@fluojs/config

Configuration loading, merging, validation, and typed runtime access for fluo applications.

Table of Contents

• Installation
• When to Use
• Quick Start
• Key Capabilities
• Public API
• Related Packages
• Example Sources

Installation

npm install @fluojs/config

When to Use

Use this package when you need to:

• Load configuration from .env files plus an explicit processEnv snapshot.
• Merge multiple configuration sources with strict precedence rules.
• Validate your application configuration at startup.
• Access configuration values through a typed ConfigService.

Quick Start

The ConfigModule handles loading and validating your configuration during bootstrap.

import { Module } from '@fluojs/core';
import { ConfigModule } from '@fluojs/config';
import { z } from 'zod';

const EnvSchema = z.object({
  DATABASE_URL: z.string().url(),
  PORT: z.coerce.number().default(3000),
});

@Module({
  imports: [
    ConfigModule.forRoot({
      envFile: '.env',
      processEnv: {
        DATABASE_URL: process.env.DATABASE_URL,
      },
      defaults: { PORT: '3000' },
      schema: EnvSchema,
    }),
  ],
})
class AppModule {}

Use envFilePath when the env file lives at an absolute or pre-resolved path, or parse when you need a custom flat-file parser instead of the default dotenv parser.

Once registered, you can inject ConfigService to access your values:

import { Inject } from '@fluojs/core';
import { ConfigService } from '@fluojs/config';

@Inject(ConfigService)
class MyService {
  constructor(private readonly config: ConfigService) {
    const port = this.config.get('PORT');
    const dbUrl = this.config.getOrThrow('DATABASE_URL');
  }
}

Key Capabilities

Source Precedence

Configuration is merged in the following order (highest precedence wins):

1. Runtime Overrides: Values passed explicitly via runtimeOverrides.
2. Process Environment Snapshot: Values passed via the processEnv option.
3. Environment File: Values from the .env file (or custom path).
4. Defaults: Values provided in the defaults option.

@fluojs/config does not scan ambient environment variables automatically. Pass an explicit processEnv snapshot at the bootstrap boundary when process-backed values should participate in precedence.

envFilePath overrides envFile, and parse lets callers replace dotenv parsing with a custom parser for flat key/value files. Missing env files are treated as empty input during load; watch mode also observes the parent directory so creating the file later can trigger a reload.

Deep Merging

Plain objects are deep-merged by key. Arrays and primitive values from higher-precedence sources completely replace lower-precedence ones.

Validation

The schema option accepts a synchronous Standard Schema-compatible validator such as Zod, Valibot, or ArkType. The schema runs after all sources are merged but before the application starts. Its validated value becomes the final config snapshot, and reported issues fail bootstrap/load/reload with INVALID_CONFIG.

@fluojs/config keeps loading and reload APIs synchronous. Async Standard Schema results are rejected with INVALID_CONFIG; use a synchronous schema for config validation.

Runtime Access and Reload Cost Model

ConfigService.get('a.b.c') resolves dot-path keys by walking each path segment, so lookup cost is proportional to path depth. When get(), getOrThrow(), or snapshot() returns an object-like value, the returned value is a detached clone; clone cost is proportional to the returned subtree size so caller mutations cannot affect the active config snapshot.

ConfigReloadManager.reload() serializes reload work. If another reload is requested while the current reload is notifying listeners, the follow-up reload is queued and applied after the active notification finishes; if the active notification fails, the previous snapshot is restored and the queued reload is discarded. The same serialization and rollback contract applies to createConfigReloader(...).reload(), including manual reloads queued during watch-triggered notifications.

Module registration and reloader creation snapshot caller-owned options before storing them. Later mutations to objects passed to ConfigModule.forRoot(...), ConfigReloadModule.forRoot(...), or createConfigReloader(...) do not affect bootstrap, manual reloads, or watch reloads. When ConfigModule.forRoot({ watch: true, ... }) is used, the module starts an env-file watcher during application bootstrap, first aligns the injected ConfigService with the watch reloader baseline, and then updates the same injected ConfigService instance after successful watch reloads. Pass onReloadError when the application needs ownership of automatic watch reload failures from ConfigModule. In watch mode, the parent directory is watched for both existing and missing env files, so creating or atomically replacing the env file can trigger reload. Watch reloads compare the final env file content with the last committed watch baseline before reloading, so unchanged saves and change-then-revert bursts do not replace the in-process config snapshot.

ConfigReloadModule is the explicit injectable reload layer, not a standalone config source. Pair it with ConfigModule or another ConfigService provider when callers need CONFIG_RELOADER for manual reloads or subscriptions. Watchers created by ConfigModule or ConfigReloadModule are created only when watch: true, and they are closed during module shutdown. Enable watch: true on one layer for a given env file: use ConfigModule for automatic ConfigService updates, or ConfigReloadModule when callers need the injected reloader contract for subscriptions/manual reloads.

Public API

| Class/Helper | Description | |---|---| | ConfigModule | Module for registering configuration globally or locally. | | ConfigReloadModule | Registers the reload manager and exports the shared CONFIG_RELOADER token for dependency injection. | | ConfigReloadManager | Coordinates reloads for the injected ConfigService, preserving service identity while replacing snapshots through the reload path. | | CONFIG_RELOADER | Injection token for the shared config reloader contract. | | ConfigService | Read-only service for typed access to configuration values. Snapshot replacement stays inside the config reload path. | | loadConfig(options) | Functional entry point for loading configuration manually. | | createConfigReloader(options) | Creates a reloader for dynamic configuration updates. |

The package also exports option and subscription types such as ConfigModuleOptions, ConfigLoadOptions, ConfigReloadSubscription, and ConfigReloadReason.

ConfigReloadManager.reload() updates the existing ConfigService instance so consumers keep their injected service identity while observing the new snapshot. If a reload listener throws, the manager restores the previous snapshot and rethrows the listener error. createConfigReloader(...).reload() follows the same listener serialization and rollback behavior for its standalone reloader snapshot.

Related Packages

• @fluojs/runtime: Calls loadConfig internally during application bootstrap.
• Standard Schema validators: Zod, Valibot, ArkType, and other compatible schema libraries can be passed through the schema option.

Example Sources

• packages/config/src/load.ts
• packages/config/src/service.ts
• packages/config/src/load.test.ts
• packages/config/src/reload-module.ts
• docs/architecture/config-and-environments.md
• docs/architecture/dev-reload-architecture.md