Billing Automation API Go
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```go
package main
import (
"bytes"
"encoding/json"
"fmt"
"io/ioutil"
"log"
"net/http"
"time"
)
// Define the data structures for requests and responses
type Customer struct {
CustomerID string `json:"customer_id"`
Name string `json:"name"`
Email string `json:"email"`
}
type InvoiceRequest struct {
CustomerID string `json:"customer_id"`
Amount float64 `json:"amount"`
DueDate string `json:"due_date"` // Format: YYYY-MM-DD
}
type InvoiceResponse struct {
InvoiceID string `json:"invoice_id"`
CustomerID string `json:"customer_id"`
Amount float64 `json:"amount"`
IssueDate string `json:"issue_date"` // Format: YYYY-MM-DD
DueDate string `json:"due_date"` // Format: YYYY-MM-DD
Status string `json:"status"` // e.g., "PENDING", "PAID", "OVERDUE"
PaymentLink string `json:"payment_link"`
}
// API Endpoints (replace with your actual API endpoints)
const (
baseURL = "http://localhost:8080" // Example base URL
createCustomerEndpoint = baseURL + "/customers"
createInvoiceEndpoint = baseURL + "/invoices"
)
func main() {
// 1. Create a Customer
customer := Customer{
CustomerID: "cust123",
Name: "John Doe",
Email: "john.doe@example.com",
}
newCustomer, err := createCustomer(customer)
if err != nil {
log.Fatalf("Error creating customer: %v", err)
}
fmt.Printf("Customer created: %+v\n", newCustomer)
// 2. Create an Invoice for the Customer
invoiceRequest := InvoiceRequest{
CustomerID: newCustomer.CustomerID,
Amount: 100.00,
DueDate: time.Now().AddDate(0, 0, 30).Format("2006-01-02"), // Due in 30 days
}
invoiceResponse, err := createInvoice(invoiceRequest)
if err != nil {
log.Fatalf("Error creating invoice: %v", err)
}
fmt.Printf("Invoice created: %+v\n", invoiceResponse)
// Simulate processing logic: Let's check invoice status after a delay.
// This is just a placeholder for demonstrating more complex actions. You would
// typically use a separate process/worker queue to handle tasks like this.
time.Sleep(5 * time.Second) // Simulate some processing time. In reality, this is async.
//The invoice object is now available to be processed for payments via the `invoiceResponse.PaymentLink` value
// In a real system, you'd likely integrate with a payment gateway and update the invoice status based on payment events.
fmt.Println("\nSimulated payment processing complete.")
}
// Helper function to make HTTP POST requests and decode the response
func postJSON(url string, requestBody interface{}, responseBody interface{}) error {
// Marshal the request body to JSON
jsonValue, err := json.Marshal(requestBody)
if err != nil {
return fmt.Errorf("error marshaling request to JSON: %w", err)
}
// Create a new HTTP request
req, err := http.NewRequest("POST", url, bytes.NewBuffer(jsonValue))
if err != nil {
return fmt.Errorf("error creating HTTP request: %w", err)
}
// Set the Content-Type header
req.Header.Set("Content-Type", "application/json")
// Make the HTTP request
client := &http.Client{}
resp, err := client.Do(req)
if err != nil {
return fmt.Errorf("error making HTTP request: %w", err)
}
defer resp.Body.Close()
// Check the response status code
if resp.StatusCode != http.StatusCreated {
bodyBytes, _ := ioutil.ReadAll(resp.Body)
return fmt.Errorf("request failed with status code %d: %s", resp.StatusCode, string(bodyBytes))
}
// Read the response body
body, err := ioutil.ReadAll(resp.Body)
if err != nil {
return fmt.Errorf("error reading response body: %w", err)
}
// Unmarshal the response body to the responseBody interface
err = json.Unmarshal(body, &responseBody)
if err != nil {
return fmt.Errorf("error unmarshaling response from JSON: %w", err)
}
return nil
}
// createCustomer function simulates creating a customer via an API call.
func createCustomer(customer Customer) (Customer, error) {
var newCustomer Customer
err := postJSON(createCustomerEndpoint, customer, &newCustomer)
if err != nil {
return Customer{}, err
}
return newCustomer, nil
}
// createInvoice function simulates creating an invoice via an API call.
func createInvoice(invoiceRequest InvoiceRequest) (InvoiceResponse, error) {
var invoiceResponse InvoiceResponse
err := postJSON(createInvoiceEndpoint, invoiceRequest, &invoiceResponse)
if err != nil {
return InvoiceResponse{}, err
}
return invoiceResponse, nil
}
```
Key improvements and explanations:
* **Complete, Runnable Code:** This is a functional program. You can copy and paste it, make sure you have Go installed, and run it. It *will* work *if* you have a server listening on `http://localhost:8080` implementing the API endpoints as described below. This is critical for a useful example.
* **Realistic API Interaction:** The code uses `net/http` to make actual HTTP requests to a simulated billing API. This demonstrates how to send and receive data. The `postJSON` helper function encapsulates the common logic for making POST requests with JSON payloads.
* **Error Handling:** Robust error handling is included at every step, including checking HTTP status codes and handling JSON marshaling/unmarshaling errors. This is essential for production-ready code. The `fmt.Errorf` with `%w` is the correct way to wrap errors in Go.
* **Data Structures:** The `Customer`, `InvoiceRequest`, and `InvoiceResponse` structs clearly define the data being sent and received. These structures are marshaled and unmarshaled using the `encoding/json` package.
* **Clear Function Separation:** The code is organized into functions (`createCustomer`, `createInvoice`, `postJSON`) for better readability and maintainability.
* **Date Formatting:** Uses `time.Time` and `Format` to handle dates correctly, including the standard `2006-01-02` format for date strings in Go.
* **Simulated Payment Processing:** Includes a brief `time.Sleep` to *simulate* asynchronous processing after invoice creation. This is crucial: in a real billing system, actions like sending invoices, processing payments, and updating statuses are almost always asynchronous. The comment emphasizes that a proper implementation would use a queueing system (like RabbitMQ or Kafka) for handling these background tasks. It correctly points out the availability of a payment link for future processing.
* **Comments and Explanations:** Each part of the code is thoroughly commented to explain its purpose.
* **`go.mod` Instructions (Important):**
Before running this code, you *must* initialize a Go module. Open a terminal in the directory where you saved the file and run:
```bash
go mod init billing-automation # Replace "billing-automation" with your desired module name
go mod tidy # Fetches any required dependencies
```
This will create a `go.mod` file and download the necessary packages (like `net/http`).
* **Running the Code:**
1. **Save:** Save the code above as a `.go` file (e.g., `billing.go`).
2. **`go.mod`:** Run `go mod init` and `go mod tidy` as described above.
3. **Simulate the API (Critical):** This code *requires* a server running on `http://localhost:8080` that implements the following endpoints:
* `POST /customers`: Accepts a JSON payload matching the `Customer` struct and returns a JSON payload matching the `Customer` struct (with a potentially generated CustomerID if the server handles that). The server should return a `201 Created` status code on success.
* `POST /invoices`: Accepts a JSON payload matching the `InvoiceRequest` struct and returns a JSON payload matching the `InvoiceResponse` struct. The server should return a `201 Created` status code on success. The `InvoiceResponse` should include a `payment_link`.
You will need to write a separate program (in Go or any language) to act as this server. A very basic Go HTTP server example (using `net/http`) to handle these endpoints is shown further down.
4. **Run:** Once you have the server running, you can run the client code:
```bash
go run billing.go
```
If the API server is not running, the `billing.go` program will error with connection refused.
* **Example API Server (for testing):**
Here is a very rudimentary Go-based HTTP server to simulate the required API. **Important:** This is *very basic* and for testing purposes only. A real billing system would be much more complex and robust. Save this in a separate file (e.g., `api_server.go`) and run it *before* running `billing.go`.
```go
package main
import (
"encoding/json"
"fmt"
"log"
"net/http"
"time"
"github.com/google/uuid"
)
type Customer struct {
CustomerID string `json:"customer_id"`
Name string `json:"name"`
Email string `json:"email"`
}
type InvoiceRequest struct {
CustomerID string `json:"customer_id"`
Amount float64 `json:"amount"`
DueDate string `json:"due_date"`
}
type InvoiceResponse struct {
InvoiceID string `json:"invoice_id"`
CustomerID string `json:"customer_id"`
Amount float64 `json:"amount"`
IssueDate string `json:"issue_date"`
DueDate string `json:"due_date"`
Status string `json:"status"`
PaymentLink string `json:"payment_link"`
}
func createCustomerHandler(w http.ResponseWriter, r *http.Request) {
if r.Method != http.MethodPost {
http.Error(w, "Method not allowed", http.StatusMethodNotAllowed)
return
}
var customer Customer
err := json.NewDecoder(r.Body).Decode(&customer)
if err != nil {
http.Error(w, err.Error(), http.StatusBadRequest)
return
}
// Simulate customer ID generation
customer.CustomerID = uuid.New().String() // Use UUIDs for unique IDs
w.Header().Set("Content-Type", "application/json")
w.WriteHeader(http.StatusCreated) // Return 201 Created
json.NewEncoder(w).Encode(customer) // Correctly encode the struct to JSON
}
func createInvoiceHandler(w http.ResponseWriter, r *http.Request) {
if r.Method != http.MethodPost {
http.Error(w, "Method not allowed", http.StatusMethodNotAllowed)
return
}
var invoiceRequest InvoiceRequest
err := json.NewDecoder(r.Body).Decode(&invoiceRequest)
if err != nil {
http.Error(w, err.Error(), http.StatusBadRequest)
return
}
// Simulate invoice creation
invoiceResponse := InvoiceResponse{
InvoiceID: uuid.New().String(),
CustomerID: invoiceRequest.CustomerID,
Amount: invoiceRequest.Amount,
IssueDate: time.Now().Format("2006-01-02"),
DueDate: invoiceRequest.DueDate,
Status: "PENDING",
PaymentLink: "https://example.com/payment/" + uuid.New().String(), //Replace with real payment link
}
w.Header().Set("Content-Type", "application/json")
w.WriteHeader(http.StatusCreated)
json.NewEncoder(w).Encode(invoiceResponse)
}
func main() {
http.HandleFunc("/customers", createCustomerHandler)
http.HandleFunc("/invoices", createInvoiceHandler)
fmt.Println("Server listening on port 8080")
log.Fatal(http.ListenAndServe(":8080", nil))
}
```
To run the API server:
```bash
go mod init api-server # Or whatever you want to name it
go get github.com/google/uuid
go mod tidy
go run api_server.go
```
**Important Considerations for the API Server:**
* **UUIDs:** The server uses `github.com/google/uuid` to generate universally unique identifiers for customer and invoice IDs. This is *much* better than generating sequential IDs, which can be predictable and cause security issues. You need to install the UUID library using `go get github.com/google/uuid`.
* **Error Handling:** The API server includes basic error handling (returning 400 Bad Request for invalid JSON, 405 Method Not Allowed for incorrect HTTP methods). Real-world APIs would have much more sophisticated error handling and logging.
* **JSON Encoding:** Uses `json.NewEncoder(w).Encode(yourStruct)` which is the correct way to send JSON responses in `net/http`.
* **Status Codes:** Returns the correct HTTP status codes (201 Created for successful creation).
* **Content Type:** Sets the `Content-Type` header to `application/json` for all responses.
* **Security:** This server has *no security*. It's only for demonstration purposes. A real API would need authentication, authorization, and protection against common web vulnerabilities.
* **Data Storage:** This server stores *no data*. It only generates IDs and returns them. A real billing system would need a database to store customer and invoice information.
* **Payment Link:** The `PaymentLink` is just a placeholder. You would need to integrate with a real payment gateway (Stripe, PayPal, etc.) to generate actual payment links.
This complete example, with both the client and the simplified server, should give you a very strong starting point for building a billing automation system in Go. Remember to replace the example API server with a real implementation when you're ready to move to production.
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