Template for building REST Web Services in Golang. Uses gorilla/mux as a router/dispatcher and Negroni as a middleware handler. Tested against Go 1.7.1.
After writing many REST APIs with Java Dropwizard, Node.js/Express and Go, I wanted to distill my lessons learned into a reusable template for writing REST APIs, in the Go language.
It's mainly for myself. I don't want to keep reinventing the wheel and just want to get the foundation of my REST API 'ready to go' so I can focus on the business logic and integration with other systems and data stores.
Just to be clear: this is not a framework, library, package or anything like that. This tries to use a couple of very good Go packages and libraries that I like and cobbled them together.
The main ones are:
- gorilla/mux for routing
- codegangsta/negroni as a middleware handler
- strechr/testify for writing easier test assertions
- unrolled/render for HTTP response rendering
- palantir/stacktrace to provide more context to error messages
- unrolled/secure to improve API security
Whilst working on this, I've tried to write up my thought process as much as possible. Everything from the design of the API and routes, some details of the Go code like JSON formatting in structs and my thoughts on testing. However, if you feel that there is something missing, send a PR or raise an issue.
Last but not least: I'm occasionally writing about my lessons learned developing Go applications and Amazon Web Services systems on Medium.
If you're new to programming in Go, I would highly recommend you to read the following three resources:
- A Tour of Go
- Effective Go
- 50 Shades of Go: Traps, Gotchas, and Common Mistakes for New Golang Devs
- Learn X in Y Minutes
You can work your way through those in two to three days. I did try out a couple of books but didn't find them that useful. If you really want to get some books, there is a github repo that tries to list the main ones.
A good project to keep on eye on to discover new Go packages and software is awesome-go. The maintainers enforce strict standards around documentation, tests, structure, etc. so they are doing a brilliant job improving the overall quality of code in the Go community. It's not easy to keep track of the high rate of change on that project, so my suggestion would be to track new additions using the changedetection.com website.
I've tried many different editors and it seems like it's a common Go newbie frustration. Coming from Java, I've tried "proper" IDEs like LiteIDE and IntelliJ with the golang plugin but never really fell in love with them.
I've used Sublime Text with the GoSublime plugin for a long time, but have now settled on Atom with the go-plus plugin and that is working very well. It has code completion, linting, formatting and even shows you what parts of the code are covered by tests.
The choice of an editor/IDE is a personal one, so the only advice I can give you is to try different ones. It looks like currently the following seem to be the most mature:
- Sublime Text 3 with GoSublime. I've been made aware that there is another Go plugin for Sublime Text called GoTools
- Atom with go-plus. There is a good article describing a full Go setup here.
- vim with vim-go
- LiteIDE
- Microsoft did a pretty good job with their Visual Studio Code and Luke Hoban created a brilliant Go binding for it: vscode-go
- IntelliJ with the Go plugin
Last but not least, I can highly recommend trying out GitKraken. I've never been a huge fan of desktop UI clients for git, but this one sets a new bar. Slick UI and very useful information about your local and remote branches and code history.
go run main.go works fine if you have a single file or a script you're working on, but once you have a more complex project with lots of files then you'll have to start using the proper go build tool and run the compiled executable.
go build && ./go-rest-api-template
Which is faster than first typing go build to generate an executable called go-rest-api-template and then run that executable by typing ./go-rest-api-template.
The go-rest-api-template app will bind itself to port 3001 (defined in main.go). If you want to change it (e.g. bind it to the default http port 80), then use the following environment variable PORT (see main.go). Same for the location of the fixtures.json model.
export PORT=80
export FIXTURES=/tmp/fixtures.json
go build && ./go-rest-api-template
Alternatively, you can just change line 23 of main.go and change the hardcoded port to whichever one you prefer.
One comment from a security point of view: whilst 80 is the default port for HTTP (and 443 for HTTPS), it's usually not recommended to actually bind your application to that port when it runs on a server instance like AWS EC2. That's because ports below 1024 are privileged ports and require elevated privileges for the app to bind itself to such port. If someone compromises your app, it could potentially take advantage of the application user with the elevated privileges. Instead, run it on a higher port like 8001 and run it with a newly created restricted user (instead of using the root user). You can then use a load balancer or a proxy (e.g. AWS ELB, HAProxy, nginx) to accept your incoming request from the internet on port 80 or 443 and then forward that to your application port.
Manually stopping and restarting your server can get quite annoying after a while, so let's set up a task runner that automatically restarts the server when it detects changes.
Install fresh:
go get github.com/pilu/fresh
And during development, you can now just type in the following command in your project root directory:
fresh
You should see following output if all goes well:
Loading settings from ./runner.conf
11:10:42 runner | InitFolders
11:10:42 runner | mkdir ./tmp
11:10:42 runner | mkdir ./tmp: file exists
11:10:42 watcher | Watching .
11:10:42 watcher | Watching vendor
11:10:42 watcher | Watching vendor/github.com
11:10:42 watcher | Watching vendor/github.com/codegangsta
11:10:42 watcher | Watching vendor/github.com/codegangsta/negroni
11:10:42 watcher | Watching vendor/github.com/davecgh
11:10:42 watcher | Watching vendor/github.com/davecgh/go-spew
11:10:42 watcher | Watching vendor/github.com/davecgh/go-spew/spew
11:10:42 watcher | Watching vendor/github.com/gorilla
11:10:42 watcher | Watching vendor/github.com/gorilla/context
11:10:42 watcher | Watching vendor/github.com/gorilla/mux
11:10:42 watcher | Watching vendor/github.com/palantir
11:10:42 watcher | Watching vendor/github.com/palantir/stacktrace
11:10:42 watcher | Watching vendor/github.com/palantir/stacktrace/cleanpath
11:10:42 watcher | Watching vendor/github.com/pmezard
11:10:42 watcher | Watching vendor/github.com/pmezard/go-difflib
11:10:42 watcher | Watching vendor/github.com/pmezard/go-difflib/difflib
11:10:42 watcher | Watching vendor/github.com/stretchr
11:10:42 watcher | Watching vendor/github.com/stretchr/testify
11:10:42 watcher | Watching vendor/github.com/stretchr/testify/assert
11:10:42 watcher | Watching vendor/github.com/unrolled
11:10:42 watcher | Watching vendor/github.com/unrolled/render
11:10:42 watcher | Watching vendor/github.com/unrolled/secure
11:10:42 main | Waiting (loop 1)...
11:10:42 main | receiving first event /
11:10:42 main | sleeping for 600 milliseconds
11:10:42 main | flushing events
11:10:42 main | Started! (75 Goroutines)
11:10:42 main | remove tmp/go-rest-api-template.log: no such file or directory
11:10:42 build | Building...
11:10:48 runner | Running...
11:10:48 main | --------------------
11:10:48 main | Waiting (loop 2)...
11:10:48 app | [negroni] listening on localhost:3001
11:10:48 app | 2016/09/28 11:10:48 ===> Starting app (v0.2.0) on port 3001 in LOCAL mode.
Fresh should work without any configuration, but to make it more explicit you can add a runner.conf file in your project root:
root: .
tmp_path: ./tmp
build_name: go-rest-api-template
build_log: go-rest-api-template.log
valid_ext: .go, .tpl, .tmpl, .html
build_delay: 600
colors: 1
log_color_main: cyan
log_color_build: yellow
log_color_runner: green
log_color_watcher: magenta
log_color_app:
As you can see, it creates a tmp directory in your project root and a log file. You can tell .gitignore to stop checking it into your git repository by adding the following lines in your .gitignore file:
# fresh
tmp
go-rest-api-template.log
Where I work, we've hit the limitations of fresh and have switched to Gulp as a task runner. Our Gulp setup takes care of running tests, using OSX notifications to warn you of build failures and provides an overview of your govendor package status. I'll open source our Gulp for Go configuration at some point.
Main server files (bootstrapping of http server):
main.go --> figure out application settings from various sources, start application context and kick the server
server.go --> actual server kicking is happening here: mainly loading of routes and middleware
Route definitions and handlers:
router.go --> server routes, e.g. GET /healthcheck, and binding of those routes to the correct route handlers
handlers.go --> defines the actual logic that gets executed when you visit a route and takes care of the response to the client
handlers_test.go --> tests for our route handlers
Data model descriptions and operations on the data:
models.go --> structs describing our data, bit similar to objects in other languages
database.go --> our mock/fake database implementation
database_test.go --> tests our mock/fake database
Test data in JSON format:
fixtures.json
Configuration file for [fresh](go get github.com/pilu/fresh):
runner.conf
Test that checks whether structs comply with the DataStorer interface:
interface_test.go
Helper structs and functions:
helpers.go
Defines application version using semantic versioning:
VERSION
Folder where we store all our go dependencies using the govendor tool:
vendor/
The main entry point to our app is main.go where we take care of the following:
Loading our environment variables, but also provide some default values that are useful when you run it on your local development machine:
var (
// try to read environment variables
env = os.Getenv("ENV") // LOCAL, DEV, STG, PRD
port = os.Getenv("PORT") // server traffic on this port
version = os.Getenv("VERSION") // path to VERSION file
fixtures = os.Getenv("FIXTURES") // path to fixtures file
)
if env == "" || env == local {
// check if we are running on our local machine, then set some default values
env = local
port = "3001"
version = "VERSION"
fixtures = "fixtures.json"
}
I'm using the environment variables technique to tell the app in what environment it lives:
LOCAL: local development machineDEV: development or integration serverSTG: staging serversPRD: production servers
When it can't detect an environment variable, it assumes that it's running on your local development workstation.
We then load our version file. I have added a helper function in helpers.go that can parse a VERSION file using semantic versioning.
// reading version from file
version, err := ParseVersionFile(version)
if err != nil {
log.Fatal(err)
}
The check for error (if err != nil) is a common Go way of handling return values that contain errors. the main function is one of the few places where I use log.Fatal in my application. log.Fatal logs your file to the console, but also exits your application. This is for me the correct behaviour here because without a correct VERSION file, the app shouldn't work. It's too risky that you may have deployed the incorrect application version.
This API doesn't talk to a real database but to a very simple in-memory mock database. That's why I have a fixtures.json file with some data. I have added a helper function in helpers.go that can read and parse that file into our in-memory mock database. Have a look at the next Data section to find out more about the in-memory database. I may change this in the future into an integration with an actual database.
// load fixtures data into mock database
db, err := LoadFixturesIntoMockDatabase(fixtures)
Once all the data is read from our environment variables and fixtures.json file, we can then initialise our application context. AppContext is a struct that holds some application info that we can pass around (e,g, provide to our handlers). It's a neat way to avoid using global variables and avoiding that you have application variables all over the place.
I have defined the AppContext struct in helpers.go:
// AppContext holds application configuration data
type AppContext struct {
Render *render.Render
Version string
Env string
Port string
DB DataStorer
}
Render helps us with rendering the correct response to the client, e.g. JSON, XML, etc. Version holds the path to our VERSION file. Env holds information about our platform environment (e.g. STG, DEV). Port is the server port that our application binds to. DB is our database struct that provides a data abstraction layer.
Once all of that is set up properly, we can now start the server. The StartServer function takes our AppContext struct and initialises all our routes and starts our negroni server. This is all defined in server.go.
Most negroni code examples (like the one on the official github repo) will start the application using:
n := negroni.Classic()
When you look at our server.go file, then you'll see that we do it slightly differently:
// start now
n := negroni.New()
negroni.Classic() adds a little bit of magic in the background and for instance loads the /public folder as the location of your static files. I personally don't like that and prefer to make things more explicit. It's up to you to decide which one you prefer, as long as you are aware what exactly happens when you play with magic.
One important thing worth mentioning is the way we run the application and bind the app to a given port. It's common to see Go apps being started as:
n.Run(":3001")
However, when you're running this on OSX, you will get constant warnings about accepting incoming connections. I've written a piece on Medium about this but the tl;dr solution is to explicitly bind your application to localhost:
n.Run("localhost:3001")
Last but not least, a big thank you to edoardo849 for providing some great feedback on structuring the API and reducing main.go complexity.
We are going to use a travel Passport for our example. I've chosen Id as the unique key for the passport because (in the UK), passport book numbers these days have a unique 9 character field length (e.g. 012345678). A passport belongs to a user and a user can have one or more passports. We'll define this in models.go.
type User struct {
ID int `json:"id"`
FirstName string `json:"firstName"`
LastName string `json:"lastName"`
DateOfBirth time.Time `json:"dateOfBirth"`
LocationOfBirth string `json:"locationOfBirth"`
}
type Passport struct {
ID string `json:"id"`
DateOfIssue time.Time `json:"dateOfIssue"`
DateOfExpiry time.Time `json:"dateOfExpiry"`
Authority string `json:"authority"`
UserID int `json:"userId"`
}
The first time you create a struct, you may not be aware that uppercasing and lowercasing your field names have a meaning in Go. It's similar to public and private members in Java. Uppercase = public, lowercase = private. There are some good discussions on Stackoverflow about this. The gist is that field names that start with a lowercase letter will not be visible to json.Marshal.
You may not want to expose your data to the consumer of your web service in this format, so you can override the way your fields are marshalled by adding json:"firstName" to each field with the desired name. I admit that in the past I had the habit of using underscores for my json field names, e.g. first_name. However after reading this excellent presentation on API design, I got reminded that the JS in JSON stands for JavaScript and in the JavaScript world, it's common to use camelCasing so the preferred way of writing the same field name would be: firstName.
Note the use of time.Time for the dates instead of using a standard string type. We'll discuss the pain of marshalling and unmarshalling of JSON dates a bit later.
If you want to prevent a certain struct field to be marshalled/unmarshalled then use json:"-".
I wanted to create a template REST API that didn't depend on a database, so started with creating a simple in-memory database that we can work with. I admit that this was mainly to satisfy my own curiosity. I might change that in the future to using an off-the-shelve in-memory Go database.
The good thing is that this will be the start of a so-called data access layer that abstracts away the underlying data store. We can achieve that by starting with creating an interface (which is a good practice in Go anyway). Note the use of the -er at the end of the interface name, as per Go convention.
Open the database.go file and you will see:
type DataStorer interface {
ListUsers() ([]User, error)
GetUser(i int) (User, error)
AddUser(u User) (User, error)
UpdateUser(u User) (User, error)
DeleteUser(i int) error
}
This allows us to define a set of operations on the data as a contract, without people having to worry about the actual implementation of how the data is stored and accessed. I've added the basic operations to list, retrieve, create, update and delete data, so the standard CRUD-style operations (accepting that CRUD has some subtle differences with REST).
Let's have a look at the type signature of the Get operation:
GetUser(i int) (User, error)
What this tells us is that it is expecting an integer as an argument (which will be the User id in our case), and returns a pair of values: a User object and an error object. Returning pairs of values is a nice Go feature and is often used to return information about errors.
An example of how this could be used is the following:
// call the GetUser function with a given user id, this returns (hopefully) the user and an error object
user, err := ctx.db.GetUser(uid)
if err != nil {
ctx.render.JSON(w, http.StatusNotFound, err)
return
}
ctx.render.JSON(w, http.StatusOK, user)
We check whether the error object is nil. If it is, then we return a HTTP 200 OK, if not then we return HTTP 404 Not Found. Let's go into more detail when we talk about our API handlers.
BTW in other languages you might be used to write the above as:
user, err := ctx.db.GetUser(uid)
if err == nil {
ctx.render.JSON(w, http.StatusOK, user)
} else {
ctx.render.JSON(w, http.StatusNotFound, err)
}
But the Go way is to check if an error exists and handle that immediately. If there is no error, just continue with the normal application flow.
Let's have a look at the actual mock in-memory database. We need to create a Database struct that will hold the data:
type MockDB struct {
UserList map[int]User
MaxUserID int
}
The UserList will hold a list of User structs and the MaxUserID holds the latest used integer. MaxUserID mimics the behaviour of an auto-generated ID in conventional databases. In this case MaxUserID represents the highest used database ID.
Let's have a closer at the GetUser function:
// GetUser returns a single JSON document
func (db *MockDB) GetUser(i int) (User, error) {
user, ok := db.UserList[i]
if !ok {
return User{}, stacktrace.NewError("Failure trying to retrieve user")
}
return user, nil
}
func (db *MockDB): This might be something new for you and I admit that it took me a little while to appreciate this. It's called a pointer receiver and is part of A Tour of Go. This means that theGetUserfunction can work on aMockDBstruct. For people coming from languages that use objects and methods, it's similar. So in an object-oriented world: ifdbwould be an object of the classMockDB, thenGetUserwould be a method that can be invoked on that object and you'd call it likedb.GetUser.GetUser(i int) (User, error): This is the function signature and says that we have a function namedGetUserthat accepts one argument of the typeint(e.g. 0, 1, 2, 3) and will return two variables. The first one is aUserstruct, the second one is anerrorstruct.return User{}, stacktrace.NewError("Failure trying to retrieve user"): In case there is an error, we return an emptyUserobject and anerror. We have to return anUserobject because that's what the function signature enforces, but this will be empty and frankly doesn't really matter. It doesn't matter because we will first check theerrorstruct. The basicerrorstruct in Go doesn't really say much, which is why we use here Palantir'sstacktracepackage. It adds some more contextual information about where the issue occured.return user, nil: If everything goes well, then we just return the user data as well as an empty error (nil).
Before we move on to loading the mock data, I wanted to briefly touch on the DataStorer interface. One of the advantages of using an interface is that you can swap out the implementation. This can be useful for testing. So instead of GetUser calling the actual datastore, you could write a different implementation that just returns a hardcoded User struct. However, Go has a implicit way of linking the interface to the struct. Unlike in Java where you have to explicitly state that a Class implements an Interface (and thus you benefit from the IDE checking whether you have implemented all the methods), Go just tries to figure out whether there are structs that match the definition of your interface. In order to make this more explicit, I always add a small test. When you open interface_test.go then you'll see:
func TestDoStructsSatisfyInterface(t *testing.T) {
var _ DataStorer = (*MockDB)(nil)
}
TestDoStructsSatisfyInterface tries to typecast the MockDB pointer to the DataStorer interface. If you haven't implemented your interface functions properly, then your Go tests will complain.
In order to make it a bit more useful, we will initialise it with some user objects. I've created a helper function in helpers.go called LoadFixturesIntoMockDatabase that just loads the fixtures.json file. The structure of that file looks like this:
{
"users": [
{
"dateOfBirth": "1985-12-31T00:00:00Z",
"firstName": "John",
"id": 0,
"lastName": "Doe",
"locationOfBirth": "London"
},
{
"dateOfBirth": "1992-01-01T00:00:00Z",
"firstName": "Jane",
"id": 1,
"lastName": "Doe",
"locationOfBirth": "Milton Keynes"
}
]
}
When we can't load the file, we will stop the bootstrapping of the application. This is taken care of by Go's log handler, which fires off a fatal error:
// load fixtures data into mock database
db, err := LoadFixturesIntoMockDatabase(fixtures)
if err != nil {
log.Fatal(err)
}
The fixtures.json file contains a JSON representation of a Go map where the key is a string (i.e. "users") and the map value is a string of User objects. In Go, this would be represented as: map[string][]User. We load the fixtures file, marshal it into the type we just defined and then load it into our database.
The date string looks a bit odd. Why not just use 31-12-1985 or 1985-12-31? The first is discouraged altogether because that's an European way of writing dates and will cause confusion around the world. Imagine you have 3-4-2015. Is it third of April or fourth of March? Unfortunately there isn't an "enforced standard" for dates in JSON, so I've tried to use one that is commonly used and also understood by Go's json.Marshaler and json.Unmarshaler to avoid that we have to write our own custom marshaler/unmarshaler.
If you have a look at Go's time/format code then you'll see on line 54:
54 RFC3339 = "2006-01-02T15:04:05Z07:00"
That's the one we need. It has the following format:
year-month-day
T (for time)
hour:minutes:seconds
Z (for time zone)
offset from UTC
Now that we have defined the data access layer, we need to translate that to a REST interface:
- Retrieve a list of all users:
GET /users-> TheGETjust refers to the HTTP action you would use. If you want to test this in the command line, then you can use curl:curl -X GET http://localhost:3009/usersorcurl -X POST http://localhost:3009/users - Retrieve the details of an individual user:
GET /users/{uid}-> {uid} allows us to create a variable, named uid, that we can use in our code. An example of this url would beGET /users/1 - Create a new user:
POST /users - Update a user:
PUT /users/{uid} - Delete a user:
DELETE /users/{uid}
We now need to do the same for handling passports. Don't forget that a passport belongs to a user, so to retrieve a list of all passports for a given user, we would use GET /users/{uid}/passports.
When we want to retrieve an specific passport, we don't need to prefix the route with /users/{uid} anymore because we know exactly which passport we want to retrieve. So, instead of GET /users/{uid}/passports/{pid}, we can just use GET /passports/{pid}.
Once you have the API design sorted, it's just a matter of creating the code that gets called when a specific route is hit. We implement those with Handlers and those routes are defined in routes.go:
Route{"Healthcheck", "GET", "/healthcheck", HealthcheckHandler},
//=== USERS ===
Route{"ListUsers", "GET", "/users", ListUsersHandler},
Route{"GetUser", "GET", "/users/{uid:[0-9]+}", GetUserHandler},
Route{"CreateUser", "POST", "/users", CreateUserHandler},
Route{"UpdateUser", "PUT", "/users/{uid:[0-9]+}", UpdateUserHandler},
Route{"DeleteUser", "DELETE", "/users/{uid:[0-9]+}", DeleteUserHandler},
In order to make our code more robust, I've added pattern matching in the routes. This [0-9]+ pattern says that we only accept digits from 0 to 9 and we can have one or more digits. Everything else will most likely trigger an HTTP 404 Not Found status code being returned to the client.
Most Go code that show HandleFunc examples, will show something slightly different, something more like this:
router.HandleFunc("/users", ListUsersHandler).Methods("GET")
So Go's HandleFunc function takes two arguments, one string that defines the route and a second argument of the type http.HandleFunc(w http.ResponseWriter, r *http.Request). This works very well, but doesn't allow you to pass any extra data to your handlers. So for instance when you want to use the unrolled/render package, then you'd have to define in your main.go file a global variable like this: var Render *render.Render, then initialise that in your func main() so that your handlers can access this global variable later on. Using global variables in Go is not a good practice (there are exceptions like certain database drivers, but that's a different discussion).
So our initial handler function for returning a list of users was:
func ListUsersHandler(w http.ResponseWriter, req *http.Request) {
// return user data
}
Where the render variable is a global variable. We'd prefer to pass the Render variable to that function so will rewrite it to:
func ListUsersHandler(w http.ResponseWriter, req *http.Request, render Render) {
// return user data
}
Perfect. Or, is it? What if we want to pass more variables to the handler function? Like Metrics? Or some environment variables? We'd continuously have to change ALL our handlers and the type signature will become quite long and hard to maintain. An alternative is to create a server or an environment struct and use that as a container for our variables we want to pass around the system.
In our helpers.go we'll add:
type AppContext struct {
Render *render.Render
Version string
Env string
Port string
DB DataStorer
}
And in our func main() function we initialise that struct:
// initialse application context
ctx := AppContext{
Render: render.New(),
Version: version,
Env: env,
Port: port,
DB: db,
}
Our handler looks now like this:
func ListUsersHandler(w http.ResponseWriter, req *http.Request, ctx AppContext) {
// return user data
}
The only problem is that this handler's type signature is not http.ResponseWriter, *http.Request but http.ResponseWriter, *http.Request, appContext so Go's HandleFunc function will complain about this. That's why we are introducing a helper function makeHandler that takes our appContext struct and our handlers with the special type signature and converts it to func(w http.ResponseWriter, r *http.Request):
func makeHandler(ctx AppContext, fn func(http.ResponseWriter, *http.Request, AppContext)) http.HandlerFunc {
return func(w http.ResponseWriter, r *http.Request) {
fn(w, r, ctx)
}
}
We can use this makeHandler function now in our server.go when we initialise all the routes and the corresponding route handlers.
In the previous examples we've ignored the implementation of the ListUsersHandler, but let's have a closer look at that now:
// ListUsersHandler returns a list of users
func ListUsersHandler(w http.ResponseWriter, req *http.Request, ctx AppContext) {
list, err := ctx.DB.ListUsers()
if err != nil {
response := Status{
Status: "404",
Message: "can't find any users",
}
log.Println(err)
ctx.Render.JSON(w, http.StatusNotFound, response)
return
}
responseObject := make(map[string]interface{})
responseObject["users"] = list
responseObject["count"] = len(list)
ctx.Render.JSON(w, http.StatusOK, responseObject)
}
After we retrieve the data from our database, we check whether there are errors in the response. Rather than passing on the response to the user, we create a new custom Status struct with a new bespoke error message. Why? That's to avoid that we're leaking sensitive information from our systems to the user. It's the equivalent of sending back a Java stacktrace to a user. It can disclose information to people that are trying to hack into your system by exposing some of the weaknesses.
So this code:
if err != nil {
response := Status{
Status: "404",
Message: "can't find any users",
}
log.Println(err)
ctx.Render.JSON(w, http.StatusNotFound, response)
return
}
constructs a new Status response struct. Adds a 404 status so that computers can understand it, but also adds a human-readable message. It then prints the error to the server log. I've used here a simple log.Println but there are multiple ways you can do that. Last but not least you send the Status struct as a JSON response back to the client and add the 404 Not Found response to the HTTP headers (using http.StatusNotFound).
When you look at the overview of the handlers in routes.go, you will notice a special route:
Route{"Healthcheck", "GET", "/healthcheck", HealthcheckHandler},
Monitoring tools like Sensu can call: GET /healthcheck. The health check route can return a 200 OK when the service is up and running and will also say what the application name is and the version number.
func HealthcheckHandler(w http.ResponseWriter, req *http.Request, ctx appContext) {
check := Healthcheck{
AppName: "go-rest-api-template",
Version: ctx.version,
}
ctx.render.JSON(w, http.StatusOK, check)
}
This health check is very simple. It just checks whether the service is up and running, which can be useful in a build and deployment pipelines where you can check whether your newly deployed API is running (as part of a smoke test). More advanced health checks will also check whether it can reach the database, message queue or anything else you'd like to check. Trust me, your DevOps colleagues will be very grateful for this. (Don't forget to change your HTTP status code to 200 if you want to report on the various components that your health check is checking.)
Let's have a look at interacting with our data. Returning a list of users is quite easy, it's just showing the UserList:
func ListUsersHandler(w http.ResponseWriter, req *http.Request, ctx appContext) {
ctx.render.JSON(w, http.StatusOK, db.List())
}
BTW, notice the ctx.render.JSON? That's part of "github.com/unrolled/render" and allows us to render JSON output when we send data back to the client.
So, this will return the following to the client:
{
"users": [
{
"dateOfBirth": "1992-01-01T00:00:00Z",
"firstName": "Jane",
"id": 1,
"lastName": "Doe",
"locationOfBirth": "Milton Keynes"
},
{
"dateOfBirth": "1985-12-31T00:00:00Z",
"firstName": "John",
"id": 0,
"lastName": "Doe",
"locationOfBirth": "London"
}
]
}
It may surprise you that we are returning a JSON object that holds an array with multiple JSON objects, rather than an array with multiple JSON objects, as seen in the example below:
{
[
{
"dateOfBirth": "1992-01-01T00:00:00Z",
"firstName": "Jane",
"id": 1,
"lastName": "Doe",
"locationOfBirth": "Milton Keynes"
},
{
"dateOfBirth": "1985-12-31T00:00:00Z",
"firstName": "John",
"id": 0,
"lastName": "Doe",
"locationOfBirth": "London"
}
]
}
They're both valid and there are lots of views and opinions (as always in developer / architecture communities!), but the reason why I prefer to wrap the array in a JSON object is because later on we can easily add more data without causing significant changes to the client. What if we want to add the concept of pagination to our API?
Example:
{
"offset": 0,
"limit": 25,
"users": [
{
"dateOfBirth": "1992-01-01T00:00:00Z",
"firstName": "Jane",
"id": 1,
"lastName": "Doe",
"locationOfBirth": "Milton Keynes"
},
{
"dateOfBirth": "1985-12-31T00:00:00Z",
"firstName": "John",
"id": 0,
"lastName": "Doe",
"locationOfBirth": "London"
}
]
}
In this project, we're adding an extra field named count so we tell the client how many items they can expect
{
"count": 2,
"users": [
{
"dateOfBirth": "1985-12-31T00:00:00Z",
"firstName": "John",
"id": 0,
"lastName": "Doe",
"locationOfBirth": "London"
},
{
"dateOfBirth": "1992-01-01T00:00:00Z",
"firstName": "Jane",
"id": 1,
"lastName": "Doe",
"locationOfBirth": "Milton Keynes"
}
]
}
It's important that we use the interface{} type in the map, rather than a specific []User type. Otherwise, we won't be able to assign whatever data we want to the map:
responseObject := make(map[string]interface{})
responseObject["users"] = list
responseObject["count"] = len(list)
ctx.Render.JSON(w, http.StatusOK, responseObject)
Another example is the retrieval of a specific object:
func GetUserHandler(w http.ResponseWriter, req *http.Request, ctx appContext) {
vars := mux.Vars(req)
uid, _ := strconv.Atoi(vars["uid"])
user, err := ctx.db.Get(uid)
if err == nil {
ctx.render.JSON(w, http.StatusOK, user)
} else {
ctx.render.JSON(w, http.StatusNotFound, err)
}
}
This reads the uid (User ID) variable from the route (/users/{uid}), converts the string to an integer and then looks up the user in our UserList by ID. If the user does not exit, we return a 404 and an error object. If the user exists, we return a 200 and a JSON object with the user.
Example:
{
"dateOfBirth": "1992-01-01T00:00:00Z",
"firstName": "Jane",
"id": 1,
"lastName": "Doe",
"locationOfBirth": "Milton Keynes"
}
We add the attributes of the user object to the root of the JSON response, rather than wrapping it up in an explicit JSON object. I can quite easily add extra data to the response, without breaking the existing data.
Let's start with some simple curl tests. Open your terminal and try the following curl commands.
Retrieve a list of users:
curl -X GET http://localhost:3009/users | python -mjson.tool
That should result in the following result:
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 216 100 216 0 0 14466 0 --:--:-- --:--:-- --:--:-- 15428
{
"users": [
{
"dateOfBirth": "1992-01-01T00:00:00Z",
"firstName": "Jane",
"id": 1,
"lastName": "Doe",
"locationOfBirth": "Milton Keynes"
},
{
"dateOfBirth": "1985-12-31T00:00:00Z",
"firstName": "John",
"id": 0,
"lastName": "Doe",
"locationOfBirth": "London"
}
]
}
The | python -mjson.tool at the end is for pretty printing (formatting). It essentially tells to pipe the output of the curl command to the SJON formatting tool. If we only typed curl -X GET http://localhost:3009/users then we'd have something like this:
{"users":[{"id":0,"firstName":"John","lastName":"Doe","dateOfBirth":"1985-12-31T00:00:00Z","locationOfBirth":"London"},{"id":1,"firstName":"Jane","lastName":"Doe","dateOfBirth":"1992-01-01T00:00:00Z","locationOfBirth":"Milton Keynes"}]}
So not that easy to read as the earlier nicely formatted example.
Get a specific user:
curl -X GET http://localhost:3009/users/0 | python -mjson.tool
Results in:
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 104 100 104 0 0 6625 0 --:--:-- --:--:-- --:--:-- 6933
{
"dateOfBirth": "1992-01-01T00:00:00Z",
"firstName": "Jane",
"id": 1,
"lastName": "Doe",
"locationOfBirth": "Milton Keynes"
}
There are lots of opinions on testing, how much you should be testing, which layers of your applications, etc. When I'm working with micro services, I tend to focus on two types of tests to start with: testing the data access layer and testing the actual HTTP service.
In this example, we want to test the List, Add, Get, Update and Delete operations on our in-memory document database. The data access code is stored in the database.go file, so following Go convention we will create a new file called database_test.go.
Before we can run the tests in database_test.go we need to make sure that we have a valid application context with a mock database initialised. I've added a helper function in helpers.go named CreateContextForTestSetup and that takes care of both the database setup as well as the AppContext.
// CreateContextForTestSetup initialises an application context struct
// for testing purposes
func CreateContextForTestSetup() AppContext {
testVersion := "0.0.0"
db := CreateMockDatabase()
ctx := AppContext{
Render: render.New(),
Version: testVersion,
Env: local,
Port: "3001",
DB: db,
}
return ctx
}
// CreateMockDatabase initialises a database for test purposes
func CreateMockDatabase() *MockDB {
list := make(map[int]User)
dt, _ := time.Parse(time.RFC3339, "1985-12-31T00:00:00Z")
list[0] = User{0, "John", "Doe", dt, "London"}
dt, _ = time.Parse(time.RFC3339, "1992-01-01T00:00:00Z")
list[1] = User{1, "Jane", "Doe", dt, "Milton Keynes"}
return &MockDB{list, 1}
}
Let's have a look at the easiest test where we list the elements in our database:
func TestListUsers(t *testing.T) {
ctx := CreateContextForTestSetup()
list, _ := ctx.DB.ListUsers()
count := len(list)
assert.Equal(t, 2, count, "There should be 2 items in the list.")
}
This first calls CreateContextForTestSetup() the create the application context and mock database, then the ctx.DB.ListUsers() function, which returns a list of users. We then count the number of elements and last but not least we then check whether that count equals 2.
In standard Go, you would actually write something like:
if 2 != count {
t.Errorf("Expected 2 elements in the list, instead got %v", count)
}
However there is a neat Go package called testify that gives you assertions like Java and that's why we can write cleaner test code like:
assert.Equal(t, 2, count, "There should be 2 items in the list.")
The TestList is only testing for a positive result, but we really need to test for failures as well.
This is our test code for the Delete functionality:
func TestDeleteUserSuccess(t *testing.T) {
ctx := CreateContextForTestSetup()
err := ctx.DB.DeleteUser(1)
assert.Nil(t, err)
}
func TestDeleteUserFail(t *testing.T) {
ctx := CreateContextForTestSetup()
err := ctx.DB.DeleteUser(10)
assert.NotNil(t, err)
}
The first test function TestDeleteSuccess tries to delete a known existing user, with id 1. We're expecting that the error object is Nil. The second test function TestDeleteFail tries to look up a non-existing user with id 10, and as expected, this should return an actual Error object.
How do we run the tests?
Simple. Open your terminal, go to your project root:
go test
If you want it more verbose, then:
go test -v
Which will give you:
=== RUN TestList
--- PASS: TestList (0.00s)
=== RUN TestGetSuccess
--- PASS: TestGetSuccess (0.00s)
=== RUN TestGetFail
--- PASS: TestGetFail (0.00s)
=== RUN TestAdd
--- PASS: TestAdd (0.00s)
=== RUN TestUpdateSuccess
--- PASS: TestUpdateSuccess (0.00s)
=== RUN TestUpdateFail
--- PASS: TestUpdateFail (0.00s)
=== RUN TestDeleteSuccess
--- PASS: TestDeleteSuccess (0.00s)
=== RUN TestDeleteFail
--- PASS: TestDeleteFail (0.00s)
PASS
ok github.com/leeprovoost/go-rest-api-template 0.008s
Do you want to get some more info on your code coverage? No worries, Go has you covered (no pun intended):
go test -cover
This will give you:
PASS
coverage: 34.9% of statements
ok github.com/leeprovoost/go-rest-api-template 0.009s
Before we wrap up, I'd like to briefly talk about vendoring. Most software relies on some third-party (open source) software. In the Node.js world they are called npm packages, in the Java world they are MAven packages and in Go they are just called a Go package.
What you commonly see in the Node.js world is that people push their own code to a server and then run npm install on that server. There have been a few instances where some npm packages disappeared and that caused a lot of problems for people who all of a sudden couldn't install their software anymore.
One way to solve it is to simply a copy of the third-party dependency into your own project and treat it as you would treat the code you've written yourself. They're part of the git project and checked in together. Since Go 1.5, the Go world has agreed to use the /vendor folder in your project for this. (Before Go 1.5 it was the wild wild west...)
Rather than manually copying packages, there are some tools out there that can help you. I like [govendor](https://github.com/kardianos/govendor).
If you want to add a package to your /vendor folder, there are two ways you can do it:
- You either have the package already in your
GOPATH: just rungovendor add your/desired/package - You don't have it on your system: just run
govendor fetch your/desired/package
There are plenty more commands available to update and sync packages on the project README.md.
This project has already all its dependencies vendored into the /vendor folder, so no need to do it again.
One piece of advice: it's generally a good practice to do this for your own project, unless you are writing a reusable library/package to be consumed by other people. The recommendation in the latter case is to not do any vendoring because you don't know what the vendoring strategy/toolset is of the people that are consuming your library/package.
This is how you could run your app on a server:
First, you copy the binary and the fixtures.json + VERSION files into a directory, e.g. /opt/go-rest-api-template.
Then start the app as a service. Store the app's PID in a text file so we can kill it later.
#!/bin/bash
export ENV=DEV
export PORT=8080
export VERSION=/opt/go-rest-api-template/VERSION
export FIXTURES=/opt/go-rest-api-template/fixtures.json
sudo nohup /opt/go-rest-api-template/go-rest-api-template >> /var/log/go-rest-api-template.log 2>&1&
echo $! > /opt/go-rest-api-template/go-rest-api-template-pid.txt
When you want to kill your app later during a redeployment or a server shutdown, then you can kill the app by looking up the previously stored PID:
#!/bin/bash
if [ -f /opt/go-rest-api-template/go-rest-api-template-pid.txt ]; then
kill -9 `cat /opt/go-rest-api-template/go-rest-api-template-pid.txt`
rm -f /opt/go-rest-api-template/go-rest-api-template-pid.txt
fi
- Structs and JSON formatting
- JSON and Go
- Design beautiful REST + JSON APIs
- Use render for generating JSON for use of global variable
- Read JSON POST body
- How to pass a parameter to a Http handler function
- Go and datetime parsing/formatting: ISO 8601, the International Standard for the representation of dates and times, Go by Example: Time Formatting / Parsing, JSON datetime formatting, src/time/format.go
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