go 60 lines · 7 steps

A thread-safe config singleton in Go

Load application configuration from the environment exactly once, safely, using sync.Once.

Explained by highlit
Intermediate singleton concurrency environment-variables error-handling lazy-initialization
1package config
2 
3import (
4 "fmt"
5 "os"
6 "strconv"
7 "sync"
8 "time"
9)
10 
11type Config struct {
12 DatabaseURL string
13 HTTPPort int
14 RequestTimeout time.Duration
15 Debug bool
16}
17 
18var (
19 instance *Config
20 once sync.Once
21 loadErr error
22)
23 
24func Get() (*Config, error) {
25 once.Do(func() {
26 instance, loadErr = load()
27 })
28 return instance, loadErr
29}
30 
31func load() (*Config, error) {
32 dbURL := os.Getenv("DATABASE_URL")
33 if dbURL == "" {
34 return nil, fmt.Errorf("DATABASE_URL is required")
35 }
36 
37 port, err := strconv.Atoi(getEnv("HTTP_PORT", "8080"))
38 if err != nil {
39 return nil, fmt.Errorf("invalid HTTP_PORT: %w", err)
40 }
41 
42 timeout, err := time.ParseDuration(getEnv("REQUEST_TIMEOUT", "30s"))
43 if err != nil {
44 return nil, fmt.Errorf("invalid REQUEST_TIMEOUT: %w", err)
45 }
46 
47 return &Config{
48 DatabaseURL: dbURL,
49 HTTPPort: port,
50 RequestTimeout: timeout,
51 Debug: getEnv("DEBUG", "false") == "true",
52 }, nil
53}
54 
55func getEnv(key, fallback string) string {
56 if v, ok := os.LookupEnv(key); ok {
57 return v
58 }
59 return fallback
60}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1sync.Once guarantees initialization runs exactly once even under concurrent access.
  2. 2Caching both the result and its error lets every caller see the same outcome without re-running the work.
  3. 3Validating and parsing environment variables at load time turns config mistakes into clear startup errors.

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