# Use Viper for configuration management

* Status: Accepted
* Deciders: Gabriel Radureau, AI Agent
* Date: 2026-04-03

## Context and Problem Statement

We needed a configuration management solution for dance-lessons-coach that provides:
- Support for multiple configuration sources (files, environment variables, defaults)
- Configuration validation
- Type-safe configuration loading
- Hot reloading capabilities
- Good error handling and reporting

## Decision Drivers

* Need for flexible configuration from multiple sources
* Desire for configuration validation
* Requirement for type-safe access to configuration
* Need for environment-specific configurations
* Desire for good error messages

## Considered Options

* Viper - Popular configuration library with many features
* Koanf - Lightweight but powerful
* envconfig - Simple environment variable loading
* Custom solution - Build our own configuration loader

## Decision Outcome

Chosen option: "Viper" because it provides comprehensive configuration management with support for multiple sources, good validation capabilities, type-safe loading, and is widely used in the Go ecosystem.

## Pros and Cons of the Options

### Viper

* Good, because supports multiple configuration sources
* Good, because good validation capabilities
* Good, because type-safe configuration loading
* Good, because widely used and well-documented
* Good, because supports hot reloading
* Bad, because slightly heavier than alternatives
* Bad, because more complex API

### Koanf

* Good, because lightweight
* Good, because good performance
* Good, because simple API
* Bad, because less feature-rich than Viper
* Bad, because smaller community

### envconfig

* Good, because very simple
* Good, because good for environment variables
* Bad, because limited to environment variables
* Bad, because no file support

### Custom solution

* Good, because tailored to our needs
* Good, because no external dependencies
* Bad, because time-consuming to develop
* Bad, because need to maintain ourselves
* Bad, because likely less feature-rich

## Implementation Example

```go
// Configuration structure
type Config struct {
    Server    ServerConfig    `mapstructure:"server"`
    Shutdown  ShutdownConfig  `mapstructure:"shutdown"`
    Logging   LoggingConfig   `mapstructure:"logging"`
}

// Loading configuration
func LoadConfig() (*Config, error) {
    v := viper.New()
    
    // Set defaults
    v.SetDefault("server.host", "0.0.0.0")
    v.SetDefault("server.port", 8080)
    
    // Read config file
    v.SetConfigName("config")
    v.SetConfigType("yaml")
    v.AddConfigPath(".")
    
    if err := v.ReadInConfig(); err != nil {
        if _, ok := err.(viper.ConfigFileNotFoundError); !ok {
            return nil, err
       }
    }
    
    // Bind environment variables
    v.AutomaticEnv()
    v.SetEnvPrefix("DLC")
    
    // Unmarshal into struct
    var config Config
    if err := v.Unmarshal(&config); err != nil {
        return nil, err
    }
    
    return &config, nil
}
```

## Configuration Priority

The implementation follows this priority order:
1. **Config file** (highest priority)
2. **Environment variables** (override defaults)
3. **Default values** (lowest priority)

## Links

* [Viper GitHub](https://github.com/spf13/viper)
* [Viper Documentation](https://github.com/spf13/viper#readme)
* [Koanf GitHub](https://github.com/knadh/koanf)
* [envconfig GitHub](https://github.com/kelseyhightower/envconfig)

## Configuration File Example

```yaml
# config.yaml
server:
  host: "0.0.0.0"
  port: 8080

shutdown:
  timeout: 30s

logging:
  json: false
  level: "trace"
```

## Environment Variables

```bash
# Set configuration via environment variables
export DLC_SERVER_HOST="0.0.0.0"
export DLC_SERVER_PORT=8080
export DLC_SHUTDOWN_TIMEOUT=30s
export DLC_LOGGING_JSON=false
```