How to implement feign in golang

In recent years, with the popularity of microservice architecture, more and more companies have begun to use microservices to build software systems. In microservices, communication between services is crucial, so an efficient and reliable way to call services is needed. In the Java ecosystem, Feign, as a commonly used microservice calling framework, has been widely used. However, there is no decent Feign framework for Golang yet, so Golang developers have to use other methods to achieve communication between microservices. This article will introduce how to use Golang to implement a microservice calling framework similar to Feign.

The following are the steps to implement Feign in Golang:

Step 1: Install and introduce the necessary libraries

In Golang, we need to use the HTTP library for HTTP communication, At the same time, some additional libraries are needed to handle HTTP communication. You can use Go Modules to manage dependencies and you can also use go get command to install these libraries. In this article, we will use go get command to install http, fmt library and other necessary libraries.

Step 2: Define the interface

In Golang, we can define an interface to define all calls in the microservice. The interface should contain all methods that need to be called, and each method should have parameters such as request URL, request method, request header, and request body. For example:

type UserService interface {
    GetUserByID(ctx context.Context, id int64) (*User, error)
    CreateUser(ctx context.Context, user *User) error
    UpdateUser(ctx context.Context, user *User) error
    DeleteUserByID(ctx context.Context, id int64) error
}

In the above code, we define a UserService interface, which contains 4 methods: GetUserByID, CreateUser, UpdateUser and DeleteUserByID. These methods will be used to call remote user services.

Step 3: Implement the interface

First, you need to create a new Client structure to implement the UserService interface, and define some necessary attributes in the structure (for example, the basic URL of the remote service, HTTP client, etc.). Then, each interface method needs to be implemented separately. Each method should build a corresponding HTTP request based on the request method (such as GET or POST) and pass all necessary parameters to the request. We can use the commonly used HTTP libraries in Golang to build the request and deserialize the response of the request into the required object. For example:

type UserClient struct {
    BaseURL    string
    HTTPClient http.Client
}

func (c *UserClient) GetUserByID(ctx context.Context, id int64) (*User, error) {
    req, _ := http.NewRequestWithContext(ctx, http.MethodGet, c.BaseURL+"/users/"+strconv.FormatInt(id, 10), nil)
    resp, err := c.HTTPClient.Do(req)
    if err != nil {
        return nil, err
    }
    defer resp.Body.Close()
    if resp.StatusCode != http.StatusOK {
        return nil, fmt.Errorf("unexpected status code: %v", resp.StatusCode)
    }
    var user User
    if err := json.NewDecoder(resp.Body).Decode(&user); err != nil {
        return nil, err
    }
    return &user, nil
}

func (c *UserClient) CreateUser(ctx context.Context, user *User) error {
    reqBody, _ := json.Marshal(user)
    req, _ := http.NewRequestWithContext(ctx, http.MethodPost, c.BaseURL+"/users", bytes.NewBuffer(reqBody))
    req.Header.Set("Content-Type", "application/json")
    resp, err := c.HTTPClient.Do(req)
    if err != nil {
        return err
    }
    defer resp.Body.Close()
    if resp.StatusCode != http.StatusCreated {
        return fmt.Errorf("unexpected status code: %v", resp.StatusCode)
    }
    return nil
}

...

In the above code, we implemented the GetUserByID and CreateUser methods. For example, the GetUserByID method generates an HTTP request based on the request method, request URL, and request parameters, and uses an HTTP client to make the request to the remote service. Then, deserialize the response into a User object and return it. The CreateUser method is similar, but adds User as the request body to the HTTP request.

Step 4: Use the implemented interface

Now, we can use the Client structure in the main application to call the methods defined in the UserService interface. For example:

func main() {
    client := &UserClient{
        BaseURL:    "http://localhost:8080",
        HTTPClient: http.Client{},
    }
    user, _ := client.GetUserByID(context.Background(), 1)
    fmt.Println(user.Name)
    newUser := &User{Name: "Bob"}
    client.CreateUser(context.Background(), newUser)
}

In the above code, we first create a UserClient instance and then use the instance to call the GetUserByID and CreateUser methods. In the GetUserByID method, we pass a context. This is to ensure that timeouts and context cancellation are handled correctly.

Summary

Through this article, we learned how to use Golang to implement a microservice calling framework similar to Feign. Using this framework, we can easily call other microservices in Golang applications without manually constructing HTTP requests. In addition, we also learned how to define and implement communication between microservices in Golang using interfaces and structs. Users can extend the framework by implementing interfaces in the Client structure to support more microservice calls.

Finally, it’s important to note that this article is just an example. In actual use, more factors need to be considered, such as security, performance and maintainability.

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