A Rust library for processing application configuration easily

Configure me

A Rust library for processing application configuration easily

About

This crate aims to help with reading configuration of application from files, environment variables and command line arguments, merging it together and validating. It auto-generates most of the parsing and deserializing code for you based on specification file. It creates a struct for you, which you can use to read configuration into. It will contain all the parsed and validated fields, so you can access the information quickly easily and idiomatically.

The generated code is formatted to be easy to read and understand.

Wait a second, why this crate doesn't use derive?

I'd love to use derive. Unfortunately it doesn't compose well with man page generation and other tooling.

For a longer version, see docs/why_not_derive.md

Example

Let's say, your application needs these parametrs to run:

• Port - this is mandatory
• IP address to bind to - defaults to 0.0.0.0
• Path to TLS certificate - optional, the server will be unsecure if not given

First create config_spec.toml configuration file specifying all the parameters:

[[param]]
name = "port"
type = "u16"
optional = false
# This text will be used in the documentation (help etc)
# It's not mandatory, but your progam will be ugly without it.
doc = "Port to listen on."

[[param]]
name = "bind_addr"
type = "::std::net::Ipv4Addr" # Yes, this works and  you can use your own types implementing Deserialize and ParseArg as well!
default = "::std::net::Ipv4Addr::new(0, 0, 0, 0)" # Rust expression that creates the value
doc = "IP address to bind to."

[[param]]
name = "tls_cert"
type = "String"
doc = "Path to the TLS certificate. The connections will be unsecure if it isn't provided."
# optional = true is the default, no need to add it here

Then, create a simple build.rs script like:

extern crate configure_me_codegen;

fn main() -> Result<(), configure_me_codegen::Error> {
    configure_me_codegen::build_script_auto()
}

Tip: use cfg_me to generate a man page for your program.

Add dependencies to Cargo.toml:

[package]
# ...
build = "build.rs"

# This tells auto build script and other tools where to look for your specificcation
[package.metadata.configure_me]
spec = "config_spec.toml"

[dependencies]
configure_me = "0.3.3"

[build-dependencies]
configure_me_codegen = "0.3.11"

And finally add appropriate incantiations into src/main.rs:

#[macro_use]
extern crate configure_me;

include_config!();

fn main() {
    // Don't worry, unwrap_or_exit() prints a nice message instead of ugly panic
    let (server_config, _remaining_args) = Config::including_optional_config_files(&["/etc/my_awesome_server/server.conf"]).unwrap_or_exit();

    // Your code here
    // E.g.:
    let listener = std::net::TcpListener::bind((server_config.bind_addr, server_config.port)).expect("Failed to bind socket");
}

If you need to generate different files for multiple binaries, create a separate file for each binary and then define them separately in Cargo.toml:

# config for binary foo
[package.metadata.configure_me.bin]
foo = "foo_config_spec.toml"

# config for binary bar
[package.metadata.configure_me.bin]
bar = "bar_config_spec.toml"

And include the file in foo like this:

include_config!("foo");

This needs to be specific because there's no way to detect binary name.

Manual page generation

The crate exports an interface for generating manual pages, but I recommend you to not worry about it. There's a tool for generating extra files (currently only man page) from your specification. You can install it using cargo.

After installing it, you can type cfg_me man to see the generated man page. Run cfg_me -o program_name.1 man to save it to a file.

Debconf generation

This crate also contains experimental debconf support behind debconf feature. It generates templates, configure and postinst files for you. If you plan to package your application, you can use it. Note that this isn't integrated with cargo-deb yet.

In order to use this feature, you must enable the flag in Cargo.toml:

configure_me_codegen = { version = "0.3.11", features = ["debconf"] }

Then add debconf options to your configuration specification:

[debconf]
# Sets the name of the package
# Enables debconf support
package_name = "my-awesome-app"

[[param]]
name = "port"
type = "u16"
optional = false
# Documentation IS mandatory for debconf!
doc = "Port to listen on."
# Priority used for debconf questions
# "high" is recommended for non-default, mandatory questions
# In case of missing priority, the option is skipped!
debconf_priority = "high"

[[param]]
name = "bind_addr"
type = "::std::net::Ipv4Addr"
default = "::std::net::Ipv4Addr::new(0, 0, 0, 0)" # Rust expression that creates the value
doc = "IP address to bind to."
debconf_priority = "low"
# The default set by debconf. While it might seem redundant, this way the user sees the
# default value when editing.
debconf_default = "0.0.0.0"

[[param]]
name = "tls_cert"
type = "String"
doc = "Path to the TLS certificate. The connections will be unsecure if it isn't provided."
debconf_priority = "medium"

Finally build your application with DEBCONF_OUT environment variable set to existing directory where configure_me should generate the files.

Planned features

This crate is unfinished and there are features I definitelly want:

• Support for documenting your configuration very well - done
• Support environment variables - done
• Generate bash completion
• Some advanced features

Comparison with clap

clap is a great crate that works well. Unfortunately, it doesn't support reading from config files. It also has stringly-typed API, which adds boilerplate and (arguably small, but non-zero) runtime overhead.

On the other hand, it's much more mature and supports some features, this crate doesn't (bash completion and native subcommands).

clap may be more suitable for programs that should be easy to work with from command line, configure_me may be better for long-running processes with a lot of configuration options.

License

MITNFA