Current chaincode development best practices and application samples leverages JSON data model, simple chaincode method routing and REST API architecture as the de-facto technology stack. This article proposes methods and tools for blockchain application development with well known specification formats like Protocol Buffers messages and gRPC services definitions.
Hyperledger Fabric network components
Blockchain network consists of multiple components, on-chain (chaincodes) and off-chain (API’s, oracles and other external applications, interacts with smart contracts).
For example, official Commercial paper chaincode example includes smart contract implementation and CLI tools for interacting with deployed сhaincodes. Since this applications can be implemented with different programming languages, it is important to have a standard way to define service interfaces and message formats.
Chaincode development tools
There are numerous libraries for development on-chain and off-chain application, written in Java,Golang and Javascript:
- SDK’s for building off-chain applications ( Node-js, Java, Golang)
- Libraries for chaincode method routing, like https://github.com/hyperledger/fabric-chaincode-node
Specification for chaincode data model and interface
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 blockchain application developer may express the interface to app 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
gRPCorRESTservice) - chaincode documentation in
markdownformat - REST API specification in swagger format
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.
Using gRPC ecosystem for chaincode development
Fundamentals of gRPC
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. Full example code can be found here.
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/s7techlab/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"
body: "*"
};
}
// Buy commercial paper
rpc Buy (schema.BuyCommercialPaper) returns (schema.CommercialPaper) {
option (google.api.http) = {
post: "/cpaper/buy"
body: "*"
};
}
// Redeem commercial paper
rpc Redeem (schema.RedeemCommercialPaper) returns (schema.CommercialPaper) {
option (google.api.http) = {
post: "/cpaper/redeem"
body: "*"
};
}
// 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 generator:
GO111MODULE=on go install github.com/s7techlab/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 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
-Iflag defines source for data mode source (.schema) or service definitiongo_outflag sets output path forprotobufstructures andgRPCservice client and servergovalidators_outflag sets output path forprotobufparameter validatorsgrpc-gateway_outflag sets output path for REST-API proxy forgRPCserviceswagger_outflag sets output for REST API swagger specificationdoc_outflag 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 —
gRPCservice 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 (
Issuemethod) - reads from the chaincode state (
List,Get,GetByExternalIdmethods) - updates commercial paper entry (
Buy,Redeemmethods) - deletes commercial paper entry (
Deletemethod)
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
}
Components for building 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/s7techlab/cckit/router"
cckit_defparam "github.com/s7techlab/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 service or REST API. CCKit contains chaincode service implementation based on hlf-sdk-go, unofficial sofware development kit (SDK) for building off-chain Hyperledger Fabric applications, 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:
gRPCservice- Chaincode SDK for using in other services (oracle, API’s etc)
RESTservice via 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/s7techlab/cckit/gateway/service"
cckit_gateway "github.com/s7techlab/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
}
}
...
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/s7techlab/cckit/examples/cpaper_asservice"
cpaperservice "github.com/s7techlab/cckit/examples/cpaper_asservice/service"
"github.com/s7techlab/cckit/gateway"
"github.com/s7techlab/cckit/gateway/service"
"github.com/s7techlab/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 API example use mocked chaincode invocation service, but for interacting with real Hyperledger Fabric network peer you just need to change chaincode invocation service to implementation using SDK, for example hlf-sdk-go.
You can run provided example using command
# cd examples/cpaper_asservice/bin/api/mock
# go run main.go
Commercial paper service REST-API specification generated in
swagger format:
Then you can use API usage examples and sample payloads:
grpc-gateway will automatically converts http request to gRPC call, input JSON payloads to protobuf, invokes chaincode service and then converts returned value from protobuf to JSON. You can also use this service as pure gRPC service. Chaincode methods 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.
This article has been published from the source link without modifications to the text. Only the headline has been changed.
