Chained Configuration Provider

These features are only available if the chained feature is activated

Although it is not a very common usage scenario, you may encounter a scenario where you need to chain multiple configurations from different sources into a unified configuration. A practical example would be distinct configurations defined by different crates.

Let’s assume that crate1 defines its default configuration as:

use config::{Configuration, Result, prelude::*};

fn default_config() -> Result<Configuration> {
    config::builder()
        .add_in_memory(&[("Mem1:KeyInMem1", "ValueInMem1")])
        .add_in_memory(&[("Mem2:KeyInMem2", "ValueInMem2")])
        .build()
}

Let’s assume that crate2 defines its default configuration as:

use config::{Configuration, Result, prelude::*};

fn default_config() -> Result<Configuration> {
    config::builder()
        .add_in_memory(&[("Mem3:KeyInMem3", "ValueInMem3")])
        .build()
}

An application can now compose the crate1 and crate2 configurations into its own configuration.

use config::prelude::*;
use crate1;
use crate2;
use std::error::Error;

fn main() -> Result<(), Box<dyn Error + 'static>> {
    let config = config::builder()
        .add_configuration(crate1::default_config()?)
        .add_configuration(crate2::default_config()?)
        .add_env_vars()
        .add_command_line()
        .build()?;

    println!("Mem1:KeyInMem1 = {}", config.get("mem1:keyinmem1").unwrap());
    println!("Mem2:KeyInMem2 = {}", config.get("Mem2:KeyInMem2").unwrap());
    println!("Mem3:KeyInMem3 = {}", config.get("MEM3:KEYINMEM3").unwrap());

    Ok(())
}

This configuration would output:

Mem1:KeyInMem1 = ValueInMem1
Mem2:KeyInMem2 = ValueInMem2
Mem3:KeyInMem3 = ValueInMem3