premortem

Know how your app will die—before it does.

A configuration library that performs a premortem on your app's config—finding all the ways it would die before it ever runs.

Why "premortem"?

The name is a bit tongue-in-cheek—but only a bit. Configuration errors are one of the leading causes of production outages. Bad config doesn't just cause bugs; it causes incidents, pages, and 3am debugging sessions.

A postmortem is what you do after something dies—gathering everyone to analyze what went wrong. Traditional config libraries give you the postmortem experience:

$ ./myapp
Error: missing field `database.host`

$ ./myapp  # fixed it, try again
Error: invalid port value

$ ./myapp  # fixed that too
Error: pool_size must be positive

# Three deaths to find three problems

premortem gives you all the fatal issues upfront:

$ ./myapp
Configuration errors (3):
  [config.toml:8] missing required field 'database.host'
  [env:APP_PORT] value "abc" is not a valid integer
  [config.toml:10] 'pool_size' value -5 must be >= 1

One run. All errors. Know how your app would die—before it does.

Try it yourself: cargo run --example error-demo

Features

• Accumulate all errors — Never stop at the first problem
• Trace value origins — Know exactly which source provided each value
• Multi-source loading — Files, environment, CLI args, remote sources
• Holistic validation — Type, range, format, cross-field, and business rules
• Derive macro — Declarative validation with #[derive(Validate)]
• Hot reload — Watch for config changes (optional feature)

Quick Start

use premortem::{Config, Toml, Env, Validate};
use serde::Deserialize;

#[derive(Debug, Deserialize, Validate)]
struct AppConfig {
    #[validate(non_empty)]
    pub host: String,

    #[validate(range(1..=65535))]
    pub port: u16,

    #[validate(range(1..=100))]
    pub pool_size: u32,
}

fn main() {
    let config = Config::<AppConfig>::builder()
        .source(Toml::file("config.toml"))
        .source(Env::prefix("APP_"))
        .build()
        .unwrap_or_else(|errors| {
            eprintln!("Configuration errors ({}):", errors.len());
            for e in &errors {
                eprintln!("  {}", e);
            }
            std::process::exit(1);
        });

    println!("Starting server on {}:{}", config.host, config.port);
}

Installation

[dependencies]
premortem = "0.6"

With optional features:

[dependencies]
premortem = { version = "0.6", features = ["json", "watch"] }

Feature Flags

Examples

See the examples/ directory for runnable examples:

Run an example:

cargo run --example error-demo
cargo run --example layered-config
cargo run --example yaml --features yaml

Documentation

• Common Patterns — Layered config, secrets, nested structs
• Testing Guide — Testing with MockEnv

Core Concepts

Source Layering

Sources are applied in order, with later sources overriding earlier ones:

let config = Config::<AppConfig>::builder()
    .source(Defaults::from(AppConfig::default()))  // Lowest priority
    .source(Toml::file("config.toml"))
    .source(Env::prefix("APP_"))                   // Highest priority
    .build()?;

Required Environment Variables

Mark environment variables as required at the source level with error accumulation:

let config = Config::<AppConfig>::builder()
    .source(
        Env::prefix("APP_")
            .require_all(&["JWT_SECRET", "DATABASE_URL", "API_KEY"])
    )
    .build()?;

All missing required variables are reported together:

Configuration errors (3):
  [env:APP_JWT_SECRET] Missing required field: jwt.secret
  [env:APP_DATABASE_URL] Missing required field: database.url
  [env:APP_API_KEY] Missing required field: api.key

This separates presence validation (does the variable exist?) from value validation (does it meet constraints?).

Testable I/O

All I/O is abstracted through ConfigEnv, enabling testing with MockEnv:

let env = MockEnv::new()
    .with_file("config.toml", "port = 8080")
    .with_env("APP_HOST", "localhost");

let config = Config::<AppConfig>::builder()
    .source(Toml::file("config.toml"))
    .source(Env::prefix("APP_"))
    .build_with_env(&env)?;

Value Tracing

Debug where configuration values came from:

let traced = Config::<AppConfig>::builder()
    .source(Defaults::from(AppConfig::default()))
    .source(Toml::file("config.toml"))
    .source(Env::prefix("APP_"))
    .build_traced()?;

// Check what was overridden
for path in traced.overridden_paths() {
    let trace = traced.trace(path).unwrap();
    println!("{}: {:?} from {}", path, trace.final_value.value, trace.final_value.source);
}

License

MIT Glen Baker iepathos@gmail.com