Configuration¶

This page covers the three ways to configure a Confii instance, every available option, and how priority resolution works between them.

Three Ways to Configure Confii¶

Confii follows a 3-tier priority model:

explicit code argument  >  self-config file  >  built-in default

If you pass confii.WithDeepMerge(false) in code, it wins over a deep_merge: true in .confii.yaml, which itself wins over the built-in default of true.

1. Self-Configuration File (Zero-Code Defaults)¶

Confii auto-discovers a self-configuration file before any loaders run. This is the best place for project-wide defaults shared by every developer.

Search Order¶

The first file found wins:

confii.yaml, confii.yml, confii.json, confii.toml in the current working directory
.confii.yaml, .confii.yml, .confii.json, .confii.toml in the current working directory
Same filenames in ~/.config/confii/

YAMLJSONTOML

.confii.yaml

# Environment
default_environment: development
env_switcher: APP_ENV          # read env name from this OS variable
env_prefix: APP                # auto-add an EnvironmentLoader with this prefix

# Loading behavior
deep_merge: true
use_env_expander: true
use_type_casting: true
sysenv_fallback: false

# Validation
validate_on_load: false
strict_validation: false
schema_path: schema.json

# Runtime
dynamic_reloading: false
freeze_on_load: false
debug_mode: false

# Error handling
on_error: raise                # raise | warn | ignore

# Logging
log_level: info

# Default files to load (in order)
default_files:
  - config/base.yaml
  - config/dev.yaml

# Declarative sources (alternative to default_files)
sources:
  - type: yaml
    path: config/base.yaml
  - type: json
    path: config/overrides.json
  - type: env
    prefix: APP

# Secret store configuration
secrets:
  provider: env               # env | dict | aws | azure | gcp | vault

.confii.json

{
  "default_environment": "development",
  "env_switcher": "APP_ENV",
  "env_prefix": "APP",
  "deep_merge": true,
  "use_env_expander": true,
  "use_type_casting": true,
  "validate_on_load": false,
  "strict_validation": false,
  "dynamic_reloading": false,
  "freeze_on_load": false,
  "debug_mode": false,
  "on_error": "raise",
  "default_files": [
    "config/base.yaml",
    "config/dev.yaml"
  ]
}

.confii.toml

default_environment = "development"
env_switcher = "APP_ENV"
env_prefix = "APP"
deep_merge = true
use_env_expander = true
use_type_casting = true
validate_on_load = false
strict_validation = false
dynamic_reloading = false
freeze_on_load = false
debug_mode = false
on_error = "raise"
default_files = ["config/base.yaml", "config/dev.yaml"]

Full Self-Config Settings Reference¶

Setting	Type	Default	Description
`default_environment`	`string`	`""`	Default active environment name
`env_switcher`	`string`	`""`	OS variable to read environment name from
`env_prefix`	`string`	`""`	Auto-add an `EnvironmentLoader` with this prefix
`default_prefix`	`string`	`""`	Default prefix for configuration lookups
`default_files`	`[]string`	`[]`	Ordered list of config files to load
`deep_merge`	`bool`	`true`	Enable recursive merge of nested maps
`use_env_expander`	`bool`	`true`	Enable `${VAR}` expansion in string values
`use_type_casting`	`bool`	`true`	Auto-convert strings to bool/int/float
`sysenv_fallback`	`bool`	`false`	Fall back to OS env vars on missing keys
`validate_on_load`	`bool`	`false`	Validate struct tags after loading
`strict_validation`	`bool`	`false`	Treat validation warnings as errors
`schema_path`	`string`	`""`	Path to a JSON Schema file for validation
`dynamic_reloading`	`bool`	`false`	Enable fsnotify file watching
`freeze_on_load`	`bool`	`false`	Make config immutable after load
`debug_mode`	`bool`	`false`	Enable full source tracking and override history
`on_error`	`string`	`"raise"`	Error policy: `raise`, `warn`, or `ignore`
`log_level`	`string`	`""`	Log level for Confii's internal logger
`sources`	`[]map`	`[]`	Declarative source definitions
`secrets`	`map`	`{}`	Declarative secret store configuration

When to use self-config

Self-configuration files are ideal for team-wide defaults that you commit to version control. Individual developers or CI pipelines can override specific settings via constructor options in code.

2. Constructor with Options¶

Pass functional options directly to confii.New:

cfg, err := confii.New[AppConfig](ctx,
    confii.WithLoaders(
        loader.NewYAML("config.yaml"),
        loader.NewEnvironment("APP"),
    ),
    confii.WithEnv("production"),
    confii.WithDeepMerge(true),
    confii.WithValidateOnLoad(true),
    confii.WithStrictValidation(true),
    confii.WithOnError(confii.ErrorPolicyWarn),
)

This is the most common approach for application code.

Complete Options Reference¶

Option	Purpose	Default
`WithLoaders(loaders...)`	Set the ordered list of configuration sources. Later loaders override earlier ones.	none
`WithEnv(name)`	Set the active environment (e.g. `"production"`, `"staging"`).	`""`
`WithEnvSwitcher(envVar)`	Read the environment name from the given OS variable at startup.	none
`WithEnvPrefix(prefix)`	Auto-add an `EnvironmentLoader` with this prefix (e.g. `"APP"` reads `APP_*` vars).	none
`WithDeepMerge(bool)`	Enable recursive deep merge of nested maps when combining sources.	`true`
`WithMergeStrategyOption(strategy)`	Set the default merge strategy for all paths.	`Merge`
`WithMergeStrategyMap(map)`	Set per-path merge strategy overrides (e.g. `"database"` uses `Replace`).	none
`WithEnvExpander(bool)`	Enable `${VAR}` expansion in string values using OS environment variables.	`true`
`WithTypeCasting(bool)`	Auto-convert string values to `bool`/`int`/`float64` when accessed.	`true`
`WithSysenvFallback(bool)`	Fall back to OS environment variables when a key is not found in config.	`false`
`WithValidateOnLoad(bool)`	Validate the typed struct (via `go-playground/validator` tags) immediately after loading.	`false`
`WithStrictValidation(bool)`	Treat validation warnings as errors (requires `WithValidateOnLoad`).	`false`
`WithSchema(schema)`	Set a validation schema (struct type or JSON Schema dict).	none
`WithSchemaPath(path)`	Set the path to a JSON Schema file for validation.	none
`WithFreezeOnLoad(bool)`	Make the config immutable after initialization. `Set()` returns `ErrConfigFrozen`.	`false`
`WithDynamicReloading(bool)`	Enable fsnotify file watching for automatic reload on change.	`false`
`WithDebugMode(bool)`	Enable full source tracking, override history, and debug reports.	`false`
`WithOnError(policy)`	Set the error handling policy for loader failures.	`ErrorPolicyRaise`
`WithLogger(logger)`	Set a custom `*slog.Logger` for Confii's internal logging.	`slog.Default()`

Defaults for WithEnvExpander and WithTypeCasting

Both of these default to true in the built-in defaults. This means string values containing ${VAR} patterns are expanded, and strings like "true", "8080", and "3.14" are automatically coerced to their native Go types unless you explicitly disable them.

3. Builder Pattern¶

The builder provides a fluent API for constructing Config instances. It is especially useful when configuration construction is conditional or spans multiple steps:

builder := confii.NewBuilder[AppConfig]()

// Conditionally add loaders
builder.AddLoader(loader.NewYAML("config/base.yaml"))

if os.Getenv("APP_ENV") == "production" {
    builder.AddLoader(loader.NewYAML("config/prod.yaml"))
    builder.WithEnv("production")
} else {
    builder.AddLoader(loader.NewYAML("config/dev.yaml"))
    builder.WithEnv("development")
}

// Chain additional settings
cfg, err := builder.
    EnableDeepMerge().
    EnableFreezeOnLoad().
    EnableDebug().
    Build(ctx)

Builder Methods Reference¶

Method	Description
`WithEnv(name)`	Set the active environment
`AddLoader(loader)`	Append a single loader to the source list
`AddLoaders(loaders...)`	Append multiple loaders
`EnableDeepMerge()`	Enable recursive deep merge
`DisableDeepMerge()`	Disable deep merge (shallow merge)
`EnableEnvExpander()`	Enable `${VAR}` expansion
`DisableEnvExpander()`	Disable `${VAR}` expansion
`EnableTypeCasting()`	Enable automatic type casting
`DisableTypeCasting()`	Disable automatic type casting
`EnableDynamicReloading()`	Enable fsnotify file watching
`DisableDynamicReloading()`	Disable file watching
`EnableDebug()`	Enable debug/source tracking mode
`EnableFreezeOnLoad()`	Freeze config after loading
`WithSchemaValidation(schema, strict)`	Set schema, enable validate-on-load, and set strict mode
`Build(ctx)`	Create the `Config` instance (loads all sources)

Builder vs Constructor

The builder calls the same confii.New constructor internally. There is no functional difference -- choose whichever style reads better in your code. The builder shines when you need to conditionally compose loaders or split setup across multiple functions.

Priority Resolution¶

Understanding how the three configuration methods interact is key to avoiding surprises. Confii resolves each setting independently using this priority:

1. Explicit code argument    (highest priority)
2. Self-config file value
3. Built-in default          (lowest priority)

Example¶

Given this self-config file:

.confii.yaml

deep_merge: false
use_env_expander: true
default_environment: staging

And this constructor call:

cfg, err := confii.New[any](ctx,
    confii.WithDeepMerge(true),   // explicit override
    confii.WithEnv("production"), // explicit override
)

The resolved values are:

Setting	Self-Config	Explicit	Resolved	Source
`deep_merge`	`false`	`true`	`true`	explicit wins
`use_env_expander`	`true`	(not set)	`true`	self-config wins over built-in
`env`	`staging`	`production`	`production`	explicit wins
`freeze_on_load`	(not set)	(not set)	`false`	built-in default

Error Policies¶

The WithOnError option controls how Confii handles loader failures (e.g., a file that doesn't exist or a cloud endpoint that times out).

confii.WithOnError(confii.ErrorPolicyRaise)   // default
confii.WithOnError(confii.ErrorPolicyWarn)
confii.WithOnError(confii.ErrorPolicyIgnore)

Policy	Behavior
`ErrorPolicyRaise`	Return the error immediately from `confii.New`. The application cannot start with a broken config source. This is the default.
`ErrorPolicyWarn`	Log a warning via `slog` and continue loading from remaining sources. Useful when some sources are optional.
`ErrorPolicyIgnore`	Silently skip failed loaders. Use with caution -- you may end up with missing configuration without any indication.

Choose Raise for production

ErrorPolicyRaise is the safest default. Use Warn only for genuinely optional sources (e.g., a local override file that may not exist in CI). Avoid Ignore unless you have other mechanisms to detect missing config.

Error Policy Example¶

// Optional local overrides -- warn if missing, don't fail
cfg, err := confii.New[any](ctx,
    confii.WithLoaders(
        loader.NewYAML("config/base.yaml"),       // required
        loader.NewYAML("config/local.yaml"),       // optional
        loader.NewEnvironment("APP"),
    ),
    confii.WithOnError(confii.ErrorPolicyWarn),    // (1)!
)

If config/local.yaml doesn't exist, Confii logs a warning and continues with the remaining sources.

Note

The error policy applies to all loaders uniformly. If you need different policies for different sources, consider loading them in separate steps using cfg.Extend() after initial construction.

Initialization Sequence¶

When confii.New is called, Confii executes these steps in order:

Apply constructor options -- merge user-provided With* options into defaults
Read self-config -- discover and parse .confii.yaml (or equivalent), apply non-overridden settings
Resolve environment -- if WithEnvSwitcher is set, read the OS variable to determine the active environment
Set up merger -- configure the merge engine (default or advanced with per-path strategies)
Register hooks -- enable env expander and type casting hooks if configured
Load all sources -- call each loader in order, compose _include/_defaults, track sources, merge results
Resolve environment sections -- merge default + active environment section
Validate -- if WithValidateOnLoad is set, decode and validate the typed struct
Freeze -- if WithFreezeOnLoad is set, lock the config against further changes
Start watcher -- if WithDynamicReloading is set, begin watching source files via fsnotify

Debug the initialization

Enable WithDebugMode(true) to get full source tracking. After loading, call cfg.Layers() to see which sources contributed which keys, or cfg.Explain("database.host") to trace a specific value back to its origin.