This document discusses using code generation to create a GraphQL API from GRPC services defined in Protobuf files. It proposes: 1. Parsing Protobuf and configuration files to normalize data about GRPC services and messages. 2. Using multiple plugins that each specialize in converting a data source (e.g. Protobuf, Swagger) into GraphQL definitions. 3. Generating GraphQL schema and resolver code from the normalized data using text templates, instead of heavy code generation tools like protoc-gen-gogoopsee. This approach aims to produce cleaner, more customizable and debuggable code generation by separating concerns between data preparation and code output.

1. GraphQL API
Gateway
Go code generation isn’t frightful.
Yaroslav Mytso
Software engineer at EGT Ukraine

2. Service 1
(GRPC API)
Service 2
(GRPC API)
Service 3
(GRPC API)
Service N
(GRPC API)
Front-end
X
X
X
X

3. API
Gateway
Service 1
(GRPC API)
Service 2
(GRPC API)
Service 3
(GRPC API)
Service N
(GRPC API)
Front-end

4. {
user(id: «1») {
name
}
dog(name: «A»){
nick
}
}
{
«user»: {
«name»: «UserName»
},
«dog»: {
«nick»: «Bobby»
}
}
Type: User
Type: Pet
Type: String
Type: String
Type: Query

5. GraphQL → GRPC Proxy writing process
GRPC Message
GraphQL
Output Object
GraphQL
Input Object
GRPC Service
GraphQL
Fields Array
GRPC Enum
GraphQL
Enum
Input Object
unmarshaler+
GRPC
Method
Method
Resolver

6. Message in .proto file VS Message in GraphQL Schema
message A {
int32 someField = 1;
int64 someAnotherField = 2;
string andAnotherOne = 3;
}
var GQLA = graphql.NewObject(graphql.ObjectConfig{
Name: "A",
Fields: graphql.Fields{
"someField": &graphql.Field{
Name: "someField",
Type: graphql.Int,
Resolve: func(p graphql.ResolveParams) (interface{}, error) {
switch src := p.Source.(type) {
case A:
return src.SomeField, nil
case *A:
return src.SomeField, nil
}
return nil, nil
},
},
"someAnotherField": &graphql.Field{
Name: "someAnotherField",
Type: graphql.Int,
Resolve: func(p graphql.ResolveParams) (interface{}, error) {
switch src := p.Source.(type) {
case A:
return src.SomeAnotherField, nil
case *A:
return src.SomeAnotherField, nil
}
return nil, nil
},
},
"andAnotherOne": &graphql.Field{
Name: "andAnotherOne",
Type: graphql.String,
Resolve: func(p graphql.ResolveParams) (interface{}, error) {
switch src := p.Source.(type) {
case A:
return src.AndAnotherOne, nil
case *A:
return src.AndAnotherOne, nil
}
return nil, nil
},
},
},
})
😞

7. protoc-gen-gogoopsee
(gogo plugin)
http://github.com/opsee/protobuf/

8. *.proto gogo
stdin
stdout
GRPC Client
GRPC Client Tests
GoGo code generation process
plugin 1plugin 1 plugin 2 plugin N

9. protoc-gen-gogoopsee
GRPC Message
GraphQL
Output Object
GraphQL
Input Object
GRPC Service
GraphQL
Fields Array
GRPC Enum
GraphQL
Enum
Input Object
unmarshaler+
GRPC
Method
Field
Resolver
X
X
X

10. p.P(`»`, field.GetName(), `": &`, graphQLPkg.Use(), `.Field{`)
p.In()
p.P(`Type: `, p.graphQLType(message, field, graphQLPkg, schemaPkg), `,`)
p.P(`Description: `, fieldGQL, `,`)
p.P(`Resolve: func(p `, graphQLPkg.Use(), `.ResolveParams) (interface{}, error) {`)
p.In()
p.P(`return nil, nil`)
p.Out()
p.P(`}}`)
p.Out()
fmt.Println() generated code

11. «{{$field.Name}}»: {{gqlPkg}}.Field{
Type: {{call $field.Type}},
Description: {{$field.Description}},
Resolve: func(p {{gqlPkg}}.ResolveParams) (interface{}, error) {
return nil, nil
}
}
Generate code using
text/template

12. GoGo plugin problems
• Only two files in output
• Generator knows about only one .proto file and can’t
make composition
• Hard to debug
• Option-based configuration

13. .proto files parser
generate.yaml
vendor_path: «./vendor»
paths:
- «$GOPATH/src»
- «./vendor»
proto_files:
- path : «./a.proto»
- …
- path: «./b.proto»
schema:
Query:
- field: «a»
type: «SERVICE»
- field: «b»
type: «SERVICE»
Parser
Query {
a : AService,
b: BService,
}
AService {
meth1(req:Req) : MethResp,
meth2(req:Req) : SomeRes,
}
BService {
meth1(req:Req) : MethResp,
}
a.proto b.proto
Template
schema.go
./services/a.go
./services/b.go
proto2gql

14. Too heavy template

15. Generator responsibilities
.proto files
GraphQL schema

16. Generator responsibilities
.proto files
GraphQL schema
swagger files

17. Custom .proto files parser
generate.yaml
vendor_path: «./vendor»
paths:
- «$GOPATH/src»
- «./vendor»
proto_files:
- path : «./a.proto»
- …
- path: «./b.proto»
schema:
Query:
- field: «a»
type: «SERVICE»
- field: «b»
type: «SERVICE»
Query {
a : AService,
b: BService,
}
AService {
meth1(req:Req) : MethResp,
meth2(req:Req) : SomeRes,
}
BService {
meth1(req:Req) : MethResp,
}
Parser
a.proto b.proto
Template
schema.go
./services/a.go
./services/b.go

18. Normalizing data
generate.yaml
vendor_path: «./vendor»
paths:
- «$GOPATH/src»
- «./vendor»
proto_files:
- path : «./a.proto»
- …
- path: «./b.proto»
schema:
Query:
- field: «a»
type: «SERVICE»
- field: «b»
type: «SERVICE»
Query {
a : AService,
b: BService,
}
AService {
meth1(req:Req) : MethResp,
meth2(req:Req) : SomeRes,
}
BService {
meth1(req:Req) : MethResp,
}
.proto
parser
a.proto
b.proto
Template
schema.go
./services/a.go
./services/b.go
swagger
Parser
data
normalizer
data
normalizer
swagger1.yml
swagger2.yml

19. generate.yaml
vendor_path: «./»
graphql:
schemas:
…
proto2gql:
proto_files:
- …
swagger2gql:
swagger_files:
- …
graphql plugin
(knows how to generate graphql
schema based on own DTO’s)
proto2gql plugin
(knows how to convert info about
*.proto file into graphql DTO’s)
proto2gql
swagger2gql plugin
(knows how to convert info
swagger file into graphql DTO’s)
Plugins

20. github.com/saturn4er/proto2gql

21. Conclusions
• Use templates in code generation
• Prepare data for templates in case of a possibility
of some other data-source
• Choose toolchain that will not frame you

22. Questions?
Yaroslav Mytso
yaroslav.mitso@egt-ua.com
github.com/saturn4er