ConfigKit/docs/roadmap/discovery.md

# Composable Configuration Discovery Roadmap

Status: proposal with the agreed direction, filename groups, and search presets
captured below. Remaining behavioral choices are under
[Open Decisions](#open-decisions).
This document describes future work, not an implemented contract. A staged
implementation plan will follow once the decisions are settled.

## Purpose And Ownership

Make ordinary file-based loading concise for applications that want conventional
configuration locations. ConfigKit supplies useful built-in candidate lists and
an ordered selection operation. Consumers continue to own main-file search
conventions: they choose which lists apply, their order, custom candidates, and
whether a configuration file is required.

A consumer can select one preset for a simple application or compose several
presets and custom path lists. The application supplies an explicit slug where
needed to construct conventional paths. ConfigKit does not infer application
identity from the executable name or enable discovery for existing explicit
sources.

Discovery selects one source for the existing loading lifecycle. It does not
create additional configuration tiers or merge every matching file. Existing
layered loading remains available when consumers intentionally combine sources.

## Current Baseline

The [source contract](../config.md#sources) provides explicit host files, native
filesystem files, bytes, readers, mappings, and `NoSource()`. A missing explicit
file fails. The zero source is invalid. There is no conventional main-file search.

[Ordinary loading](../config.md#single-source-loading) infers a selected overlay's
filesystem context from the main source. `NoSource()` currently rejects enabled
imports, active overlays, and filesystem context. In
[layered loading](../config.md#layered-loading), an absent main input can coexist
with underlays; a selected overlay requires an explicit `OverlayFilesystem`.
These distinctions matter when an optional search finds nothing.

The [architecture policy](../policy/architecture.md#responsibility-boundaries)
assigns search conventions to consumers and currently excludes implicit main-file
discovery. The addition should clarify that consumers may explicitly select
library-provided search conventions. Consumer ownership remains unchanged.

## Agreed Direction

### Filename Groups, Directory Searches, And Search Presets

Organize discovery around three composable descriptions. These are ordered lists:
their order determines precedence.

**Filename groups** provide reusable, ordered filename templates. The built-in
groups are:

| Group | Filenames, in order |
| --- | --- |
| Named | `<appname>.yml`, `<appname>.yaml` |
| Generic | `config.yml`, `config.yaml`, `main.yml`, `main.yaml` |

`<appname>` is the application slug supplied by the consumer.

**Directory searches** pair a directory with an ordered list of filename groups.
The default selection is `[Named, Generic]`, producing the six filenames above
in that order. A directory search may select different groups or reverse their
precedence, such as `[Generic, Named]`, while retaining each group's internal
order. Consumers can also supply custom filenames.

**Search presets** bundle ordered directory searches. Consumers can select a
complete preset for the simple case or compose presets, custom directory searches,
and explicit file candidates in their desired order. Explicit file candidates
are literal paths and receive no filename expansion.

### Ordered Expansion And Concatenation

The selection model is **ordered expansion and concatenation**, followed by
first-match selection:

1. Expand the selected presets into directory searches in their supplied order.
2. For each directory search, concatenate its selected filename groups in order,
   substitute the application slug where needed, and join each filename to that
   directory.
3. Concatenate the resulting candidate lists, preserving the positions of custom
   directory searches and explicit file candidates.
4. Try candidates in order, stopping at the first selected file or an error as
   described below.

Directory priority comes before filename priority: exhaust one directory's
filenames before trying the next directory. When several directories share a
filename list, their expansion is a Cartesian product traversed directory first.
Different directories may select different lists, so ordered expansion and
concatenation describes the complete model. Set union does not express its
precedence semantics.

The eventual documentation should explain this model using the applicable
mathematical nomenclature: ordered lists, ordered concatenation, and Cartesian
product where a shared filename list applies. Retain these names in relevant
implementation comments where they help readers understand the operations. The
model describes ordering without requiring eager construction of every candidate.

### Unix System Preset

The agreed Unix system preset is:

| Directory, in search order | Filename groups, in order |
| --- | --- |
| `/usr/local/etc/<appname>` | `[Named, Generic]` |
| `/etc/<appname>` | `[Named, Generic]` |
| `/etc` | `[Named]` |

This produces 14 candidates: six in each application-specific directory, then
`/etc/<appname>.yml` and `/etc/<appname>.yaml`. For example,
`/usr/local/etc/<appname>/main.yaml` precedes
`/etc/<appname>/<appname>.yml`. Selecting only `Named` for `/etc` is an ordinary
directory-search description; the selection algorithm needs no special exception.

### Additional Presets (OD-1 — Settled)

Provide the following independently selectable presets alongside Unix system.
Within each preset, search directories in the order shown:

| Preset | Directory | Filename groups, in order |
| --- | --- | --- |
| User configuration | `<user-config-dir>/<appname>` | `[Named, Generic]` |
| User configuration | `<user-config-dir>` | `[Named]` |
| Home directory | `<home>/.<appname>` | `[Named, Generic]` |
| Home directory | `<home>` | `[Named]` |
| Working directory | Current working directory | `[Named]` |

Resolve the user-configuration base with
[`os.UserConfigDir`](https://pkg.go.dev/os#UserConfigDir), the home base with
[`os.UserHomeDir`](https://pkg.go.dev/os#UserHomeDir), and the working-directory
base with [`os.Getwd`](https://pkg.go.dev/os#Getwd). Adopt the Go helpers'
platform-specific directory and environment behavior, including their errors;
the settled OD-2 rules below govern how discovery handles resolution errors.

The goal is to support familiar conventions through idiomatic Go. This feature
does not add a separate XDG compatibility layer or XDG system preset. A selected
user-configuration base has no additional hard-coded `<home>/.config` fallback.
The Home directory preset is a separate, explicit choice.

The working-directory preset searches only that directory and uses application-named
files; it does not walk ancestor directories. The consumer chooses precedence
between all presets. Selecting one does not automatically select any others.

### Illustrative Consumer API

Names below are illustrative and are not implemented Go API:

```go
// A simple system-wide application.
source := configkit.SearchConfig("myapp", configkit.UnixSystemDirectories())
```

```go
// A consumer-selected working-directory, user, then system search.
source := configkit.SearchConfig(
    "myapp",
    configkit.WorkingDirectory(),
    configkit.UserConfigDirectory(),
    configkit.UnixSystemDirectories(),
)
```

```go
// Explicit custom candidates followed by a selected conventional fallback.
source := configkit.SearchConfig(
    "myapp",
    configkit.Paths("/run/myapp/config.yml", "/opt/myapp/config.yaml"),
    configkit.UnixSystemDirectories(),
)
```

Use the resulting source with ordinary loading or as a layered main input. Custom
searches and file lists participate at their specified position; they do not
silently append unselected presets. Reordering presets or their directory
searches changes search priority.

### Selection And Failure Behavior

| Situation | Selected behavior |
| --- | --- |
| Candidate is absent | Continue to the next candidate. |
| A candidate file is selected | Stop searching and load that file. |
| Candidate is inaccessible, unreadable, or not an accepted file type | Fail using normal source diagnostics. |
| Selected file fails parsing, version checks, composition, decoding, or validation | Fail without trying another candidate. |
| Entire search has no match | Absence is permitted by default; follow the shared absent-main lifecycle settled under OD-3 below. |
| Entire search has no match and is required | Fail with a diagnostic explaining the unsuccessful search. |
| Caller explicitly uses `File(path)` | Retain existing exclusive-file behavior, including errors for a missing file. |

Requiredness applies to the complete search, not to each preset or directory.
Permitting an absent file does not waive application version, schema, or domain
requirements; the remaining lifecycle may still reject an incomplete configuration.

Perform discovery before parsing the chosen main file. Do not implement fallback
by repeatedly running complete loads. Defaults, application resolution, secret
lookup, and final validation retain their existing lifecycle execution rules.
Once a file has been selected, later failure does not restart discovery.

### Construction And Filesystem Behavior

Constructors describe the search without filesystem access. Resolve dynamic base
directories and access candidate files during loading, so reusable descriptions
can observe changes on subsequent loads. Do not change the process working
directory or environment.

Reuse the host-file acquisition rules for selected files, including regular-file
checks, symlink handling, resource ownership, and source limits. The feature does
not require a new filesystem traversal engine, recursive directory search, or
new file-size limits. Candidate filenames and groups are bounded by their
explicit descriptions, rather than by scanning unrelated directories.

The exact Go names, constructors, and representation of search-wide options
belong in the implementation plan. Keep the public surface root-owned and keep
the single-preset case concise.

### Unavailable Bases And Platform Applicability (OD-2 — Settled)

Resolve directory bases only as the ordered search reaches them. Once an earlier
candidate is selected, later searches do not resolve their bases. Static invalid
search arguments still fail ordinary option validation before source acquisition.

If a selected directory's base cannot be resolved, stop and report the error.
A missing directory or candidate file beneath a resolved base allows the search
to continue; a directory-resolution error does not. This distinction also applies
to optional searches: permitting no match does not suppress errors encountered
while searching. Preserve the agreed errors for inaccessible or unreadable
candidates.

Treat using the Unix system preset on an unsupported platform as an explicit
configuration error when that preset is reached. Specify its supported platforms
in the implementation plan. Do not translate the Unix preset into a different
platform's convention. User, home, and working-directory presets use their native
Go directory helpers.

This keeps search priority predictable when directory resolution fails. Consumers
select the presets appropriate to their application and environment; this proposal
does not add an option to skip unavailable bases or unsupported presets.

### No Match, Imports, And Selected Overlays (OD-3 — Settled)

Use a shared absent-main lifecycle for optional no-match and explicit `NoSource()`.
Allow imports to be enabled when the main file is absent: there are no main-file
import declarations to process, and enablement does not introduce automatic
directory scanning. Other actual sources retain their normal import behavior.
In layered loads, process underlays normally and retain the existing explicit
`OverlayFilesystem` requirement for an active overlay.

For ordinary loads with an active overlay and no main file, require an explicit
filesystem context to locate the overlay. Do not infer an overlay root from the
first or last searched directory. A missing selected overlay remains an error;
absence of a main file does not silently disable a requested overlay.

Defaults, CLI overrides, application resolution, and validation continue through
the normal lifecycle. Main-version checking receives an absent main input and
may reject it under the application's existing policy. Create no synthetic empty
document or source record for the absent main file.

This deliberately relaxes the current `NoSource()` restrictions on imports and
on overlays with explicit context. Both ways of obtaining an absent main input
have the same behavior. Implementation must cover this lifecycle change as well
as candidate selection.

### Placement, Provenance, And Existing Features

The selected file participates as the actual file source. Preserve its physical
identity, effective placement, source origin, and passive resource context.
Relative imports and relative application resources use the selected file's
normal context. Discovery does not relocate overlay or CLI coordinates.

Missing candidates are not acquired sources and do not create source IDs,
configuration history, or explicit input presence. An entirely absent search
contributes no synthetic empty configuration file. Diagnostics should distinguish
an exhausted required search from a failed candidate. Existing selected-source
inspection remains the foundation; a new public per-attempt trace is not needed
for this proposal.

Preserve collection policies, inheritance, source-version rules, CLI ordering,
final validation, and frozen inspection after selection. Search-list order does
not change precedence among underlays, the main input, an overlay, and CLI values.
An explicitly selected overlay is not another main-file fallback candidate.

## Work Needed From The Baseline

The subsequent implementation plan should cover:

- Define filename-group, directory-search, preset, and search-source representations,
  slug validation, and a search-wide requiredness option. Keep construction
  separate from acquisition and the simple preset selection concise.
- Implement ordered expansion and concatenation for presets, filename groups,
  custom searches, and explicit file candidates, followed by first-match selection
  through existing file access. Resolve each source once per consumption before
  entering its parse and composition lifecycle.
- Integrate selected and absent outcomes with ordinary and layered main inputs,
  placement, imports, application version checking, and overlay contexts according
  to OD-3. Preserve ownership and close acquired handles on success and failure.
- Preserve real selected-source metadata and safe diagnostics without treating
  absent candidates as configuration contributions.
- Exercise preset, directory, filename-group, and filename ordering, reversed
  group precedence, the Unix system preset's 14 candidates and named-only `/etc`
  search, custom-list composition, both filename extensions, no-match behavior,
  required searches, unavailable preset bases, errors that must stop fallback,
  repeated loads, and untouched explicit `File`
  behavior. Cover imports, overlays, underlays, defaults, CLI, source versions,
  source contexts, and source-list/inspection evidence at their boundaries.
- Clarify the architecture policy's consumer-selected discovery boundary. Record
  any warranted durable ownership decision in an ADR; keep preset filenames and
  API mechanics in their canonical contract when implemented. Update consumer
  examples and the configuration contract when the feature lands. Explain the
  three-part discovery model and its mathematical terminology in the eventual
  documentation, with simple preset selection and custom composition examples.

This roadmap does not change implementation, policy, or ADRs by itself. Coordinate
implementation with the ongoing sequence integration audit rather than assuming
its in-progress changes are part of a fixed implementation baseline.

## Open Decisions

### OD-4 — Empty Composition And Implicit Preset Selection

**Recommendation:** require explicit preset or search selection to activate a
convention. An empty search composition has no candidates: optional search contributes no
source, and required search fails as exhausted. Empty custom lists, including
empty filename-group selections, follow the same rule. This supports programmatic
list construction and introduces no hidden
system, user, or working-directory fallback. One explicitly selected preset
still gives the simple application a short call.

**Alternative:** let omission of searches select a documented default preset.
This shortens the simplest call, but omission and an intentionally empty computed
list would need distinguishable representations. It also requires choosing a
default convention across platforms and application styles. Another alternative
is to reject an empty composition as invalid; that catches accidental empty lists
but adds a special case beyond the search-wide requiredness rule.

### OD-5 — Slugs And Custom Path Interpretation

**Recommendation:** require a nonempty single-component slug when a selected
preset uses it to construct filenames. Reject path separators, `.` and `..`,
and other forms that cannot be used as that component on the target platform.
Preserve spelling; do not silently lowercase, trim, or impose a kebab-case naming
scheme. A composition made only of explicit custom paths need not require an
application slug.

Treat custom paths as literal host file paths. Resolve relative paths against
the working directory at load time; do not expand shell variables, `~`, or glob
syntax. Consumers can construct expanded paths themselves. Selected presets
provide the convenience for user and working-directory bases.

**Alternative:** require a restricted portable slug grammar, such as lowercase
ASCII letters, digits, and hyphens. This is easy to document and portable, but
unnecessarily excludes otherwise usable application names. Automatic shell-like
custom-path expansion is also viable, but adds escaping and environment semantics
that ordinary `File(path)` does not supply. Literal paths keep both interfaces
consistent.