goa is a framework for building microservices in Go using a unique design-first approach.

Why goa?

There are a number of good Go packages for writing modular web services out there so why build another one? Glad you asked! The existing packages tend to focus on providing small and highly modular frameworks that are purposefully narrowly focused. The intent is to keep things simple and to avoid mixing concerns.

This is great when writing simple APIs that tend to change rarely. However there are a number of problems that any non trivial API implementation must address. Things like request validation, response media type definitions or documentation are hard to do in a way that stays consistent and flexible as the API surface evolves.

goa takes a different approach to building these applications: instead of focusing solely on helping with implementation, goa makes it possible to describe the design of an API in an holistic way. goa then uses that description to provide specialized helper code to the implementation and to generate documentation, API clients, tests, even custom artifacts.

The goa design language allows writing self-explanatory code that describes the resources exposed by the API and for each resource the properties and actions. goa comes with the goagen tool which runs the design language and generates various types of artifacts from the resulting metadata.

One of the goagen output is glue code that binds your code with the underlying HTTP server. This code is specific to your API so that for example there is no need to cast or "bind" any handler argument prior to using them. Each generated handler has a signature that is specific to the corresponding resource action. It's not just the parameters though, each handler also has access to specific helper methods that generate the possible responses for that action. The metadata can also include validation rules so that the generated code also takes care of validating the incoming request parameters and payload prior to invoking your code.

The end result is controller code that is terse and clean, the boilerplate is all gone. Another big benefit is the clean separation of concern between design and implementation: on bigger projects it's often the case that API design changes require careful review, being able to generate a new version of the documentation without having to write a single line of implementation is a big boon.

This idea of separating design and implementation is not new, the excellent Praxis framework from RightScale follows the same pattern and was an inspiration to goa.

Other Whys and Hows

If you are new to goa I can't recommend enough that you read the Gopher Academy blog post. goa may look a little bit different at first, the post explains the thinking behind it so that you can better take advantage of the framework.

Installation

Assuming you have a working Go setup:

go get github.com/goadesign/goa/goagen

Teaser

1. Design

Create the file $GOPATH/src/goa-adder/design/design.go with the following content:

package design

import (
        . "github.com/goadesign/goa/design"
        . "github.com/goadesign/goa/design/apidsl"
)

var _ = API("adder", func() {
        Title("The adder API")
        Description("A teaser for goa")
        Host("localhost:8080")
        Scheme("http")
})

var _ = Resource("operands", func() {
        Action("add", func() {
                Routing(GET("add/:left/:right"))
                Description("add returns the sum of the left and right parameters in the response body")
                Params(func() {
                        Param("left", Integer, "Left operand")
                        Param("right", Integer, "Right operand")
                })
                Response(OK, "plain/text")
        })

})

This file contains the design for an adder API which accepts HTTP GET requests to /add/:x/:y where :x and :y are placeholders for integer values. The API returns the sum of x and y in its body.

2. Implement

Now that the design is done, let's run goagen on the design package:

$ cd $GOPATH/src/goa-adder
$ goagen bootstrap -d goa-adder/design

This produces the following outputs:

• main.go and operands.go contain scaffolding code to help bootstrap the implementation. running goagen again does no recreate them so that it's safe to edit their content.
• an app package which contains glue code that binds the low level HTTP server to your implementation.
• a client package with a Client struct that implements a AddOperands function which calls the API with the given arguments and returns the http.Response. The client directory also contains the complete source for a client CLI tool (see below).
• a swagger package with implements the GET /swagger.json API endpoint. The response contains the full Swagger specificiation of the API.

3. Run

First let's implement the API - edit the file operands.go and replace the content of the Add function with:

// Add runs the add action.
func (c *OperandsController) Add(ctx *app.AddOperandsContext) error {
        sum := ctx.Left + ctx.Right
        return ctx.OK([]byte(strconv.Itoa(sum)))
}

Now let's compile and run the service:

$ cd $GOPATH/src/goa-adder
$ go build
$ ./goa-adder
2016/02/16 16:39:27 [INFO] mount        ctrl: Operands  action: Add     route: GET /add/:left/:right
2016/02/16 16:39:27 [INFO] mount file   filename: swagger/swagger.json  path: GET /swagger.json
2016/02/16 16:39:27 [INFO] listen       address: :8080

Open a new console and compile the generated CLI tool:

cd $GOPATH/src/goa-adder/client/adder-cli
go build

The tool includes contextual help:

$ ./adder-cli --help
CLI client for the adder service

Usage:
  adder-cli [command]

Available Commands:
  add         add returns the sum of the left and right parameters in the response body

Flags:
      --dump[=false]: Dump HTTP request and response.
  -H, --host="localhost:8080": API hostname
      --pp[=false]: Pretty print response body
  -s, --scheme="http": Set the requests scheme
  -t, --timeout=20s: Set the request timeout, defaults to 20s

Use "adder-cli [command] --help" for more information about a command.

$ ./adder-cli add operands --help
Usage:
  adder-cli add operands [flags]

Global Flags:
      --dump[=false]: Dump HTTP request and response.
  -H, --host="localhost:8080": API hostname
      --pp[=false]: Pretty print response body
  -s, --scheme="http": Set the requests scheme
  -t, --timeout=20s: Set the request timeout, defaults to 20s

Now let's run it:

$ ./adder-cli add operands /add/1/2
2016/02/16 11:04:30 [INFO] started  id: fHAaZ9tx  GET: http://localhost:8080/add/1/2
2016/02/16 11:04:30 [INFO] completed  id: fHAaZ9tx  status: 200 time: 811.575µs
3⏎

The console running the service shows the request that was just handled:

2016/02/16 16:42:01 [INFO] started      app: API        ctrl: operands  action: Add     id: FloJa8uAbm-1        GET: /add/1/2
2016/02/16 16:42:01 [INFO] params       app: API        ctrl: operands  action: Add     id: FloJa8uAbm-1        left: 1 right: 2
2016/02/16 16:42:01 [INFO] completed    app: API        ctrl: operands  action: Add     id: FloJa8uAbm-1        status: 200     bytes: 1        time: 91.858µs

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

./adder-cli add operands add/1/d
2016/02/24 11:05:15 [INFO] started  id: 3F1FKVRR  GET: http://localhost:8080/add/1/d
2016/02/24 11:05:15 [INFO] completed  id: 3F1FKVRR  status: 400 time: 1.044759ms
error: 400: "[{\"id\":1,\"title\":\"invalid parameter value\",\"msg\":\"invalid value \\\"d\\\" for parameter \\\"right\\\", must be a integer\"}]"

As you can see the generated code validated the incoming request state against the types defined in the design.

4. Document

The swagger directory contains the entire Swagger specification in the swagger.json file. The specification can also be accessed through the service:

$ curl localhost:8080/swagger.json

For open source services hosted on github swagger.goa.design provides a free service that renders the Swagger representation dynamically from goa design packages.

Resources

GoDoc

• Package goa contains the data structures and algorithms used at runtime.
• Package apidsl contains the implementation of the goa design language.
• Package design defines the output data structures of the design language.
• Package dslengine is a tool to parse and process any arbitrary DSL

Website

http://goa.design contains further information on goa.

Getting Started

Can't wait to give it a try? the easiest way is to follow the short getting started guide.

Middleware

The middleware package provides a number of middlewares covering common needs. It also provides a good source of examples for writing new middlewares.

Examples

The goa-cellar repo contains the implementation for a goa service which demonstrates many aspects of the design language. It is kept up-to-date and provides a reference for testing functionality.

Contributing

Did you fix a bug? write docs or additional tests? or implement some new awesome functionality? You're a rock star!! Just make sure that make succeeds (or that TravisCI is green) and send a PR over.

The issues contain entries tagged with help wanted: beginners which provide a great way to get started!