Rust Application Configuration: config and dotenv |…

Rust Application Configuration: `config` and `dotenv` Libraries

Any modern application requires a flexible configuration system: database settings, API keys, server ports, logging levels, and other parameters. In the Rust ecosystem, there are powerful tools for this — the config and dotenv crates. They allow you to combine settings from different sources (files, environment variables, command-line arguments) into a single type-safe structure.

Why a separate library?

Rust is a language with strong static typing. Simply reading strings from std::env::var() leads to numerous manual checks and conversions. The config library automates this process:

Format support: YAML, JSON, TOML, HJSON, INI, RON
Hierarchical configuration structure with override capability
Automatic deserialization into custom structures via serde
Merging multiple sources with priorities (file < env < CLI)

Installation

Add to Cargo.toml:

[dependencies]config = "0.14"serde = { version = "1.0", features = ["derive"] }dotenv = "0.15"

Basic example: reading from file and env

Let's create a structure for the application configuration:

use serde::Deserialize;use config::Config;

#[derive(Debug, Deserialize)]struct AppConfig {    server: ServerConfig,    database: DatabaseConfig,}

#[derive(Debug, Deserialize)]struct ServerConfig {    host: String,    port: u16,    workers: u8,}

#[derive(Debug, Deserialize)]struct DatabaseConfig {    url: String,    pool_size: u32,}

fn main() -> Result<(), config::ConfigError> {    // Load configuration    let config = Config::builder()        .add_source(config::File::with_name("config/default"))        .add_source(config::Environment::with_prefix("APP"))        .build()?;

    let app_config: AppConfig = config.try_deserialize()?;    println!("{:#?}", app_config);    Ok(())}

File config/default.toml:

[server]host = "127.0.0.1"port = 8080workers = 4

[database]url = "postgres://localhost/mydb"pool_size = 10

Environment variables with the APP_ prefix override values from the file. For example, APP_DATABASE__URL will change the database URL.

Working with `dotenv`

For development convenience, a .env file is often used. The dotenv crate loads it into environment variables before initializing config:

use dotenv::dotenv;

fn main() {    dotenv().ok(); // load .env    // then initialize config as in the example above}

Contents of .env:

APP_SERVER__PORT=3000APP_DATABASE__POOL_SIZE=20

Advanced scenario: multiple sources

Real projects require combining settings from different places. config allows you to specify a chain of sources with priority:

let config = Config::builder()    // Base settings (low priority)    .add_source(config::File::with_name("config/default"))    // Override for a specific environment    .add_source(config::File::with_name(&format!("config/{}", env::var("APP_ENV").unwrap_or("development"))))    // Environment variables (medium priority)    .add_source(config::Environment::with_prefix("APP"))    // Command-line arguments (high priority)    .add_source(config::File::from(std::env::args().nth(1).unwrap_or_default()))    .build()?;