Viper Config Sources and File Formats

Supported file formats

Viper detects format from file extension. Supported extensions:

Format	Extensions
YAML	`.yaml`, `.yml`
TOML	`.toml`
JSON	`.json`
HCL	`.hcl`
INI	`.ini`
Properties	`.properties`
dotenv	`.env`

Force a format when there is no extension:

viper.SetConfigType("yaml")
viper.SetConfigFile("/etc/myapp/config")  // no extension — type required

Config file search

viper.SetConfigName("config")            // file name without extension
viper.SetConfigType("yaml")              // required when no extension
viper.AddConfigPath("$HOME/.myapp")      // search path 1 (highest priority when multiple)
viper.AddConfigPath("/etc/myapp/")       // search path 2
viper.AddConfigPath(".")                 // search path 3 (lowest priority)

// viper searches paths in order, stops at the first match
if err := viper.ReadInConfig(); err != nil {
    var notFound *viper.ConfigFileNotFoundError
    if !errors.As(err, &notFound) {
        return err  // real error (permission denied, malformed YAML, etc.)
    }
    // not found — continue with flags/env/defaults
}

// After reading, this returns the resolved path:
fmt.Println("Using config:", viper.ConfigFileUsed())

Merging multiple config files

MergeInConfig merges a second config file into the current state. Later values override earlier ones for the same key.

viper.SetConfigFile("base.yaml")
viper.ReadInConfig()

viper.SetConfigFile("override.yaml")
viper.MergeInConfig()  // keys from override.yaml win on collision

Pattern: ship a base config with the binary, let users drop an override in ~/.myapp/override.yaml.

Multiple config files via SetConfigFile

For environment-based config loading:

env := os.Getenv("APP_ENV")
if env == "" {
    env = "development"
}
viper.SetConfigFile(fmt.Sprintf("config.%s.yaml", env))
viper.ReadInConfig()

Remote KV stores (etcd, Consul)

Viper supports remote KV stores via the viper/remote sub-package. This keeps remote config behind an opt-in import:

import _ "github.com/spf13/viper/remote"

// etcd
viper.AddRemoteProvider("etcd3", "http://127.0.0.1:2379", "/config/myapp.yaml")
viper.SetConfigType("yaml")
viper.ReadRemoteConfig()

// Consul
viper.AddRemoteProvider("consul", "localhost:8500", "myapp/config")
viper.SetConfigType("json")
viper.ReadRemoteConfig()

Caution: Remote config adds network latency to startup and a runtime dependency. Use it only when you need centralized config across many service instances. For most applications, files + env vars are sufficient.

Watch for remote changes:

go func() {
    for {
        time.Sleep(5 * time.Second)
        viper.WatchRemoteConfig()
        // re-read values after watching
    }
}()

Embedding config with go:embed

Load config from embedded assets (for self-contained binaries):

//go:embed config.yaml
var defaultConfig []byte

func init() {
    viper.SetConfigType("yaml")
    viper.ReadConfig(bytes.NewReader(defaultConfig))
}

ReadConfig accepts any io.Reader. This is useful for shipping default config inside the binary, then layering user overrides on top via MergeInConfig.

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