#merge #derive #configuration


Layered configuration, can be expressed in a single line of code. The Converge trait merges instances of structures into a new instance of a structure. The Converge trait can be derived, and when needed with field attributes for nested structures, or a custom strategy.

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#69 in Configuration

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Used in converge_test


123 lines


Layered configuration system for Rust applications using little code. By only converging configuration, flexibility of the configuration source and how it converges is under control of the user.


Applications which have layered configuration of command line arguments, environment and configuration file, are what users expect. This library allows these sources to be layered with minimal effort and code.


Both The Unix philosophy and the idea of microservices lead to writing lots of applications. It is reasonable to expect these applications will take configuration from the command line arguments, environment and often a configuration file. As the applications mature the number of configuration options tends to grow. A naive approach to managing this is repetitive and error prone, and adding testing to this increases the repetition.


This library leaves the user free to decide configuration file format, command line parsing method, and has minimal dependencies. It works with structures holding the configuration items as fields. These structures must implement a trait with a single method that is commonly derivable for most structures.

This method works with immutable data structures, is very low in code, but with the cost of cloning the configuration during the layering process. This is a reasonable compromise as it usually happens only once in an applications lifecycle.


It is based on a trait Converge with a single method converge this is in the crate converge.

pub trait Converge<Rhs = Self> {
    fn converge(self, default: Rhs) -> Self;

This trait can be derived using converge_derive as shown below.

pub struct Config {
    pub config_file: Option<String>,
    pub loglevel: Option<i8>,
    pub xunit_local_globs: Option<Vec<String>>,
    pub environment_sk: Option<String>,
    pub environment_keys: Option<Vec<String>>,
    pub project_sk: Option<String>,
    pub project_identifier: Option<String>,
    pub project_human_name: Option<String>,
    pub run_identifier: Option<String>,
    pub run_sk: Option<String>,
    pub service_url: Option<String>,

These structures populated from can be nested and can then be coupled together to derive new instances with clear and simple prescience.

let config_commandline : Config = parse_commandline_to_config();
let config_file : Config = parse_file_to_config();
let config_env : Config = parse_env_to_config();
let cfg = config_commandline.converge(config_file).converge(config_env);

It is possible to have T typed fields that are not Optional values, but this not usual as then converge just with take the left hand side value.

Designing your configuration structure

The structure implementing the trait Converge should contain a logical grouping of fields to represent each setting you may wish to use as part of your layered configuration.

Example deriving nested Converge structures

pub struct ConfigRabbitMqCredentials {
    pub username: Option<String>,
    pub password: Option<String>,

pub struct ConfigRabbitMQ {
    pub host: Option<String>,
    pub port: Option<i32>,
    pub credentials: Option<ConfigRabbitMqCredentials>,

When using Fields types that also implement Converge you can mark fields as also supporting Converge with the #[converge(nest)] attribute, this then allows converge to be used on this structure by converge.

Deriving with custom field converge strategies

pub struct ConfigRabbitMqCredentials {
    pub username: Option<String>,
    pub password: Option<String>,

pub struct ConfigRabbitMQ {
    pub host: Option<String>,
    pub port: Option<i32>,
    #[converge(strategy = converge::strategies::vec::replace_empty)]
    pub credentials: Vec<ConfigRabbitMqCredentials>,

Fields may alternatively be marked with a custom strategy for a custom converge. Where the strategy is the path to a function in the form:

fn custom_function<T>(lhs: T, rhs: T) -> T

Where T matches the type of the field. The converge crate includes some common generic strategies sorted in modules by container type.

How to integrate data sources

This library is expected to be used in combination with other libraries to parse configuration file formats, the command line, and the execution environment variables. In practice the resultant structure presented by these libraries is often closely bound to the input source. The Converge trait requires that each source provides a common data structure. By implementing the From trait or alternatively the TryFrom trait for your common data format Converge can be applied to these data sources.

Associated libraries

A non exhaustive list of data source libraries we can recommend to provide configuration data.


A non exhaustive list of possible alternatives.

  • merge
    • Similar idea to converge.
  • twelf
    • Configuration solution for Rust including 12-Factor support.


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