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	diff --git a/Cargo.toml b/Cargo.toml
	index a1e9b53..777f044 100644
	--- a/Cargo.toml
	+++ b/Cargo.toml
	@@ -1,27 +1,33 @@
	[package]
	name = "limiting-factor"
	version = "0.1.0"
	authors = [
	"Sébastien Santoro <dereckson@espace-win.org>",
	]
	description = "Library to create a REST API with Diesel and Rocket"
	readme = "README.md"
	keywords = [
	"Diesel",
	"API",
	"Rocket",
	"REST",
	]
	categories = [
	"web-programming",
	]
	license = "BSD-2-Clause"
	repository = "https://devcentral.nasqueron.org/source/limiting-factor/"

	[dependencies]
	-diesel = { version = "^1.0.0", features = ["postgres", "r2d2", "chrono"] }
	+diesel = { version = "^1.0.0", features = ["postgres", "r2d2", "chrono"], optional = true }
	dotenv = "0.9.0"
	log = "^0.4.4"
	-r2d2 = "^0.8.2"
	+r2d2 = { version = "^0.8.2", optional = true }
	rocket = "^0.3.16"
	rocket_contrib = { version = "^0.3.16", features = [ "json" ] }
	+
	+[features]
	+default = ["pgsql"]
	+minimal = []
	+
	+pgsql = ["diesel", "r2d2"]
	diff --git a/src/api/replies.rs b/src/api/replies.rs
	index 9f15880..189d0e3 100644
	--- a/src/api/replies.rs
	+++ b/src/api/replies.rs
	@@ -1,128 +1,133 @@
	//! # API standard and JSON responses.
	//!
	//! This module provides useful traits and methods to craft API replies from an existing type.

	-use std::error::Error;
	-
	-use diesel::result::DatabaseErrorInformation;
	-use diesel::result::DatabaseErrorKind;
	+#[cfg(feature = "pgsql")]
	+use diesel::result::{DatabaseErrorInformation, DatabaseErrorKind, QueryResult};
	+#[cfg(feature = "pgsql")]
	use diesel::result::Error as ResultError;
	-use diesel::result::QueryResult;
	+
	use rocket::http::Status;
	use rocket::response::Failure;
	use rocket_contrib::Json;

	+#[cfg(feature = "pgsql")]
	+use std::error::Error;
	+
	/* -------------------------------------------------------------
	Custom types
	- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

	pub type ApiJsonResponse<T> = Result<Json<T>, Failure>;

	/* -------------------------------------------------------------
	API Response

	:: Implementation for QueryResult (Diesel ORM)
	- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

	/// This trait allows to consume an object into an HTTP response.
	pub trait ApiResponse<T> {
	/// Consumes the value and creates a JSON or a Failure result response.
	fn into_json_response(self) -> ApiJsonResponse<T>;
	}

	+#[cfg(feature = "pgsql")]
	impl<T> ApiResponse<T> for QueryResult<T> {
	/// Prepares an API response from a query result.
	///
	/// The result is the data structure prepared by the Diesel ORM after a SELECT query
	/// with one result, for example using `first` method. You can also you use it to
	/// parse the returning result (... RETURNING *), which is a default for Diesel after
	/// an INSERT query.
	///
	/// So result can be:
	/// - Ok(T)
	/// - Err(any database error)
	///
	/// # Examples
	///
	/// To offer a /player/foo route to serve player information from the player table:
	///
	/// ```
	/// use limiting_factor::api::ApiResponse;
	/// use limiting_factor::api::ApiJsonResponse;
	///
	/// #[get("/player/<name>")]
	/// pub fn get_player(connection: DatabaseConnection, name: String) -> ApiJsonResponse<Player> {
	/// players
	/// .filter(username.eq(&name))
	/// .first::<Player>(&*connection)
	/// .into_json_response()
	/// }
	/// ```
	///
	/// This will produce a JSON representation when the result is found,
	/// a 404 error when no result is found, a 500 error if there is a database issue.
	fn into_json_response(self) -> ApiJsonResponse<T> {
	match self {
	// CASE I - The query returns one value, we return a JSON representation fo the item
	Ok(item) => Ok(Json(item)),

	Err(error) => match error {
	// Case II - The query returns no result, we return a 404 Not found response
	ResultError::NotFound => Err(Failure::from(Status::NotFound)),

	// Case III - We need to handle a database error, which could be a 400/409/500
	ResultError::DatabaseError(kind, details) => Err(build_database_error_response(kind, details)),

	// Case IV - The error is probably server responsbility, log it and throw a 500
	_ => Err(error.into_failure_response()),
	}
	}
	}
	}

	/* -------------------------------------------------------------
	Failure response

	:: Implementation for diesel::result::Error
	- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

	/// This trait allows to consume an object into an HTTP failure response.
	pub trait FailureResponse {
	/// Consumes the variable and creates a Failure response .
	fn into_failure_response(self) -> Failure;
	}

	+#[cfg(feature = "pgsql")]
	impl FailureResponse for ResultError {
	/// Consumes the error and creates a Failure 500 Internal server error response.
	fn into_failure_response(self) -> Failure {
	build_internal_server_error_response(self.description())
	}
	}

	/* -------------------------------------------------------------
	Helper methods to prepare API responses
	- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

	pub fn build_internal_server_error_response(message: &str) -> Failure {
	warn!(target:"api", "{}", message);

	Failure::from(Status::InternalServerError)
	}

	+#[cfg(feature = "pgsql")]
	fn build_database_error_response(error_kind: DatabaseErrorKind, info: Box<dyn DatabaseErrorInformation>) -> Failure {
	match error_kind {
	// Case IIIa - The query tries to do an INSERT violating an unique constraint
	// e.g. two INSERT with the same unique value
	// We return a 409 Conflict
	DatabaseErrorKind::UniqueViolation => Failure::from(Status::Conflict),

	// Case IIIb - The query violated a foreign key constraint
	// e.g. an INSERT referring to a non existing user 1004
	// when there is no id 1004 in users table
	// We return a 400 Bad request
	DatabaseErrorKind::ForeignKeyViolation => Failure::from(Status::BadRequest),

	// Case IIIc - For other databases errors, the client responsibility isn't involved.
	_ => build_internal_server_error_response(info.message()),
	}
	}
	diff --git a/src/config.rs b/src/config.rs
	index 1c4fb58..44a084c 100644
	--- a/src/config.rs
	+++ b/src/config.rs
	@@ -1,98 +1,187 @@
	//! # Service configuration.
	//!
	//! This module allows to configure the service.
	//!
	//! It provides a Config trait to build custom configuration implementation.
	//!
	//! It also provides a `DefaultConfig` implementation of this `Config` trait to
	//! extract variables from an .env file or environment.

	use dotenv::dotenv;
	+#[cfg(feature = "pgsql")]
	+use kernel::DefaultService;
	+use kernel::{MinimalService, Service};
	+use rocket::Route;
	use std::env;
	use std::error::Error;
	use ErrorResult;

	/* -------------------------------------------------------------
	Config trait
	- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

	/// This trait allows to provide a configuration for the resources needed by the API.
	pub trait Config {
	fn get_database_url(&self) -> &str;
	fn get_entry_point(&self) -> &str;
	fn get_database_pool_size(&self) -> u32;
	+ fn with_database(&self) -> bool;
	+ fn into_service(self, routes: Vec<Route>) -> Box<dyn Service>;
	+}
	+
	+/* -------------------------------------------------------------
	+ EnvironmentConfigurable
	+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
	+
	+/// This trait allows to configure the object from the environment
	+pub trait EnvironmentConfigurable {
	+ fn parse_environment() -> ErrorResult<Self> where Self: Sized;
	}

	/* -------------------------------------------------------------
	DefaultConfig

	:: Config
	:: sui generis implementation
	- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

	/// This is a default implementation of the `Config` trait, which extracts the following variables
	/// from an .env file or environment:
	///
	/// - `API_ENTRY_POINT` (facultative, by default `/`): the mouting point of the API methods
	/// - `DATABASE_URL` (mandatory): the URL to connect to your database
	/// - `DATABASE_POOL_SIZE` (facultative, by default 4): the number of connections to open
	+#[cfg(feature = "pgsql")]
	pub struct DefaultConfig {
	database_url: String,
	entry_point: String,
	database_pool_size: u32,
	+ with_database: bool,
	+}
	+
	+#[cfg(feature = "pgsql")]
	+impl DefaultConfig {
	+ const DEFAULT_DATABASE_POOL_SIZE: u32 = 4;
	}

	+#[cfg(feature = "pgsql")]
	impl Config for DefaultConfig {
	- fn get_database_url(&self) -> &str {
	- &self.database_url
	- }
	+ fn get_database_url(&self) -> &str { &self.database_url }

	- fn get_entry_point(&self) -> &str {
	- &self.entry_point
	- }
	+ fn get_entry_point(&self) -> &str { &self.entry_point }

	- fn get_database_pool_size(&self) -> u32 {
	- self.database_pool_size
	+ fn get_database_pool_size(&self) -> u32 { self.database_pool_size }
	+
	+ fn with_database(&self) -> bool { self.with_database }
	+
	+ fn into_service(self, routes: Vec<Route>) -> Box<dyn Service> {
	+ let service = DefaultService {
	+ config: self,
	+ routes: Box::new(routes),
	+ };
	+
	+ Box::new(service)
	}
	}

	-impl DefaultConfig {
	- pub const DEFAULT_DATABASE_POOL_SIZE: u32 = 4;
	-
	- pub fn parse_environment() -> ErrorResult<Self> {
	+#[cfg(feature = "pgsql")]
	+impl EnvironmentConfigurable for DefaultConfig {
	+ fn parse_environment() -> ErrorResult<Self> {
	if let Err(error) = dotenv() {
	warn!(target: "config", "Can't parse .env: {}", error.description());
	};

	+ let with_database = env::var("LF_DISABLE_DATABASE").is_err();
	+
	let database_url = match env::var("DATABASE_URL") {
	Ok(url) => url,
	Err(e) => {
	- error!(target: "config", "You need to specify a DATABASE_URL variable in the environment (or .env file).");
	- return Err(Box::new(e));
	+ if with_database {
	+ error!(target: "config", "You need to specify a DATABASE_URL variable in the environment (or .env file).");
	+ return Err(Box::new(e));
	+ }
	+
	+ String::new()
	}
	};

	let entry_point = env::var("API_ENTRY_POINT").unwrap_or(String::from("/"));

	let database_pool_size = match env::var("DATABASE_POOL_SIZE") {
	Ok(variable) => {
	match variable.parse::<u32>() {
	Ok(size) => size,
	Err(_) => {
	warn!(target: "config", "The DATABASE_POOL_SIZE variable must be an unsigned integer.");

	DefaultConfig::DEFAULT_DATABASE_POOL_SIZE
	},
	}
	},
	Err(_) => DefaultConfig::DEFAULT_DATABASE_POOL_SIZE,
	};

	Ok(DefaultConfig {
	database_url,
	entry_point,
	database_pool_size,
	+ with_database,
	})
	}
	}

	+/* -------------------------------------------------------------
	+ MinimalConfig
	+
	+ :: Config
	+ :: sui generis implementation
	+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
	+
	+/// This is a minimal implementation of the `Config` trait, which extracts the following variables
	+/// from an .env file or environment:
	+///
	+/// - `API_ENTRY_POINT` (facultative, by default `/`): the mouting point of the API methods
	+///
	+/// It sets the server not to use a database.
	+pub struct MinimalConfig {
	+ entry_point: String,
	+}
	+
	+impl Config for MinimalConfig {
	+ fn get_database_url(&self) -> &str {
	+ ""
	+ }
	+
	+ fn get_entry_point(&self) -> &str {
	+ &self.entry_point
	+ }
	+
	+ fn get_database_pool_size(&self) -> u32 {
	+ 0
	+ }
	+
	+ fn with_database(&self) -> bool { false }
	+
	+ fn into_service(self, routes: Vec<Route>) -> Box<dyn Service> {
	+ let service = MinimalService {
	+ config: self,
	+ routes: Box::new(routes),
	+ };
	+
	+ Box::new(service)
	+ }
	+}
	+
	+impl EnvironmentConfigurable for MinimalConfig {
	+ fn parse_environment() -> ErrorResult<Self> {
	+ if let Err(error) = dotenv() {
	+ warn!(target: "config", "Can't parse .env: {}", error.description());
	+ };
	+
	+ let entry_point = env::var("API_ENTRY_POINT").unwrap_or(String::from("/"));
	+
	+ Ok(MinimalConfig {
	+ entry_point,
	+ })
	+ }
	+}
	diff --git a/src/kernel.rs b/src/kernel.rs
	index 46382fa..2f49cef 100644
	--- a/src/kernel.rs
	+++ b/src/kernel.rs
	@@ -1,135 +1,240 @@
	//! # Service execution utilities.
	//!
	//! Provides methods to start the server and handle the application

	-use config::Config;
	+use config::{Config, MinimalConfig};
	+#[cfg(feature = "pgsql")]
	use config::DefaultConfig;
	-use database::initialize_database_pool;
	-use database::test_database_connection;
	+#[cfg(feature = "pgsql")]
	+use database::{initialize_database_pool, test_database_connection};
	use ErrorResult;
	use rocket::Route;
	use rocket::ignite;
	use std::process;
	+use std::marker::PhantomData;
	+use config::EnvironmentConfigurable;

	/* -------------------------------------------------------------
	- Application
	+ Service

	Allow to define config and routes. Launch a server.
	- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

	-pub trait Application {
	+pub trait Service {
	fn get_config(&self) -> &dyn Config;

	fn get_routes(&self) -> &[Route];

	+ fn launch_server(&mut self) -> ErrorResult<()>;
	+
	+ fn check_service_configuration(&self) -> ErrorResult<()>;
	+
	+ fn run (&mut self) -> ErrorResult<()> {
	+ info!(target: "runner", "Server started.");
	+
	+ {
	+ self.check_service_configuration()?
	+ }
	+
	+ self.launch_server()?;
	+
	+ Ok(())
	+ }
	+}
	+
	+/* -------------------------------------------------------------
	+ Default service
	+
	+ Allow to define config and routes. Launch a server.
	+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
	+
	+/// The default service offers a pgsql database connection with Diesel and r2d2.
	+#[cfg(feature = "pgsql")]
	+pub struct DefaultService {
	+ pub config: DefaultConfig,
	+ pub routes: Box<Vec<Route>>,
	+}
	+
	+#[cfg(feature = "pgsql")]
	+impl Service for DefaultService {
	+ fn get_config(&self) -> &dyn Config { &self.config }
	+
	+ fn get_routes(&self) -> &[Route] { self.routes.as_slice() }
	+
	fn launch_server(&mut self) -> ErrorResult<()> {
	let config = self.get_config();
	let routes = self.get_routes();

	- ignite()
	- .manage(
	+ let mut server = ignite();
	+
	+ if config.with_database() {
	+ server = server.manage(
	initialize_database_pool(config.get_database_url(), config.get_database_pool_size())?
	- )
	+ );
	+ }
	+
	+ server
	.mount(config.get_entry_point(), routes.to_vec())
	.launch();

	Ok(())
	}

	- fn run (&mut self) -> ErrorResult<()> {
	- info!(target: "runner", "Server started.");
	-
	- // Initial connection to test if the database configuration works
	- {
	- let config = self.get_config();
	+ fn check_service_configuration(&self) -> ErrorResult<()> {
	+ let config = self.get_config();
	+ if config.with_database() {
	test_database_connection(config.get_database_url())?;
	info!(target: "runner", "Connection to database established.");
	}

	- self.launch_server()?;
	+ Ok(())
	+ }
	+}
	+
	+/* -------------------------------------------------------------
	+ Minimal service
	+
	+ Allow to define config and routes. Launch a server.
	+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
	+
	+/// The minimal service allows to spawn a server without any extra feature.
	+pub struct MinimalService {
	+ pub config: MinimalConfig,
	+ pub routes: Box<Vec<Route>>,
	+}
	+
	+impl Service for MinimalService {
	+ fn get_config(&self) -> &dyn Config { &self.config }
	+
	+ fn get_routes(&self) -> &[Route] { self.routes.as_slice() }
	+
	+ fn launch_server(&mut self) -> ErrorResult<()> {
	+ let config = self.get_config();
	+ let routes = self.get_routes();
	+
	+ ignite()
	+ .mount(config.get_entry_point(), routes.to_vec())
	+ .launch();

	Ok(())
	}
	+
	+ fn check_service_configuration(&self) -> ErrorResult<()> { Ok(()) }
	}

	/* -------------------------------------------------------------
	- Default application
	+ Base application as concrete implementation

	:: Application
	:: sui generis implementation
	- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

	+/// The application structure allows to encapsulate the service into a CLI application.
	+///
	+/// The application takes care to run the service and quits with a correct exit code.
	+///
	+/// It also takes care of initialisation logic like parse the environment to extract
	+/// the configuration.
	+pub struct Application<U>
	+ where U: Config
	+{
	+ service: Box<dyn Service>,
	+ config_type: PhantomData<U>,
	+}
	+
	+impl<U> Application<U>
	+ where U: Config + EnvironmentConfigurable
	+{
	+ pub fn new (config: U, routes: Vec<Route>) -> Self {
	+ Application {
	+ service: config.into_service(routes),
	+ config_type: PhantomData,
	+ }
	+ }
	+
	+ /// Starts the application
	+ ///
	+ /// # Exit codes
	+ ///
	+ /// The software will exit with the following error codes:
	+ ///
	+ /// - 0: Graceful exit (currently not in use, as the application never stops)
	+ /// - 1: Error during the application run (e.g. routes conflict or Rocket fairings issues)
	+ /// - 2: Error parsing the configuration (e.g. no database URL has been defined)
	+ pub fn start (&mut self) {
	+ info!(target: "runner", "Server initialized.");
	+
	+ if let Err(error) = self.service.run() {
	+ error!(target: "runner", "{}", error.description());
	+ process::exit(1);
	+ }
	+
	+ process::exit(0);
	+ }
	+
	+ pub fn start_application (routes: Vec<Route>) {
	+ let config = <U>::parse_environment().unwrap_or_else(\|_error\| {
	+ process::exit(2);
	+ });
	+
	+ let mut app = Application::new(config, routes);
	+ app.start();
	+ }
	+}
	+
	+/* -------------------------------------------------------------
	+ Default application
	+
	+ :: Application
	+ :: sui generis implementation, wrapper for Application
	+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
	+
	/// The default application implements CLI program behavior to prepare a configuration from the
	/// `DefaultConfig` implementation, test if it's possible to connect to the database, and if so,
	/// launch a Rocket server.
	///
	/// # Examples
	///
	/// To run an application with some routes in a `requests` module:
	///
	/// ```
	/// use limiting_factor::kernel::DefaultApplication;
	/// use requests::*;
	///
	/// pub fn main () {
	/// let routes = routes![
	/// status,
	/// favicon,
	/// users::register,
	/// users::get_player,
	/// ];
	///
	/// DefaultApplication::start_application(routes);
	/// }
	/// ```
	///
	/// The default configuration will be used and the server started.
	-pub struct DefaultApplication {
	- config: DefaultConfig,
	- routes: Box<Vec<Route>>,
	-}
	-
	-impl Application for DefaultApplication {
	- fn get_config(&self) -> &dyn Config {
	- &self.config
	- }
	-
	- fn get_routes(&self) -> &[Route] {
	- self.routes.as_slice()
	- }
	-}
	+#[cfg(feature = "pgsql")]
	+pub struct DefaultApplication {}

	+#[cfg(feature = "pgsql")]
	impl DefaultApplication {
	- pub fn new (config: DefaultConfig, routes: Vec<Route>) -> Self {
	- DefaultApplication {
	- config,
	- routes: Box::new(routes),
	- }
	- }
	-
	- /// Starts the application, prepares default configuration
	- ///
	- /// # Exit codes
	- ///
	- /// The software will exit with the following error codes:
	- ///
	- /// - 0: Graceful exit (currently not in use, as the application never stops)
	- /// - 1: Error during the application run (e.g. routes conflict or Rocket fairings issues)
	- /// - 2: Error parsing the configuration (e.g. no database URL has been defined)
	pub fn start_application (routes: Vec<Route>) {
	- info!(target: "runner", "Server initialized.");
	+ Application::<DefaultConfig>::start_application(routes);
	+ }
	+}

	- let config = DefaultConfig::parse_environment().unwrap_or_else(\|_error\| {
	- process::exit(2);
	- });
	+/* -------------------------------------------------------------
	+ Minimal application

	- let mut app = Self::new(config, routes);
	+ :: Application
	+ :: sui generis implementation, wrapper for Application
	+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

	- if let Err(error) = app.run() {
	- error!(target: "runner", "{}", error.description());
	- process::exit(1);
	- }
	+pub struct MinimalApplication {}

	- process::exit(0);
	+impl MinimalApplication {
	+ pub fn start_application (routes: Vec<Route>) {
	+ Application::<MinimalConfig>::start_application(routes);
	}
	}
	diff --git a/src/lib.rs b/src/lib.rs
	index 12c774c..9a5d291 100644
	--- a/src/lib.rs
	+++ b/src/lib.rs
	@@ -1,52 +1,64 @@
	//! A library with components to implement a REST API.
	//!
	//! The goal of this crate is to provide:
	//!
	//! - boilerplate to parse environment and run a Rocket server
	//! - glue code for Rocket and Diesel to use a database in the web service
	//! - standard API replies
	//!
	//! That allows an API or a back-end web server to focus on requests and data model.
	//!
	//! # Examples
	//!
	//! A simple server serving a 200 ALIVE response on /status :
	//!
	//! ```no_run
	//! use limiting_factor::kernel::DefaultApplication;
	//!
	//! pub fn run () {
	//! let routes = routes![
	//! status,
	//! ];
	//!
	//! DefaultApplication::start_application(routes);
	//! }
	//!
	//! #[get("/status")]
	//! pub fn status() -> &'static str {
	//! "ALIVE"
	//! }
	//! ```
	+//!
	+//! Replacing `DefaultApplication` by `MinimalApplication` allows to use a lighter version
	+//! of the library without Diesel dependencies or database use.

	+#[cfg(feature = "pgsql")]
	extern crate diesel;
	extern crate dotenv;
	-#[macro_use] extern crate log;
	+#[macro_use]
	+extern crate log;
	+#[cfg(feature = "pgsql")]
	extern crate r2d2;
	extern crate rocket;
	extern crate rocket_contrib;

	/* -------------------------------------------------------------
	Public modules offered by this crate
	- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

	pub mod api;
	pub mod config;
	-pub mod database;
	pub mod kernel;

	+/* -------------------------------------------------------------
	+ Optional public features modules offered by this crate
	+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
	+
	+#[cfg(feature = "pgsql")]
	+pub mod database;
	+
	/* -------------------------------------------------------------
	Custom types
	- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

	pub type ErrorResult<T> = Result<T, Box<dyn std::error::Error>>;

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