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

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.

Assuming you have a working Go setup:

go get github.com/goadesign/goa
go get github.com/goadesign/goa/goagen

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, "text/plain")
        })

})

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.

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 not 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.
• a tool directory that contains the complete source for a client CLI tool.
• a swagger package with implements the GET /swagger.json API endpoint. The response contains the full Swagger specificiation of the API.

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

// Add import for strconv
import "strconv"

// 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
2016/04/05 20:39:10 [INFO] mount ctrl=Operands action=Add route=GET /add/:left/:right
2016/04/05 20:39:10 [INFO] listen transport=http addr=:8080

Open a new console and compile the generated CLI tool:

cd $GOPATH/src/goa-adder/tool/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               Dump HTTP request and response.
  -H, --host string        API hostname (default "localhost:8080")
  -s, --scheme string      Set the requests scheme
  -t, --timeout duration   Set the request timeout (default 20s)

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

To get information on how to call a specific API use:

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

Flags:
      --left int    Left operand
      --pp          Pretty print response body
      --right int   Right operand

Global Flags:
      --dump               Dump HTTP request and response.
  -H, --host string        API hostname (default "localhost:8080")
  -s, --scheme string      Set the requests scheme
  -t, --timeout duration   Set the request timeout (default 20s)

Now let's run it:

./adder-cli add operands /add/1/2
2016/04/05 20:43:18 [INFO] started id=HffVaGiH GET=http://localhost:8080/add/1/2
2016/04/05 20:43:18 [INFO] completed id=HffVaGiH status=200 time=1.028827ms
3⏎

This also works:

$ ./adder-cli add operands --left=1 --right=2
2016/04/25 00:08:59 [INFO] started id=ouKmwdWp GET=http://localhost:8080/add/1/2
2016/04/25 00:08:59 [INFO] completed id=ouKmwdWp status=200 time=1.097749ms
3⏎     

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

2016/06/06 10:23:03 [INFO] started req_id=rLAtsSThLD-1 GET=/add/1/2 from=::1 ctrl=OperandsController action=Add
2016/06/06 10:23:03 [INFO] params req_id=rLAtsSThLD-1 right=2 left=1
2016/06/06 10:23:03 [INFO] completed req_id=rLAtsSThLD-1 status=200 bytes=1 time=66.25µs

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

./adder-cli add operands add/1/d
2016/06/06 10:24:22 [INFO] started id=Q2u/lPUc GET=http://localhost:8080/add/1/d
2016/06/06 10:24:22 [INFO] completed id=Q2u/lPUc status=400 time=1.301083ms
error: 400: {"code":"invalid_request","status":400,"detail":"invalid value \"d\" for parameter \"right\", must be a integer"}

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

The swagger directory contains the API Swagger specification in both YAML and JSON format.

For open source projects hosted on github swagger.goa.design provides a free service that renders the Swagger representation dynamically from goa design packages. Simply set the url query string with the import path to the design package. For example displaying the docs for github.com/goadesign/goa-cellar/design is done by browsing to:

http://swagger.goa.design/?url=goadesign%2Fgoa-cellar%2Fdesign

Note that the above generates the swagger spec dynamically and does not require it to be present in the Github repo.

The Swagger JSON can also easily be served from the documented service itself using a simple Files definition in the design. Edit the file design/design.go and add:

var _ = Resource("swagger", func() {
        Origin("*", func() {
               Methods("GET") // Allow all origins to retrieve the Swagger JSON (CORS)
        })
        Files("/swagger.json", "swagger/swagger.json")
})

Re-run goagen bootstrap -d goa-adder/design and note the new file swagger.go containing the implementation for a controller that serves the swagger.json file.

Mount the newly generated controller by adding the following two lines to the main function in main.go:

cs := NewSwaggerController(service)
app.MountSwaggerController(service, cs)

Recompile and restart the service:

^C
go build
./goa-adder
2016/06/06 10:31:14 [INFO] mount ctrl=Operands action=Add route=GET /add/:left/:right
2016/06/06 10:31:14 [INFO] mount ctrl=Swagger files=swagger/swagger.json route=GET /swagger.json
2016/06/06 10:31:14 [INFO] listen transport=http addr=:8080

Note the new route /swagger.json. Requests made to it return the Swagger specification. The generated controller also takes care of adding the proper CORS headers so that the JSON may be retrieved from browsers using JavaScript served from a different origin (e.g. via Swagger UI). The client also has a new download action:

cd tool/adder-cli
go build
./adder-cli download --help
Download file with given path

Usage:
  adder-cli download [PATH] [flags]

Flags:
      --out string   Output file

Global Flags:
      --dump               Dump HTTP request and response.
  -H, --host string        API hostname (default "localhost:8080")
  -s, --scheme string      Set the requests scheme
  -t, --timeout duration   Set the request timeout (default 20s)

Which can be used like this to download the file swagger.json in the current directory:

./adder-cli download swagger.json
2016/06/06 10:36:24 [INFO] started file=swagger.json id=ciHL2VLt GET=http://localhost:8080/swagger.json
2016/06/06 10:36:24 [INFO] completed file=swagger.json id=ciHL2VLt status=200 time=1.013307ms

We now have a self-documenting API and best of all the documentation is automatically updated as the API design changes.

Consult the following resources to learn more about goa.

goa.design contains further information on goa including a getting started guide, detailed DSL documentation as well as information on how to implement a goa service.

The examples repo contains simple examples illustrating basic concepts.

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.

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!