Design First!

Overview

Goa takes a different approach to building services by making it possible to describe the design of the service API using a simple Go DSL. Goa uses the description to generate specialized service helper code, client code and documentation. Goa is extensible via plugins, for example the goakit plugin generates code that leverage the Go kit library.

The service design describes the transport independent layer of the services in the form of simple methods that accept a context and a payload and return a result and an error. The design also describes how the payloads, results and errors are serialized in the transport (HTTP or gRPC). For example a service method payload may be built from an HTTP request by extracting values from the request path, headers and body. This clean separation of layers makes it possible to expose the same service using multiple transports. It also promotes good design where the service business logic concerns are expressed and implemented separately from the transport logic.

The Goa DSL consists of Go functions so that it may be extended easily to avoid repetition and promote standards. The design code itself can easily be shared across multiple services by simply importing the corresponding Go package again promoting reuse and standardization across services.

Code Generation

The Goa tool accepts the Go design package import path as input and produces the interface as well as the glue that binds the service and client code with the underlying transport. The code is specific to the API so that for example there is no need to cast or "bind" any data structure prior to using the request payload or response result. The design may define validations in which case the generated code takes care of validating the incoming request payload prior to invoking the service method on the server, and validating the response prior to invoking the client code.

Installation

go install goa.design/goa/v3/cmd/goa@v3

Current Release: v3.13.2

Getting Started

1. Design

Create a new Goa project:

mkdir -p calcsvc/design
cd calcsvc
go mod init calcsvc

Create the file design.go in the design directory with the following content:

package design

import . "goa.design/goa/v3/dsl"

// API describes the global properties of the API server.
var _ = API("calc", func() {
        Title("Calculator Service")
        Description("HTTP service for multiplying numbers, a goa teaser")
        Server("calc", func() {
                Host("localhost", func() { URI("http://localhost:8088") })
        })
})

// Service describes a service
var _ = Service("calc", func() {
        Description("The calc service performs operations on numbers")
        // Method describes a service method (endpoint)
        Method("multiply", func() {
                // Payload describes the method payload
                // Here the payload is an object that consists of two fields
                Payload(func() {
                        // Attribute describes an object field
                        Attribute("a", Int, "Left operand")
                        Attribute("b", Int, "Right operand")
                        // Both attributes must be provided when invoking "multiply"
                        Required("a", "b")
                })
                // Result describes the method result
                // Here the result is a simple integer value
                Result(Int)
                // HTTP describes the HTTP transport mapping
                HTTP(func() {
                        // Requests to the service consist of HTTP GET requests
                        // The payload fields are encoded as path parameters
                        GET("/multiply/{a}/{b}")
                        // Responses use a "200 OK" HTTP status
                        // The result is encoded in the response body
                        Response(StatusOK)
                })
        })
})

This file contains the design for a calc service which accepts HTTP GET requests to /multiply/{a}/{b} where {a} and {b} are placeholders for integer values. The API returns the product of a multiplied by b in the HTTP response body.

2. Implement

Now that the design is done, let's run goa on the design package. In the calcsvc directory run:

goa gen calcsvc/design

This produces a gen directory with the following directory structure:

gen
├── calc
│   ├── client.go
│   ├── endpoints.go
│   └── service.go
└── http
    ├── calc
    │   ├── client
    │   │   ├── cli.go
    │   │   ├── client.go
    │   │   ├── encode_decode.go
    │   │   ├── paths.go
    │   │   └── types.go
    │   └── server
    │       ├── encode_decode.go
    │       ├── paths.go
    │       ├── server.go
    │       └── types.go
    ├── cli
    │   └── calc
    │       └── cli.go
    ├── openapi.json
    └── openapi.yaml

7 directories, 15 files

calc contains the service endpoints and interface as well as a service client.
http contains the HTTP transport layer. This layer maps the service endpoints to HTTP handlers server side and HTTP client methods client side. The http directory also contains a complete OpenAPI 3.0 spec for the service.

The goa tool can also generate example implementations for both the service and client. These examples provide a good starting point:

goa example calcsvc/design

calc.go
cmd/calc-cli/http.go
cmd/calc-cli/main.go
cmd/calc/http.go
cmd/calc/main.go

The tool generated the main functions for two commands: one that runs the server and one the client. The tool also generated a dummy service implementation that prints a log message. Again note that the example command is intended to generate just that: an example, in particular it is not intended to be re-run each time the design changes (as opposed to the gen command which should be re-run each time the design changes).

Let's implement our service by providing a proper implementation for the multiply method. Goa generated a payload struct for the multiply method that contains both fields. Goa also generated the transport layer that takes care of decoding the request so all we have to do is to perform the actual multiplication. Edit the file calc.go and change the code of the multiply function as follows:

// Multiply returns the multiplied value of attributes a and b of p.
func (s *calcsrvc) Multiply(ctx context.Context, p *calc.MultiplyPayload) (res int, err error) {
        return p.A * p.B, nil
}

That's it! we have now a full-fledged HTTP service with a corresponding OpenAPI specification and a client tool.

3. Run

Now let's compile and run the service:

cd cmd/calc
go build
./calc
[calcapi] 16:10:47 HTTP "Multiply" mounted on GET /multiply/{a}/{b}
[calcapi] 16:10:47 HTTP server listening on "localhost:8088"

Open a new console and compile the generated CLI tool:

cd calcsvc/cmd/calc-cli
go build

and run it:

./calc-cli calc multiply -a 2 -b 3
6

The tool includes contextual help:

./calc-cli --help

Help is also available on each command:

./calc-cli calc multiply --help

Now let's see how robust our code is and try to use non integer values:

./calc-cli calc multiply -a 1 -b foo
invalid value for b, must be INT
run './calccli --help' for detailed usage.

The generated code validates the command line arguments against the types defined in the design. The server also validates the types when decoding incoming requests so that your code only has to deal with the business logic.

The service now returns an integer, but most OpenAPI services expect JSON. Lets fix that now!

In design.go, change Result(Int) so it reads like this:

Result(func() {
    Attribute("result", Int)
    Required("result")
})

Inside of calc.go, replace the func block:

func (s *calcsrvc) Multiply(ctx context.Context, p *calc.MultiplyPayload) (res *calc.MultiplyResult, err error) {
	return &calc.MultiplyResult{Result: p.A * p.B}, nil
}

Finally rebuild the app by running the build parts again:

goa gen calcsvc/design
cd cmd/calc
go build
./calc

You can now test and verify that your service is returning JSON:

curl -X 'GET' 'http://localhost:8088/multiply/10/10' -H 'accept: application/json' | jq .

If all goes well, you should see:

{
  "result": 100
}

4. Document

The http directory contains OpenAPI 2.0 and 3.0 specifications in both YAML and JSON format.

The specification can easily be served from the service itself using a file server. The Files DSL function makes it possible to serve a static file. Edit the file design/design.go and add:

var _ = Service("openapi", func() {
	// Serve the file gen/http/openapi3.json for requests sent to
	// /openapi.json. The HTTP file system is created below.
	Files("/openapi.json", "openapi3.json")
})

Re-run goa gen calcsvc/design and note the new directory gen/openapi and gen/http/openapi which contain the implementation for a HTTP handler that serves the openapi.json file.

All we need to do is mount the handler on the service mux. Add the corresponding import statement to cmd/calc/http.go:

import openapisvr "calcsvc/gen/http/openapi/server"

and mount the handler by adding the following line in the same file and after the mux creation (e.g. one the line after the // Configure the mux. comment):

svr := openapisvr.New(nil, mux, dec, enc, nil, nil, http.Dir("../../gen/http"))
openapisvr.Mount(mux, svr)

That's it! we now have a self-documenting service. Stop the running service with CTRL-C. Rebuild and re-run it then make requests to the newly added /openapi.json endpoint:

^C[calcapi] 16:17:37 exiting (interrupt)
[calcapi] 16:17:37 shutting down HTTP server at "localhost:8088"
[calcapi] 16:17:37 exited
go build
./calc

In a different console:

curl localhost:8088/openapi.json
{"openapi":"3.0.3","info":{"title":"Calculator Service","description":...

Resources

Docs

The goa.design website provides a high level overview of Goa and the DSL.

In particular the page Implementing a Goa Service explains how to leverage the generated code to implement an HTTP or gRPC service.

The DSL Go Doc contains a fully documented reference of all the DSL functions.

Instrumentation and System Example

The clue project provides observability packages that work in tandem with Goa. The packages cover logging, tracing, metrics, health checks and service client mocking. clue also includes a fully featured example consisting of three instrumented Goa microservices that communicate with each other.

Getting Started Guides

A couple of Getting Started guides produced by the community.

Joseph Ocol from Pelmorex Corp. goes through a complete example writing a server and client service using both HTTP and gRPC transports.

Gleidson Nascimento goes through how to create a complete service that using both CORS and JWT based authentication to secure access.

Examples

The examples directory contains simple examples illustrating basic concepts.

Troubleshooting

Q: I'm seeing an error that says:

generated code expected goa.design/goa/v3/codegen/generator to be present in the vendor directory, see documentation for more details

How do I fix this?

A: If you are vendoring your dependencies Goa will not attempt to satisfy its dependencies by retrieving them with go get. If you see the above error message, it means that the goa.design/goa/v3/codegen/generator package is not included in your vendor directory.

To fix, ensure that goa.design/goa/v3/codegen/generator is being imported somewhere in your project. This can be as a bare import (e.g. import _ "goa.design/goa/v3/codegen/generator") in any file or you can use a dedicated tools.go file (see Manage Go tools via Go modules and golang/go/issues/25922 for more details.) Finally, run go mod vendor to ensure the imported packages are properly vendored.

Contributing

See CONTRIBUTING.

server-may-cry/goa