"
	cckit_gateway "github.com/KompiTech/cckit/gateway"
)
	
// gateway implementation
// gateway can be used as kind of SDK, GRPC or REST server ( via grpc-gateway or clay )
type CPaperGateway struct {
	Gateway cckit_gateway.Chaincode
}

// NewCPaperGateway creates gateway to access chaincode method via chaincode service
func NewCPaperGateway(ccService cckit_ccservice.Chaincode, channel, chaincode string, opts ...cckit_gateway.Opt) *CPaperGateway {
	return &CPaperGateway{Gateway: cckit_gateway.NewChaincode(ccService, channel, chaincode, opts...)}
}

type ValidatorInterface interface {
	Validate() error
}

func (c *CPaperGateway) Issue(ctx context.Context, in *schema.IssueCommercialPaper) (*schema.CommercialPaper, error) {
	var inMsg interface{} = in
	if v, ok := inMsg.(ValidatorInterface); ok {
		if err := v.Validate(); err != nil {
			return nil, err
		}
	}

	if res, err := c.Gateway.Invoke(ctx, CPaperChaincode_Issue, []interface{}{in}, &schema.CommercialPaper{}); err != nil {
		return nil, err
	} else {
		return res.(*schema.CommercialPaper), nil
	}
}

...

API

Using generated chaincode gateway you can easily build external to chaincode application. For example, to create API application you need to create entry point for the HTTP reverse-proxy server and use generated gateway in gRPC server:

package main

import (
	"context"
	"io/ioutil"
	"log"
	"net"
	"net/http"
	"time"

	"github.com/grpc-ecosystem/grpc-gateway/runtime"
	"github.com/KompiTech/cckit/examples/cpaper_asservice"
	cpaperservice "github.com/KompiTech/cckit/examples/cpaper_asservice/service"
	"github.com/KompiTech/cckit/gateway"
	"github.com/KompiTech/cckit/gateway/service"
	"github.com/KompiTech/cckit/testing"
	"google.golang.org/grpc"
)

const (
	chaincodeName = `cpaper`
	channelName   = `cpaper`

	grpcAddress = `:8080`
	restAddress = `:8081`
)

func main() {
	ctx := context.Background()
	ctx, cancel := context.WithCancel(ctx)
	defer cancel()

	// Create mock for commercial paper chaincode invocation

	// Commercial paper chaincode instance
	cc, err := cpaper_asservice.NewCC()
	if err != nil {
		log.Fatalln(err)
	}

	// Mockstub for commercial paper
	cpaperMock := testing.NewMockStub(chaincodeName, cc)

	// Chaincode invocation service mock. For real network you can use example with hlf-sdk-go
	cpaperMockService := service.NewMock().WithChannel(channelName, cpaperMock)

	// default identity for signing requests to peeer (mocked)
	apiIdentity, err := testing.IdentityFromFile(`MSP`, `../../../testdata/admin.pem`, ioutil.ReadFile)
	if err != nil {
		log.Fatalln(err)
	}
	// Generated gateway for access to chaincode from external application
	cpaperGateway := cpaperservice.NewCPaperGateway(
		cpaperMockService, // gateway use mocked chaincode access service
		channelName,
		chaincodeName,
		gateway.WithDefaultSigner(apiIdentity))

	grpcListener, err := net.Listen("tcp", grpcAddress)
	if err != nil {
		log.Fatalf("failed to listen grpc: %v", err)
	}

	// Create gRPC server
	s := grpc.NewServer()
	cpaperservice.RegisterCPaperServer(s, cpaperGateway)

	// Runs gRPC server in goroutine
	go func() {
		log.Printf(`listen gRPC at %s`, grpcAddress)
		if err := s.Serve(grpcListener); err != nil {
			log.Fatalf("failed to serve gRPC: %v", err)
		}
	}()

	// wait for gRPC service stared
	time.Sleep(3 * time.Second)

	// Register gRPC server endpoint
	mux := runtime.NewServeMux()
	opts := []grpc.DialOption{grpc.WithInsecure()}
	err = cpaperservice.RegisterCPaperHandlerFromEndpoint(ctx, mux, grpcAddress, opts)
	if err != nil {
		log.Fatalf("failed to register handler from endpoint %v", err)
	}

	log.Printf(`listen REST at %s`, restAddress)

	// Start HTTP server (and proxy calls to gRPC server endpoint)
	if err = http.ListenAndServe(restAddress, mux); err != nil {
		log.Fatalf("failed to serve REST: %v", err)
	}
}

Provided example use mocked chaincode invocation service, but for interacting with real Hyperledger Fabric network you just need to change chaincode invocation service to implementation using SDK, from example hlf-sdk-go

You can run provide example using command

cd examples/cpaper_asservice/bin/api/mock
go run main.go

Commercial paper service REST-API specification generated in swagger format:

You can use API usage examples and sample payloads by sending HTTP requests to grpc-gateway HTTP proxy:

Access to chaincode methods also available via gRPC service, that can be called with generated gRPC client. Service and schema documentation also auto-generated.

Conclusion

Provided tools allows to specify chaincode data model and interface and then generate code for building on-chain (chaincode) and off-chain (API, Oracles, SDK etc) application in consistent manner.