Configuration with viper & envconfig

Q: How do I test configuration loading?

Set env vars in t.Setenv, write temp YAML files, or build AppConfig literals in tests without touching global viper when possible.

Q: Should CLIs use the same stack as services?

Often yes for shared AppConfig structs; CLIs may add pflag bindings where services rely on env only.

Q: What about validation beyond required?

Unmarshal into structs and run go-playground/validator or hand-rolled checks on ports, URLs, and durations.

Q: Is viper too heavy for small tools?

For single-binary CLIs with three flags, stdlib flag is simpler and avoids a dependency.

Q: Should config structs live in internal/config?

Yes - one package owns loading, validation, and redacted String() for logs.

Services and CLIs need configuration that changes per environment without recompilation.

spf13/viper merges files, flags, and environment variables with precedence rules; kelseyhightower/envconfig maps env vars directly into typed structs for smaller binaries.

Recipe

Quick-reference recipe card - copy-paste ready.

viper.SetConfigName("config")
viper.SetConfigType("yaml")
viper.AddConfigPath(".")
viper.AutomaticEnv()
viper.SetDefault("http.port", 8080)
_ = viper.ReadInConfig()
port := viper.GetInt("http.port")

type Config struct {
    Port int    `envconfig:"PORT" default:"8080"`
    DSN  string `envconfig:"DATABASE_URL" required:"true"`
}
var cfg Config
envconfig.Process("", &cfg)

When to reach for this:

Kubernetes-deployed services with ConfigMaps, Secrets, and env overrides
CLIs that read a config file locally but accept flags for automation
Teams documenting required env vars in README and Helm charts
Gradual migration from ad hoc os.Getenv scattered across packages

Working Example

package main
 
import (
    "fmt"
    "log"
    "strings"
 
    "github.com/spf13/viper"
)
 
type AppConfig struct {
    HTTPPort int
    LogLevel string
    DSN      string
}
 
func loadViper() AppConfig {
    viper.SetConfigName("config")
    viper.SetConfigType("yaml")
    viper.AddConfigPath(".")
    viper.SetEnvKeyReplacer(strings.NewReplacer(".", "_"))
    viper.AutomaticEnv()
 
    viper.SetDefault("http.port", 8080)
    viper.SetDefault("log.level", "info")
 
    if err := viper.ReadInConfig(); err != nil {
        log.Printf("no config file: %v", err)
    }
 
    return AppConfig{
        HTTPPort: viper.GetInt("http.port"),
        LogLevel: viper.GetString("log.level"),
        DSN:      viper.GetString("database.dsn"),
    }
}
 
func main() {
    cfg := loadViper()
    if cfg.DSN == "" {
        log.Fatal("database.dsn or DATABASE_DSN required")
    }
    fmt.Printf("port=%d level=%s\n", cfg.HTTPPort, cfg.LogLevel)
}

What this demonstrates:

YAML defaults with env override via AutomaticEnv and key replacer
Fail-fast validation for secrets in main
Typed AppConfig struct separate from viper's stringly API
Safe logging when config file is optional in dev

Deep Dive

How It Works

viper stores a key-value map populated from multiple sources; later reads use precedence (flag > env > config file > default).
envconfig reflects struct tags at startup and errors on missing required fields.
Both run once at process start; hot reload needs fsnotify (viper) or process restart (simplest ops model).
Keep config loading in main or a config package; domain packages receive typed structs, not global viper calls.

Precedence (viper)

Source	Typical use	Override strength
Explicit `Set` in code	Tests	Highest when used
Flags (`viper.BindPFlags`)	CLI overrides	High
Environment	K8s Secrets, 12-factor	High
Config file	Defaults per environment	Medium
`SetDefault`	Safe fallbacks	Low

envconfig Tags

Tag	Effect
`envconfig:"PORT"`	Env var name (with optional prefix)
`required:"true"`	Process fails if unset
`default:"8080"`	Value when env missing
`split_words:"true"`	`HTTP_PORT` maps to `HTTPPort`

Go Notes

// Prefer unmarshaling into a struct once viper keys stabilize
var cfg AppConfig
if err := viper.Unmarshal(&cfg); err != nil {
    log.Fatal(err)
}

Mapstructure tags on struct fields align viper keys with nested YAML.
Never log full DSNs or API keys; print redacted snapshots at info level only.

Gotchas

Global viper in libraries - packages calling viper.GetString hide dependencies. Fix: load in main, pass AppConfig structs.
Key naming drift - http.port vs HTTP_PORT breaks silently without SetEnvKeyReplacer. Fix: document env names in Helm and README tables.
Missing required secrets - empty string passes GetString checks. Fix: validate with explicit if cfg.DSN == "" or envconfig required.
Hot reload without readiness - viper WatchConfig reloads mid-request. Fix: restart pods or gate reload behind atomic pointer swap plus drain.
Secrets in ConfigMaps - YAML files committed to git leak credentials. Fix: mount Secrets as env vars; keep files for non-secret defaults only.
Flag parsing order - reading viper before flag.Parse misses CLI overrides. Fix: parse flags first, then bind with viper.BindPFlags.

Alternatives

Alternative	Use When	Don't Use When
`flag` + `os.Getenv` only	Two env vars and one port flag	Dozens of keys across files
`caarlos0/env`	Struct tags without envconfig's age	You already standardized on envconfig
`koanf`	Explicit merge layers without viper globals	Team knows viper and wants one tool
Helm values only	Config never local	Developers need offline `go run`

FAQs

viper or envconfig for a new microservice?

Start with envconfig when all config comes from Kubernetes env and Secrets.

Add viper when you need YAML defaults for local dev and file-based feature flags.

How do I test configuration loading?

Set env vars in t.Setenv, write temp YAML files, or build AppConfig literals in tests without touching global viper when possible.

Should CLIs use the same stack as services?

Often yes for shared AppConfig structs; CLIs may add pflag bindings where services rely on env only.

How does this relate to the observability config article?

That page covers flags and env patterns broadly; this page compares viper and envconfig libraries specifically.

Can I mix viper and envconfig?

Yes - load file defaults with viper, then overlay with envconfig for secrets - but prefer one loader to avoid precedence confusion.

What about validation beyond required?

Unmarshal into structs and run go-playground/validator or hand-rolled checks on ports, URLs, and durations.

How do I document env vars for operators?

Generate a table from struct tags in README, duplicate in Helm values.yaml comments, and fail startup with actionable error messages.

Is viper too heavy for small tools?

For single-binary CLIs with three flags, stdlib flag is simpler and avoids a dependency.

How do feature flags fit?

Store flag keys in viper files or a remote provider; keep defaults safe when the provider is unreachable.

Should config structs live in `internal/config`?

Yes - one package owns loading, validation, and redacted String() for logs.

Configuration Management: Env, Flags & Viper - 12-factor overview
Essential Libraries Basics - viper quick start
Input Validation & Safe HTML/JSON Handling - validate loaded values
Graceful Shutdown & Signal Handling - config-driven drain timeouts
Module & Dependency Rules - minimal dependency hygiene

Stack versions: This page was written for Go 1.26.x (Green Tea GC default, go fix modernizers - verify patch at build), chi (latest - verify at build), gin (latest - verify at build), echo (latest - verify at build), google.golang.org/grpc (latest - verify at build), sigs.k8s.io/controller-runtime (latest - verify at build), kubebuilder (latest - verify at build), tinygo (latest - verify board targets at build), wazero (latest - verify at build), and golangci-lint (latest - verify linter set at build).

spf13/viper merges files, flags, and environment variables with precedence rules; kelseyhightower/envconfig maps env vars directly into typed structs for smaller binaries.

Recipe

Quick-reference recipe card - copy-paste ready.

viper.SetConfigName("config")
viper.SetConfigType("yaml")
viper.AddConfigPath(".")
viper.AutomaticEnv()
viper.SetDefault("http.port", 8080)
_ = viper.ReadInConfig()
port := viper.GetInt("http.port")

type Config struct {
    Port int    `envconfig:"PORT" default:"8080"`
    DSN  string `envconfig:"DATABASE_URL" required:"true"`
}
var cfg Config
envconfig.Process("", &cfg)

When to reach for this:

Kubernetes-deployed services with ConfigMaps, Secrets, and env overrides
CLIs that read a config file locally but accept flags for automation
Teams documenting required env vars in README and Helm charts
Gradual migration from ad hoc os.Getenv scattered across packages

Working Example

package main
 
import (
    "fmt"
    "log"
    "strings"
 
    "github.com/spf13/viper"
)
 
type AppConfig struct {
    HTTPPort int
    LogLevel string
    DSN      string
}
 
func loadViper() AppConfig {
    viper.SetConfigName("config")
    viper.SetConfigType("yaml")
    viper.AddConfigPath(".")
    viper.SetEnvKeyReplacer(strings.NewReplacer(".", "_"))
    viper.AutomaticEnv()
 
    viper.SetDefault("http.port", 8080)
    viper.SetDefault("log.level", "info")
 
    if err := viper.ReadInConfig(); err != nil {
        log.Printf("no config file: %v", err)
    }
 
    return AppConfig{
        HTTPPort: viper.GetInt("http.port"),
        LogLevel: viper.GetString("log.level"),
        DSN:      viper.GetString("database.dsn"),
    }
}
 
func main() {
    cfg := loadViper()
    if cfg.DSN == "" {
        log.Fatal("database.dsn or DATABASE_DSN required")
    }
    fmt.Printf("port=%d level=%s\n", cfg.HTTPPort, cfg.LogLevel)
}

What this demonstrates:

YAML defaults with env override via AutomaticEnv and key replacer
Fail-fast validation for secrets in main
Typed AppConfig struct separate from viper's stringly API
Safe logging when config file is optional in dev

Deep Dive

How It Works

viper stores a key-value map populated from multiple sources; later reads use precedence (flag > env > config file > default).
envconfig reflects struct tags at startup and errors on missing required fields.
Both run once at process start; hot reload needs fsnotify (viper) or process restart (simplest ops model).
Keep config loading in main or a config package; domain packages receive typed structs, not global viper calls.

Precedence (viper)

Source	Typical use	Override strength
Explicit `Set` in code	Tests	Highest when used
Flags (`viper.BindPFlags`)	CLI overrides	High
Environment	K8s Secrets, 12-factor	High
Config file	Defaults per environment	Medium
`SetDefault`	Safe fallbacks	Low

envconfig Tags

Tag	Effect
`envconfig:"PORT"`	Env var name (with optional prefix)
`required:"true"`	Process fails if unset
`default:"8080"`	Value when env missing
`split_words:"true"`	`HTTP_PORT` maps to `HTTPPort`

Go Notes

// Prefer unmarshaling into a struct once viper keys stabilize
var cfg AppConfig
if err := viper.Unmarshal(&cfg); err != nil {
    log.Fatal(err)
}

Mapstructure tags on struct fields align viper keys with nested YAML.
Never log full DSNs or API keys; print redacted snapshots at info level only.

Gotchas

Global viper in libraries - packages calling viper.GetString hide dependencies. Fix: load in main, pass AppConfig structs.
Key naming drift - http.port vs HTTP_PORT breaks silently without SetEnvKeyReplacer. Fix: document env names in Helm and README tables.
Missing required secrets - empty string passes GetString checks. Fix: validate with explicit if cfg.DSN == "" or envconfig required.
Hot reload without readiness - viper WatchConfig reloads mid-request. Fix: restart pods or gate reload behind atomic pointer swap plus drain.
Secrets in ConfigMaps - YAML files committed to git leak credentials. Fix: mount Secrets as env vars; keep files for non-secret defaults only.
Flag parsing order - reading viper before flag.Parse misses CLI overrides. Fix: parse flags first, then bind with viper.BindPFlags.

Alternatives

Alternative	Use When	Don't Use When
`flag` + `os.Getenv` only	Two env vars and one port flag	Dozens of keys across files
`caarlos0/env`	Struct tags without envconfig's age	You already standardized on envconfig
`koanf`	Explicit merge layers without viper globals	Team knows viper and wants one tool
Helm values only	Config never local	Developers need offline `go run`

FAQs

viper or envconfig for a new microservice?

Start with envconfig when all config comes from Kubernetes env and Secrets.

Add viper when you need YAML defaults for local dev and file-based feature flags.

How do I test configuration loading?

Set env vars in t.Setenv, write temp YAML files, or build AppConfig literals in tests without touching global viper when possible.

Should CLIs use the same stack as services?

Often yes for shared AppConfig structs; CLIs may add pflag bindings where services rely on env only.

How does this relate to the observability config article?

That page covers flags and env patterns broadly; this page compares viper and envconfig libraries specifically.

Can I mix viper and envconfig?

Yes - load file defaults with viper, then overlay with envconfig for secrets - but prefer one loader to avoid precedence confusion.

What about validation beyond required?

Unmarshal into structs and run go-playground/validator or hand-rolled checks on ports, URLs, and durations.

How do I document env vars for operators?

Generate a table from struct tags in README, duplicate in Helm values.yaml comments, and fail startup with actionable error messages.

Is viper too heavy for small tools?

For single-binary CLIs with three flags, stdlib flag is simpler and avoids a dependency.

How do feature flags fit?

Store flag keys in viper files or a remote provider; keep defaults safe when the provider is unreachable.

Should config structs live in `internal/config`?

Yes - one package owns loading, validation, and redacted String() for logs.

Configuration Management: Env, Flags & Viper - 12-factor overview
Essential Libraries Basics - viper quick start
Input Validation & Safe HTML/JSON Handling - validate loaded values
Graceful Shutdown & Signal Handling - config-driven drain timeouts
Module & Dependency Rules - minimal dependency hygiene

Stack versions: This page was written for Go 1.26.x (Green Tea GC default, go fix modernizers - verify patch at build), chi (latest - verify at build), gin (latest - verify at build), echo (latest - verify at build), google.golang.org/grpc (latest - verify at build), sigs.k8s.io/controller-runtime (latest - verify at build), kubebuilder (latest - verify at build), tinygo (latest - verify board targets at build), wazero (latest - verify at build), and golangci-lint (latest - verify linter set at build).

Configuration with viper & envconfig

Search Documentation

Configuration with viper & envconfig