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Kohei
Kohei

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Go tutorial: 4. Sample Application -TODO List API-

#go #beginners

This article was originally published on GitHub.

We are going to develop a tiny RESTful API for the TODO application that contains the following features:

  • Return all TODO
  • Save new TODO
  • Delete a TODO

Before building the API, please create a new directory.

$ mkdir todo
$ cd todo
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Data schema

The API deal TODO lists with clients as a JSON files such as:

[
  {
    "id": 1,
    "title": "Do dishes",
    "note": "That will be done by Gopher.",
    "due_date": "2000-01-01T00:00:00Z"
  }
]
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First of all, let's define the schema with Go.

$ mkdir schema
$ touch schema/model.go
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// schema/model.go
package schema

import "time"

type Todo struct {
    ID      int       `json:"id"`
    Title   string    `json:"title"`
    Note    string    `json:"note"`
    DueDate time.Time `json:"due_date"`
}
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It's a really common pattern to define schemas as a structure. The Todo structure has four fields, ID, Title, Note, and DueDate. They respectively have their types and json tags. The tags will be the field names when the Todo structure is encoded. If you don't write the tags, json field names will be the same as the structure's name.

Repositories

Create the db directory and the db/repository.go.

$ mkdir db
$ touch db/repository.go
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// db/repository.go
package db

import (
    "context"

    "../schema"
)

const keyRepository = "Repository"

type Repository interface {
    Close()
    Insert(todo *schema.Todo) (int, error)
    Delete(id int) error
    GetAll() ([]schema.Todo, error)
}

func SetRepository(ctx context.Context, repository Repository) context.Context {
    return context.WithValue(ctx, keyRepository, repository)
}

func Close(ctx context.Context) {
    getRepository(ctx).Close()
}

func Insert(ctx context.Context, todo *schema.Todo) (int, error) {
    return getRepository(ctx).Insert(todo)
}

func Delete(ctx context.Context, id int) error {
    return getRepository(ctx).Delete(id)
}

func GetAll(ctx context.Context) ([]schema.Todo, error) {
    return getRepository(ctx).GetAll()
}

func getRepository(ctx context.Context) Repository {
    return ctx.Value(keyRepository).(Repository)
}
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This Repository interface has four methods. We can access our DB through the interface in the context. The interface can divide the application logic and implementations for each middleware such as PostgreSQL or MySQL. The SetRepository function sets structures implementing the Repository interface to the context.

Sample TODO list

At first, we'll create a structure that returns static TODO list as samples instead of dynamic values in DB.

$ touch db/samples.go
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// db/samples.go
package db

import "../schema"

type Sample struct{}

func (s *Sample) Close() {}

func (s *Sample) Insert(todo *schema.Todo) (int, error) {
    return 0, nil
}

func (s *Sample) Delete(id int) error {
    return nil
}

func (s *Sample) GetAll() ([]schema.Todo, error) {
    return nil, nil
}
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The Sample structure has Close, Insert, Delete, and GetAll methods, so we can use it as a Repository interface type. But these methods do nothing yet. Create tests for them first and implement them after that.

Test for samples

Create db/sample_test.go and write the tests.

$ touch db/sample_test.go
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// db/sample_test.go
package db

import (
    "reflect"
    "testing"
    "time"

    "../schema"
)

func TestClose(t *testing.T) {
    sample := Sample{}

    sample.Close()
}

func TestInsert(t *testing.T) {
    sample := Sample{}

    todo := &schema.Todo{}

    got, err := sample.Insert(todo)
    if err != nil {
        t.Error(err)
    }

    if got != 0 {
        t.Fatal("Want: 0, Got: ", got)
    }
}

func TestDelete(t *testing.T) {
    sample := Sample{}

    err := sample.Delete(1)
    if err != nil {
        t.Error(err)
    }
}

func TestGetAll(t *testing.T) {
    sample := Sample{}

    got, err := sample.GetAll()
    if err != nil {
        t.Error(err)
    }

    want := []schema.Todo{
        {
            ID:      1,
            Title:   "Do dishes",
            Note:    "",
            DueDate: time.Date(2000, 1, 1, 0, 0, 0, 0, time.UTC),
        },
        {
            ID:      2,
            Title:   "Do homework",
            Note:    "",
            DueDate: time.Date(2000, 1, 1, 0, 0, 0, 0, time.UTC),
        },
        {
            ID:      2,
            Title:   "Twitter",
            Note:    "",
            DueDate: time.Date(2000, 1, 1, 0, 0, 0, 0, time.UTC),
        },
    }

    if !reflect.DeepEqual(got, want) {
        t.Fatalf("Want: %v, Got: %v\n", want, got)
    }
}
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Our sample repository won't have any completed features for Close, Insert, and Delete. So the tests for them are meaningless and already pass. Of course, TestGetAll will fail yet.

$ go test ./db/*
-------- FAIL: TestGetAll (0.00s)
        sample_test.go:71: Want: [{1 Do dishes  2000-01-01 00:00:00 +0000 UTC} {2 Do homework  2000-01-01 00:00:00 +0000 UTC} {2 Twitter  2000-01-01 00:00:00 +0000 UTC}], Got: []
FAIL
FAIL    command-line-arguments  0.041s
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Implementation for samples 

// db/samples.go
package db

import (
    "time"

    "../schema"
)

// ...

func (s *Sample) GetAll() ([]schema.Todo, error) {
    todoList := []schema.Todo{
        {
            ID:      1,
            Title:   "Do dishes",
            Note:    "",
            DueDate: time.Date(2000, 1, 1, 0, 0, 0, 0, time.UTC),
        },
        {
            ID:      2,
            Title:   "Do homework",
            Note:    "",
            DueDate: time.Date(2000, 1, 1, 0, 0, 0, 0, time.UTC),
        },
        {
            ID:      2,
            Title:   "Twitter",
            Note:    "",
            DueDate: time.Date(2000, 1, 1, 0, 0, 0, 0, time.UTC),
        },
    }

    return todoList, nil
}
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Execute the tests. They will pass.

$ go test ./db/*
ok      command-line-arguments  0.182s
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Service

Create the service directory and service/todo.go. This service has only simple functions that call functions with the same name in the db package. If you'd like to upgrade this TODO API, the service will have a lot of complex function.

$ mkdir service
$ touch service/todo.go
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// service/todo.go
package service

import (
    "context"

    "../db"
    "../schema"
)

func Close(ctx context.Context) {
    db.Close(ctx)
}

func Insert(ctx context.Context, todo *schema.Todo) (int, error) {
    return db.Insert(ctx, todo)
}

func Delete(ctx context.Context, id int) error {
    return db.Delete(ctx, id)
}

func GetAll(ctx context.Context) ([]schema.Todo, error) {
    return db.GetAll(ctx)
}
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Routing and Handlers

Sample Handler 

$ mkdir handler
$ touch handler/todo.go
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// handler/todo.go
package handler

import (
    "encoding/json"
    "net/http"

    "../db"
    "../service"
)

type todoHandler struct {
    samples  *db.Sample
}

func (handler *todoHandler) GetSamples(w http.ResponseWriter, r *http.Request) {
    ctx := db.SetRepository(r.Context(), handler.samples)

    todoList, err := service.GetAll(ctx)
    if err != nil {
        responseError(w, http.StatusInternalServerError, err.Error())
        return
    }

    responseOk(w, todoList)
}
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The package have the GetSamples method that sets a db.Sample structure to the context. So the TODO service will return sample TODO list.

Create utility functions responseOk and responseError.

// handler/todo.go
func responseOk(w http.ResponseWriter, body interface{}) {
    w.WriteHeader(http.StatusOK)
    w.Header().Set("Content-Type", "application/json")

    json.NewEncoder(w).Encode(body)
}

func responseError(w http.ResponseWriter, code int, message string) {
    w.WriteHeader(code)
    w.Header().Set("Content-Type", "application/json")

    body := map[string]string{
        "error": message,
    }
    json.NewEncoder(w).Encode(body)
}
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Routing

Create handler/routes.go. The routings will be written in the file.

$ touch handler/routes.go
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// handler/routes.go
package handler

import (
    "net/http"

    "../db"
)

func SetUpRouting() *http.ServeMux {
    todoHandler := &todoHandler{
        samples:  &db.Sample{},
    }

    mux := http.NewServeMux()
    mux.HandleFunc("/samples", todoHandler.GetSamples)

    return mux
}
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Functional test

We've completed building the /samples endpoint the should return the sample TODO list as a Json file. Let's write the functional test to confirm it.

$ mkdir functional
$ touch functional/todo_test.go
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// functional/todo_test.go
package functional

import (
    "net/http"
    "strings"
    "testing"

    "../handler"
)

func TestGetSamples(t *testing.T) {
    testServer := setupServer()

    req, err := http.NewRequest(http.MethodGet, "http://localhost:8080/samples", nil)
    if err != nil {
        t.Fatal(err)
    }

    rec := httptest.NewRecorder()
    testServer.ServeHTTP(rec, req)

    got := strings.TrimSpace(rec.Body.String())

    want := `[{"id":1,"title":"Do dishes","note":"","due_date":"2000-01-01T00:00:00Z"},{"id":2,"title":"Do homework","note":"","due_date":"2000-01-01T00:00:00Z"},{"id":2,"title":"Twitter","note":"","due_date":"2000-01-01T00:00:00Z"}]`

    if got != want {
        t.Fatalf("Want: %v, Got: %v", want, got)
    }
}

func setupServer() *http.ServeMux {
    return handler.SetUpRouting()
}
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This test is very simple. It just calls the endpoint and compares the response body with the expected value. It should pass.

$ go test ./functional/todo_test.go
ok      command-line-arguments  0.099s
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main.go

It's finally time to make main.go that call handler.SetUpRouting() and set up the HTTP server.

$ touch main.go
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// main.go
package main

import (
    "fmt"
    "log"
    "net/http"

    "./handler"
)

func main() {
    mux := handler.SetUpRouting()

    fmt.Println("http://localhost:8080")
    log.Fatal(http.ListenAndServe(":8080", mux))
}
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Run main.go and visit http://localhost:8080/samples. The sample Json file will be returned.

$ go run main.go
$ curl "http://localhost:8080/samples" | jq
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   221  100   221    0     0  27949      0 --:--:-- --:--:-- --:--:-- 31571
[
  {
    "id": 1,
    "title": "Do dishes",
    "note": "",
    "due_date": "2000-01-01T00:00:00Z"
  },
  {
    "id": 2,
    "title": "Do homework",
    "note": "",
    "due_date": "2000-01-01T00:00:00Z"
  },
  {
    "id": 2,
    "title": "Twitter",
    "note": "",
    "due_date": "2000-01-01T00:00:00Z"
  }
]
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PostgreSQL

We've created API that returns TODO list. But it can only return a static file so we can't add a new task, update an existed task, and delete them. We have to implement Postgres repository and new endpoints to call its method.

DB for tests 

$ mkdir testdb
$ touch testdb/testdb.go
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// testdb/testdb.go
package testdb

import (
    "database/sql"
)

const createTable = `
DROP TABLE IF EXISTS todo;
Alter SEQUENCE todo_id RESTART WITH 1;
CREATE TABLE todo (
  ID serial PRIMARY KEY,
  TITLE TEXT NOT NULL,
  NOTE TEXT,
  DUE_DATE TIMESTAMP WITH TIME ZONE
);
`

type TestDB struct {
    db *sql.DB
}

func Setup() *sql.DB {
    db, err := connectPostgresForTests()
    if err != nil {
        panic(err)
    }

    if _, err = db.Exec(createTable); err != nil {
        panic(err)
    }

    return db
}

func connectPostgresForTests() (*sql.DB, error) {
    connStr := "postgres://postgres:postgres@localhost:5432/postgres?sslmode=disable"
    db, err := sql.Open("postgres", connStr)
    if err != nil {
        return nil, err
    }

    err = db.Ping()
    if err != nil {
        return nil, err
    }

    return db, nil
}
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Test for Postgres

Create db/postgres.go and add methods to implement the Repository interface.

$ touch db/postgres.go
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// db/postgres.go
package db

import (
    "database/sql"

    "github.com/cohhei/go-to-the-handson/04/schema"
)

type Postgres struct {
    DB *sql.DB
}

func (p *Postgres) Close() {}

func (p *Postgres) Insert(todo *schema.Todo) (int, error) {
    return 0, nil
}

func (p *Postgres) Delete(id int) error {
    return nil
}

func (p *Postgres) GetAll() ([]schema.Todo, error) {
    return nil, nil
}
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Then create db/postgres_test.go.

$ touch db/postgres_test.go
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// db/postgres_test.go
package db

import (
    "reflect"
    "testing"
    "time"

    "../schema"
    "../testdb"
    _ "github.com/lib/pq"
)

func TestPostgres_Insert(t *testing.T) {
    postgres := &Postgres{testdb.Setup()}
    defer postgres.Close()

    todo := &schema.Todo{
        Title:   "title1",
        Note:    "note1",
        DueDate: time.Date(2000, 1, 1, 0, 0, 0, 0, time.Local),
    }

    got, err := postgres.Insert(todo)
    if err != nil {
        t.Fatal(err)
    }

    want := 1

    if got != want {
        t.Fatal(err)
    }
}

func TestPostgres_GetAll(t *testing.T) {
    postgres := &Postgres{testdb.Setup()}
    defer postgres.Close()

    todo := &schema.Todo{
        Title:   "title1",
        Note:    "note1",
        DueDate: time.Date(2000, 1, 1, 0, 0, 0, 0, time.Local),
    }

    _, err := postgres.Insert(todo)
    if err != nil {
        t.Fatal(err)
    }

    got, err := postgres.GetAll()
    if err != nil {
        t.Fatal(err)
    }

    want := []schema.Todo{
        {
            ID:      1,
            Title:   "title1",
            Note:    "note1",
            DueDate: time.Date(2000, 1, 1, 0, 0, 0, 0, time.Local),
        },
    }

    if equal(got, want) {
        t.Fatalf("Want: %v, Got: %v", want, got)
    }
}

func TestPostgres_Delete(t *testing.T) {
    postgres := &Postgres{testdb.Setup()}
    defer postgres.Close()

    todo := &schema.Todo{
        Title:   "title1",
        Note:    "note1",
        DueDate: time.Date(2000, 1, 1, 0, 0, 0, 0, time.Local),
    }

    id, err := postgres.Insert(todo)
    if err != nil {
        t.Fatal(err)
    }

    err = postgres.Delete(id)
    if err != nil {
        t.Fatal(err)
    }

    got, err := postgres.GetAll()
    if err != nil {
        t.Fatal(err)
    }

    if len(got) > 0 {
        t.Fatal("The record is not deleted.")
    }
}

func equal(got interface{}, want interface{}) bool {
    return reflect.DeepEqual(got, want)
}
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The test functions contain other methods in the Postgres structure for instance the TestPostgres_Delete has not only the Postgres.Delete method but Insert and GetAll. That's too bad pattern. When Insert has any bugs, TestPostgres_Delete will fail even if Delete has been correctly implemented. You should build functions to input and output to DB if you can.

Of course, the tests will fail yet.

$ go test ./db/postgres*
-------- FAIL: TestPostgres_Insert (0.03s)
        postgres_test.go:31: <nil>
FAIL
FAIL    command-line-arguments  0.077s
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Implementation for Postgres 

// db/postgres.go
package db

import (
    "database/sql"

    "../schema"
    _ "github.com/lib/pq"
)

type Postgres struct {
    DB *sql.DB
}

func (p *Postgres) Close() {
    p.DB.Close()
}

func (p *Postgres) Insert(todo *schema.Todo) (int, error) {
    query := `
        INSERT INTO todo (id, title, note, due_date)
        VALUES (nextval('todo_id'), $1, $2, $3)
        RETURNING id;
    `

    rows, err := p.DB.Query(query, todo.Title, todo.Note, todo.DueDate)
    if err != nil {
        return -1, err
    }

    var id int
    for rows.Next() {
        if err := rows.Scan(&id); err != nil {
            return -1, err
        }
    }

    return id, nil
}

func (p *Postgres) Delete(id int) error {
    query := `
        DELETE FROM todo
        WHERE id = $1;
    `

    if _, err := p.DB.Exec(query, id); err != nil {
        return err
    }

    return nil
}

func (p *Postgres) GetAll() ([]schema.Todo, error) {
    query := `
        SELECT *
        FROM todo
        ORDER BY id;
    `

    rows, err := p.DB.Query(query)
    if err != nil {
        return nil, err
    }

    var todoList []schema.Todo
    for rows.Next() {
        var t schema.Todo
        if err := rows.Scan(&t.ID, &t.Title, &t.Note, &t.DueDate); err != nil {
            return nil, err
        }
        todoList = append(todoList, t)
    }

    return todoList, nil
}
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To pass the tests, you should set PostgreSQL up. If you've already installed Docker, you can easily build it by using the one-line command.

$ docker run -d --name docker-postgres -e POSTGRES_PASSWORD=postgres -p 5432:5432 postgres:alpine
$ go test ./db/postgres*
ok      command-line-arguments  0.250s
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Functional test for Postgres 

// functional/todo_test.go

// ...
import (
    // ...
    "bytes"
    "fmt"
    "net/http/httptest"
    "reflect"
    "time"
    // ...
    "../db"
    "../schema"
    "../testdb"
    // ...
)
// ...
func TestGetSamples(t *testing.T) {
    testServer := setupServer(nil)
//...
func setupServer(postgres *db.Postgres) *http.ServeMux {
    return handler.SetUpRouting(postgres)
}
// ...
func TestGetAllTodo(t *testing.T) {
    postgres := &db.Postgres{testdb.Setup()}
    testServer := setupServer(postgres)

    todo := &schema.Todo{
        Title:   "My Task1",
        DueDate: time.Date(2000, 1, 1, 0, 0, 0, 0, time.Local),
    }

    _, err := postgres.Insert(todo)
    if err != nil {
        t.Fatal(err)
    }

    req, err := http.NewRequest(http.MethodGet, "http://localhost:8080/todo", nil)
    if err != nil {
        t.Fatal(err)
    }

    rec := httptest.NewRecorder()
    testServer.ServeHTTP(rec, req)

    got := strings.TrimSpace(rec.Body.String())

    want := `[{"id":1,"title":"My Task1","note":"","due_date":"2000-01-01T00:00:00+09:00"}]`

    if got != want {
        t.Fatalf("Want: %v, Got: %v", want, got)
    }
}

func TestSaveTodo(t *testing.T) {
    postgres := &db.Postgres{testdb.Setup()}
    testServer := setupServer(postgres)

    body := []byte(`{"id":1,"title":"My Task1","note":"","due_date":"2000-01-01T00:00:00+09:00"}`)

    req, err := http.NewRequest(http.MethodPost, "http://localhost:8080/todo", bytes.NewReader(body))
    if err != nil {
        t.Fatal(err)
    }

    rec := httptest.NewRecorder()
    testServer.ServeHTTP(rec, req)

    got := strings.TrimSpace(rec.Body.String())
    want := "1"

    if got != want {
        t.Fatalf("Want: %v, Got: %v", want, got)
    }

    gotTodo, err := postgres.GetAll()
    if err != nil {
        t.Fatal(err)
    }

    wantTodo := []schema.Todo{
        {
            Title:   "My Task1",
            DueDate: time.Date(2000, 1, 1, 0, 0, 0, 0, time.Local),
        },
    }

    if !reflect.DeepEqual(got, want) {
        t.Fatalf("Want: %v, Got: %v\n", wantTodo, gotTodo)
    }
}

func TestDeleteTodo(t *testing.T) {
    postgres := &db.Postgres{testdb.Setup()}
    testServer := setupServer(postgres)

    todo := &schema.Todo{
        Title:   "My Task1",
        DueDate: time.Date(2000, 1, 1, 0, 0, 0, 0, time.Local),
    }

    id, err := postgres.Insert(todo)
    if err != nil {
        t.Fatal(err)
    }

    body := []byte(fmt.Sprintf(`{"id":%d}`, id))

    req, err := http.NewRequest(http.MethodDelete, "http://localhost:9999/todo", bytes.NewReader(body))
    if err != nil {
        t.Fatal(err)
    }

    rec := httptest.NewRecorder()
    testServer.ServeHTTP(rec, req)

    got := rec.Body.String()

    want := ""

    if got != want {
        t.Fatalf("Want: %v, Got: %v", want, got)
    }

    gotTodo, err := postgres.GetAll()
    if err != nil {
        t.Fatal(err)
    }

    if len(gotTodo) > 0 {
        t.Fatalf("Should return the empty slice, Got: %v\n", gotTodo)
    }
}
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$ go test ./functional/todo_test.go
# command-line-arguments
functional/todo_test.go:164:22: too many arguments in call to handler.SetUpRouting
        have (*db.Postgres)
        want ()
FAIL    command-line-arguments [build failed]
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New routing and handlers 

// handler/todo.go
import (
    // ...
    "io/ioutil"
    // ...
    "../schema"
)

type todoHandler struct {
    postgres *db.Postgres
    samples  *db.Sample
}
//...
func (handler *todoHandler) saveTodo(w http.ResponseWriter, r *http.Request) {
    ctx := db.SetRepository(r.Context(), handler.postgres)

    b, err := ioutil.ReadAll(r.Body)
    if err != nil {
        responseError(w, http.StatusInternalServerError, err.Error())
        return
    }

    var todo schema.Todo
    if err := json.Unmarshal(b, &todo); err != nil {
        responseError(w, http.StatusBadRequest, err.Error())
        return
    }

    id, err := service.Insert(ctx, &todo)
    if err != nil {
        responseError(w, http.StatusInternalServerError, err.Error())
        return
    }

    responseOk(w, id)
}

func (handler *todoHandler) deleteTodo(w http.ResponseWriter, r *http.Request) {
    ctx := db.SetRepository(r.Context(), handler.postgres)

    b, err := ioutil.ReadAll(r.Body)
    if err != nil {
        responseError(w, http.StatusInternalServerError, err.Error())
        return
    }

    var req struct {
        ID int `json:"id"`
    }
    if err := json.Unmarshal(b, &req); err != nil {
        responseError(w, http.StatusBadRequest, err.Error())
        return
    }

    if err := service.Delete(ctx, req.ID); err != nil {
        responseError(w, http.StatusInternalServerError, err.Error())
        return
    }

    w.WriteHeader(http.StatusOK)
}

func (handler *todoHandler) getAllTodo(w http.ResponseWriter, r *http.Request) {
    ctx := db.SetRepository(r.Context(), handler.postgres)

    todoList, err := service.GetAll(ctx)
    if err != nil {
        responseError(w, http.StatusInternalServerError, err.Error())
        return
    }

    responseOk(w, todoList)
}
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We are setting a repository at each handler function, but that's just in order to explain interfaces in Go. So you shouldn't use the pattern in your production code.

// handler/routes.go
package handler

import (
    "net/http"

    "../db"
)

// - func SetUpRouting() {
func SetUpRouting(postgres *db.Postgres) *http.ServeMux {
    todoHandler := &todoHandler{
        postgres: postgres,
        samples:  &db.Sample{},
    }

    mux := http.NewServeMux()
    mux.HandleFunc("/samples", todoHandler.GetSamples)
    mux.HandleFunc("/todo", func(w http.ResponseWriter, r *http.Request) {
        switch r.Method {
        case http.MethodGet:
            todoHandler.getAllTodo(w, r)
        case http.MethodPost:
            todoHandler.saveTodo(w, r)
        case http.MethodDelete:
            todoHandler.deleteTodo(w, r)
        default:
            responseError(w, http.StatusNotFound, "")
        }
    })

    return mux
}
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$ go test ./functional/todo_test.go
ok      command-line-arguments  0.179s
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After passing functional tests, you can stop the docker container.

$ docker ps
CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS                    NAMES
530507a60b58        postgres:alpine     "docker-entrypoint.s…"   19 hours ago        Up 19 hours         0.0.0.0:5432->5432/tcp   docker-postgres

$ docker stop 530507a60b58
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Updating main.go 

// main.go

import (
    //...
    "time"
    "./db"
    //...
)

func main() {
    var postgres *db.Postgres
    var err error
    for i := 0; i < 10; i++ {
        time.Sleep(3 * time.Second)
        postgres, err = db.ConnectPostgres()
    }
    if err != nil {
        panic(err)
    } else if postgres == nil {
        panic("postgres is nil")
    }

    mux := handler.SetUpRouting(postgres)

    fmt.Println("http://localhost:8080")
    log.Fatal(http.ListenAndServe(":8080", mux))
}
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We don't create the db/ConnectPostgres function. Add it to db/postgres.go.

// db/postgres.go
func ConnectPostgres() (*Postgres, error) {
    connStr := "postgres://postgres:postgres@postgres/postgres?sslmode=disable"
    db, err := sql.Open("postgres", connStr)
    if err != nil {
        return nil, err
    }

    err = db.Ping()
    if err != nil {
        return nil, err
    }

    return &Postgres{db}, nil
}
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What's the difference between the function and testdb.connectPostgresForTests? It's the connStr string. localhost:5432 was configured as a host name in testdb.connectPostgresForTests. But postgres is configured here. Docker will resolve the name.

Docker

Postgres 

$ mkdir postgres
$ touch postgres/up.sql
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-- postgres/up.sql
DROP TABLE IF EXISTS todo;
CREATE SEQUENCE todo_id START 1;
CREATE TABLE todo (
  ID serial PRIMARY KEY,
  TITLE TEXT NOT NULL,
  NOTE TEXT,
  DUE_DATE TIMESTAMP WITH TIME ZONE
);
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$ touch postgres/Dockerfile
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# postgres/Dockerfile
FROM postgres:10.3
COPY up.sql /docker-entrypoint-initdb.d/1.sql
CMD ["postgres"]
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TODO API 

$ touch Dockerfile
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# Dockerfile
# build stage
FROM golang:1.10.2-alpine AS build
ARG dir=/todo
ADD . ${dir}
RUN apk update && \
    apk add --virtual build-dependencies build-base git && \
    cd ${dir} && \
    go get -u github.com/lib/pq && \
    go build -o todo-api

# final stage
FROM alpine:3.7
ARG dir=/todo
WORKDIR /app
COPY --from=build ${dir}/todo-api /app/
EXPOSE 8080
CMD ./todo-api
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docker-compose.yml 

$ touch docker-compose.yml
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# docker-compose.yml
version: "3.6"

services:
  postgres:
    build: "./postgres"
    restart: "always"
    environment:
      POSTGRES_DB: "postgres"
      POSTGRES_USER: "postgres"
      POSTGRES_PASSWORD: "postgres"
  todo-api:
    build: "."
    depends_on:
      - postgres
    ports:
      - 8080:8080
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$ docker-compose up -d
Creating todo_postgres_1 ... done
Creating todo_todo-api_1 ... done

 $ docker-compose ps
     Name                    Command              State           Ports
-------------------------------------------------------------------------------------
todo_postgres_1   docker-entrypoint.sh postgres   Up      5432/tcp
todo_todo-api_1   /bin/sh -c ./todo-api           Up      0.0.0.0:8080->8080/tcp
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Requests

We've completed the TODO-API but it can't be used from the browser. Create requests/request.go, a cli tool to use our API.

$ mkdir requests
$ touch requests/requests.go
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// requests/requests.go
package main

import (
    "bytes"
    "encoding/json"
    "fmt"
    "io/ioutil"
    "net/http"
    "os"
    "strconv"
)

const usage = `
usage: todo COMMAND

Commands:
  samples     Get sample todo tasks
  all         Get all todo tasks
  add         Add new todo task
  delete      Remove a todo task
`

func main() {
    if len(os.Args) < 2 {
        fmt.Print(usage)
        return
    }

    switch command := os.Args[1]; command {
    case "samples":
        get("samples")
    case "all":
        get("todo")
    case "add":
        add()
    case "delete":
        del()
    default:
        fmt.Printf("'%s' is not a todo command.", command)
    }
}

func get(path string) {
    res, err := http.Get(fmt.Sprintf("http://localhost:8080/%s", path))
    if err != nil {
        panic(err)
    }
    defer res.Body.Close()

    b, err := ioutil.ReadAll(res.Body)
    if err != nil {
        panic(err)
    }

    if len(b) == 0 {
        fmt.Println("The response is empty.")
        return
    }
    fmt.Println(string(b))
}

const usage_add = `
usage: todo add TODO_NAME TODO_NOTE DUE_DATE
`

func add() {
    if len(os.Args) < 3 {
        fmt.Print(usage_add)
        return
    }

    name := os.Args[2]
    var note string
    var date string

    if len(os.Args) > 3 {
        note = os.Args[3]
        if len(os.Args) > 4 {
            date = os.Args[4]
        }
    }

    todo := struct {
        Title   string `json:"title"`
        Note    string `json:"note,omitempty"`
        DueDate string `json:"due_date,omitempty"`
    }{
        name, note, date,
    }

    b, err := json.Marshal(todo)
    if err != nil {
        panic(err)
    }
    fmt.Println(string(b))

    res, err := http.Post("http://localhost:8080/todo", "application/json", bytes.NewReader(b))
    if err != nil {
        panic(err)
    }
    defer res.Body.Close()

    b, err = ioutil.ReadAll(res.Body)
    if err != nil {
        panic(err)
    }

    fmt.Println(string(b))
}

func del() {
    if len(os.Args) < 3 {
        fmt.Print("usage: todo delete TASK_ID")
        return
    }

    id, err := strconv.Atoi(os.Args[2])
    if err != nil {
        fmt.Println("TASK_ID should be number")
        return
    }

    b, err := json.Marshal(map[string]int{"id": id})
    if err != nil {
        panic(err)
    }

    req, err := http.NewRequest(http.MethodDelete, "http://localhost:8080/todo", bytes.NewReader(b))
    if err != nil {
        panic(err)
    }

    client := &http.Client{}
    res, err := client.Do(req)
    if err != nil {
        panic(err)
    }

    res.Body.Close()
}
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You can call all API's endpoints by sub commands, samples, all, add, and delete.

$ go run requests/requests.go

usage: todo COMMAND

Commands:
  samples     Get sample todo tasks
  all         Get all todo tasks
  add         Add new todo task
  delete      Remove a todo task
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For instance, add a new task by add $TASK_NAME $NOTE and get all tasks by all.

$ go run requests/requests.go add Task1 'this is the first task.'
$ go run requests/requests.go all | jq
[
  {
    "id": 1,
    "title": "Task1",
    "note": "this is the first task.",
    "due_date": "2000-01-01T00:00:00Z"
  }
]
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