ConfigKit/docs/roadmap/sources-and-layering.md

# Filesystem Sources and Explicit Layered Loading

Status: proposed target for design agreement; not implemented. Significant API
and semantic choices are collected under Open Decisions. A later implementation
plan will own staged execution, detailed signatures, and acceptance commands.

## Purpose and priorities

Implement the two prerequisites in the
[PromptKit request](feature-request-promptkit.md) as one coherent extension:
native `fs.FS` sources and public, explicitly ordered configuration layers,
including programmatic input. Neither capability should require temporary files,
a generated root YAML document, or serialization through YAML merely to transfer
already available values.

Design priorities, in order:

1. Clear, standardized behavior consistent with ConfigKit's existing conventions.
   PromptKit may adapt its formats and APIs; its historical conventions are not
   new ConfigKit compatibility obligations.
2. Efficient loading and retained state, aiming for linear or near-linear work
   in actual input and output size. Prefer clarity and maintainability where a
   more elaborate optimization would compromise the first priority.
3. A small public surface with cohesive private implementation, usable by simple
   applications without requiring them to learn the layered-loading machinery.

Consumers load once and receive a complete configuration and its inspection
metadata. They should not need a second generic merge/defaulting pass, a separate
presence tracker, or reconstructed provenance. Domain-specific resolution remains
consumer-owned, but fits into the recorded loading lifecycle before the result
is validated and returned. Subject to consistent semantics and correct presence
and provenance, minimizing consumer integration code is an explicit design goal.

The [named-inheritance request](feature-request-promptkit-inheritance.md) remains
separate and optional. This roadmap neither implements it nor adds speculative
infrastructure for it.

## Current baseline

Planning baseline: `ec26591b275bd74f89120bdea69c5f6459dfcafb`, following v0.4.2.
Current behavior is owned by the [configuration contract](../config.md), not by
these proposals.

- `Load[T]` takes one `File`, `Bytes`, `Reader`, or `NoSource` input. Imports
  strictly compose documents; one named overlay uses a root-relative directory
  convention; explicit CLI overrides follow it.
- Filesystem context uses host directories and an `os.Root`-based implementation.
  Byte and reader labels do not imply filesystem paths. There is no native
  `fs.FS` source or public batch of heterogeneous layers.
- Composition rejects non-mapping overlaps, including equal values. Overlay
  recursively merges mappings and replaces other values. Empty mappings do not
  clear existing children; null is an explicit value, not omission.
- An explicitly configured schema-version path currently belongs to the selected
  source. Its checker runs before overlay/CLI input, which cannot alter that path;
  imported fragments cannot contribute it. This roadmap preserves those ownership
  and timing rules.
- CLI overrides already convert supported programmatic scalars, lists, and
  string-keyed maps directly to the private document representation. They have
  CLI-specific paths, labels, restrictions, and origins, not a general source API.
- Source records and resolution queries expose acquired-source identity and
  provenance, but do not provide an `fs.FS` resource context.
- Root-owned public wrappers delegate to `internal/engine`; parser-independent
  trees belong to `internal/document`, and YAML parsing stays in `internal/yaml`.
- Current mapping assembly uses repeated linear key searches. Overlay application
  clones the accumulated document and provenance; subtree-origin removal scans
  the whole origin index. Straightforward repetition across many layers would
  amplify these costs.

Existing source/lifecycle contracts, ownership, secret handling, and diagnostics
remain the compatibility baseline. This work is not permission to silently
reinterpret current `Load`, imports, selected overlays, or CLI overrides.
Redesigning schema-version policy is outside this roadmap's scope.

## Target source model

### Native filesystem input

A caller can supply a filesystem, an explicit file location within it, and a
meaningful diagnostic label/context. `embed.FS` is an ordinary supported example,
not a separate parsing mode. Use standard slash-separated `io/fs` path conventions;
do not apply host drive-letter or separator rules to virtual paths.

The source retains the filesystem context needed to acquire its imports, glob
matches, and selected overlay. Relative imports resolve from the importing
document; named overlay lookup remains relative to that configuration context's
root. References must never fall back to the host filesystem simply because a
virtual path is missing. Imports/globs retain their explicit opt-in, deterministic
ordering, composition semantics, and actionable errors.

Filesystem namespace identity and a normalized path identify a resource; labels
are presentation, not uniqueness keys. Identical names in different filesystems
must not collide. Use a reusable load-local context identity, not comparison or
hashing of arbitrary `fs.FS` interface values. Repeated-import and cycle checks
operate within the relevant input composition graph; selecting explicit layers
is distinct from accidentally importing a file twice.

Keep the host-file backend's established canonicalization and containment behavior.
For generic `fs.FS`, describe namespace-relative resolution and the supplied
filesystem's actual behavior. `fs.FS` does not universally provide symlink
canonicalization or a security sandbox; do not invent such guarantees or reject
all implementations that cannot provide host-specific operations. The caller
chooses the filesystem, its acquisition behavior, and its lifetime. ConfigKit
closes handles it opens, not a caller-owned filesystem as a whole.

### Origins and downstream resources

Locations remain meaningful for host files, filesystem-relative files, labeled
bytes, and programmatic values. Preserve deterministic source IDs and traversal
order while distinguishing source identity, input kind, and precedence role.
Do not invent host filenames or line/column coordinates for synthetic inputs.

A consumer can follow an effective field's origin to its contributing source and
obtain the context needed to resolve a relative resource. For a field inherited
through layering, that is the contributing source's context, not simply the last
layer's directory. Composite origins remain composite; ConfigKit must not choose
one contributing directory arbitrarily.

Resource-context access is explicit and separate from serializable diagnostic
records. Querying metadata itself does not read resources. PromptKit owns opening
and interpreting message text, rendering, and handling resource errors. Context
retention must not imply that external resources are snapshotted with the loaded
configuration or remain valid after the caller closes their filesystem.

## Target layering model

### Explicit operations, not discovery order

Expose a public loading entry point for an explicit ordered set of sources and
operations, using the same typed definition/result lifecycle as ordinary `Load`.
Support native filesystem, host-file, byte, reader, and programmatic inputs.
The order is supplied by the caller; ConfigKit does not discover priorities or
know about PromptKit's catalog tiers.

The selected operation determines how values combine, not their source format
or loading entry point. Equivalent file, embedded, byte/reader, and programmatic
input uses the same presence and combination rules. Explicitly choosing compose,
overlay, or replacement expresses different intentions; merely changing the input
medium must not switch between them. This applies to both ordinary `Load` and
the new layered-loading interface wherever they offer the same operation.
The existing opt-in version-path ownership rule remains an explicit metadata
constraint, independent of whether its owning source is a file or programmatic
input; it is not a source-format-dependent merge rule.

Keep strict composition and intentional overriding visibly distinct:

| Operation | Desired semantics |
| --- | --- |
| Compose independent contributions | Existing strict composition: recurse through mappings and report non-mapping conflicts with relevant source locations. |
| Overlay settings | Existing recursive mapping overlay: omitted children retain lower input; scalar, sequence, and null values replace. |
| Replace a definition | Replace the selected input subtree as a unit, without carrying over omitted lower-input children. The inclusion and public expression of this operation are Decision 1. |

Each independently supplied root may bring its explicitly enabled imports and
source context. Its import graph is strictly composed before intentional
precedence is applied. An overlay must not hide malformed YAML or a composition
conflict in an earlier contribution. There is no automatic matching by an `id`
field, implicit sequence merge, universal profile registry, or merge callback.

Preserve version checking after base composition and before intentional overlays
and CLI contributions. Assemble the effective input before typed defaults/decoding,
application resolution, secret resolution, semantic validation, and final
snapshotting. Each lifecycle phase runs once per load, not once per layer.
Higher layers may replace values that would not fit the final typed schema;
effective typed validation is not validation of each intermediate snapshot.
Input syntax, source access, and composition remain validated at acquisition.

The standard precedence relationship is typed defaults below assembled input,
then the existing selected-overlay convenience and explicit CLI overrides where
those are used, followed by recorded application resolution. Named-overlay lookup
has an unambiguous filesystem context, separate from version ownership: ordinary
loading retains its source/root convention; a multi-context load explicitly
identifies the lookup context when that convenience is used. Other sources retain
their own import and resource contexts. Callers do not manufacture a primary
document merely to combine layers, and no overlay lookup context is needed when
no named overlay is selected.

Replacing lower input does not mean resetting or deleting typed application
defaults. Existing defaults and null rules still apply during final decoding.
The distinction must be visible in examples, especially empty maps and partially
specified definitions. No new reset/delete operator is implied by replacement.
For example, replacing input `{timeout: 10, retries: 2}` with `{timeout: 20}`
removes the lower input's `retries: 2`; a typed default of `retries: 3` can still
supply the final value. The metadata then attributes `retries` to defaults, not
to the replaced input. An empty replacement entry likewise removes lower-input
children without suppressing applicable typed defaults.

### Preserve existing schema-version semantics

An application opts into version checking by configuring a version path. A key
spelled `version` has no automatic special status. When checking is configured,
one designated input owns that declaration; fragments cannot contribute it and
overlays or CLI overrides cannot alter it, even to an equal value. New layering
operations must respect that ownership rather than bypass it through subtree
replacement. Preserve the current version-path and ancestor protections.

Keep the checker at its existing lifecycle point: after base composition and
schema construction, before intentional overlays, CLI overrides, defaults, and
typed decoding. Preserve supported scalar forms, absent/null distinction,
diagnostics, failure propagation, and version-path integer-to-string decoding.
An absent declaration remains absent for the checker; an invalid version type
fails without invoking it. Later input cannot mask a version-check failure.

Filesystem sources and layered loading do not require a different version
policy. The layered API needs a way to identify the version-owning input when a
`VersionRule` is configured; Decision 4 concerns that integration only. Without
version checking, no version owner or synthetic primary document is required.
Named-overlay lookup context is a separate concern, not a reason to impose
version ownership on unversioned loads.

Consumers continue to define version meaning and acceptance. Effective-version
last-wins validation, independent per-source version checks, schema selection,
and migration are not added by this feature. A future concrete need can motivate
a separate version-policy proposal; preserving the current contract here does
not settle that broader question permanently.

### Programmatic presence

Programmatic input has a non-secret source label and uses explicit structure to
represent presence. Omitted mapping keys are absent; present zero, false, empty
string, empty collection, and nil/null values remain explicit. Use ConfigKit's
existing canonical scalar and collection conventions, not YAML round trips or
Go zero-value inference. Decision 2 selects the initial input representation.

Do not label generic programmatic sources as CLI input or misuse synthetic flag
names to fit the existing override API. Preserve parser-independent conversion,
safe errors, deterministic map ordering, and existing secret-reference semantics;
this is not a new route for embedding resolved credentials in document trees.

Caller-owned mutable input is detached at a clearly documented ingestion
boundary. Results and inspection remain independently owned under existing
contracts. This does not promise language-enforced immutability of Go values or
an immutable filesystem.

A final Go value alone need not encode presence: an omitted boolean and an
explicit `false` can both decode to `false`. The returned configuration together
with its presence/provenance metadata preserves that distinction through
defaulting, layering, and recorded resolution. Null, empty values, zero, and
absence must not be collapsed by source conversion or merging.

### Provenance and operational boundaries

Origins, presence, history, explanation, and effective rendering agree with the
final typed result. Overridden and replaced definitions remain explainable without
retaining stale effective descendants. Replacements identify their scope and
displaced sources; equal-value explicit overrides are still meaningful history.
Include typed defaults and application-derived values, not only source-supplied
values, in final-value provenance. Retain redaction and callback-lifetime behavior.

Apply existing resource-accounting principles across the whole load rather than
resetting allowances for every layer. Audit how bytes, programmatic nodes,
accepted source occurrences, and glob work are counted without accidental double
charging. Do not add arbitrary new budgets or infer that efficiency work waives
existing ones; any needed adjustment must be explicit in the later design.

## Efficiency target

Measure cost against total acquired bytes/nodes, reference and path data, final
materialized values, and retained provenance/history. Layer count alone is not
a useful size measure. Large history or output can legitimately dominate cost.

Aim for O(N + H + R) processing and storage where N is total input/visited-path
work, H is retained history/provenance, and R is produced result data. Account
separately for deterministic sorting, filesystem traversal, schema processing,
and caller callbacks. This is an engineering target, not an unconditional public
complexity guarantee or a fixed performance gate.

The target implementation:

- Acquires/parses each accepted source occurrence once and directly converts
  programmatic input. It does not decode or serialize accumulated configurations
  between layers. Respect single-consumption readers and mutable file sources;
  no implicit cross-load cache is needed.
- Uses indexed lookup alongside ordered mapping storage, retaining deterministic
  iteration and diagnostics without repeatedly scanning growing mappings.
- Builds one load-owned working assembly instead of cloning the accumulated
  configuration and history for every added layer. Detach at meaningful ownership
  boundaries; safely share immutable private history where appropriate.
- Updates affected subtrees and their provenance rather than scanning all fields
  for every replacement. Avoid repeatedly copying ever-growing ancestor-origin
  lists or rechecking the same conflict group for every member.
- Reuses one schema/lifecycle evaluation per load and freezes the final result
  once. It does not require global caches, copy-on-write public APIs, or a general
  persistent-tree framework merely to meet an asymptotic target.

Use representative benchmarks and allocation measurements for broad keyed
catalogs, many small layers, repeated replacement, imported filesystem defaults,
and retained history. Compare growth as inputs scale, including small common
loads. Favor changes addressing demonstrated scaling problems; preserve stable
diagnostic order and ownership rather than trading them for a superficial O(n)
claim. No timing threshold or additional release gate is requested.

## Work needed from the baseline

These are responsibility areas, not implementation stages:

- Extend the root-owned source/context facade and private acquisition boundary
  for `fs.FS`; adapt import, glob, overlay, identity, and resource-context handling
  without exporting parser or engine types.
- Add explicit layered loading and programmatic input, sharing one engine path
  with ordinary loading rather than building a parallel configuration system.
- Resolve the fixed operation and ownership decisions below, then extend source
  records, histories, diagnostics, and rendering consistently across input kinds.
- Integrate version-owner selection into layered loading while preserving the
  existing version lifecycle and protections. Keep filesystem-context selection
  independent of version checking.
- Refactor mapping assembly and provenance updates where current algorithms
  would scale poorly under layered workloads. Keep optimization scope tied to
  the new workflow rather than rewriting unrelated library subsystems.
- Add focused behavioral coverage for mixed sources, relative resources,
  namespace collisions, import failures, composition conflicts, explicit empty
  and null values, caller/result independence, and source-correct redaction and
  explanation. Add comparative benchmarks without making performance a brittle
  pass/fail contract.
- Compare equivalent logical inputs across supported source kinds and loading
  interfaces. Assert consistent values, presence, operation outcomes, and origin
  meaning while allowing genuine location/label differences. Cover defaulted and
  derived values, replacement followed by typed defaults, and preservation of
  version ownership/check timing across both loading interfaces. Verify that
  unversioned layered loads need no synthetic version-owning root.
- Provide maintained simple and PromptKit-shaped examples: embedded defaults,
  operator configuration from another context, and labeled programmatic input.
  Demonstrate exact operations and resource origins without provider calls or
  recreating PromptKit itself. The consumer supplies sources, explicit operations,
  types, and domain rules; it does not implement generic merging, presence
  tracking, or origin reconstruction after loading.
- Update Go declaration documentation, the configuration contract, consumer
  guidance, component overview, and relevant architecture/deferred-scope records
  as behavior lands. Record significant boundary changes in an ADR without
  rewriting accepted ADR history. Keep named inheritance explicitly separate.

Plan an additive minor release for the new public capability, preserving current
version semantics. If another design choice requires changing documented behavior,
identify that change expressly and provide migration guidance rather than calling
it an internal optimization.
No release, implementation change, or current-policy revision occurs in this pass.

## Completion criteria

A consumer can load embedded configuration with relative imports, combine it
with independently located operator configuration and explicit programmatic
values, and obtain one typed result with accurate presence, sources, history,
and resource context. No temporary configuration files or serialization round
trip or subsequent generic configuration processing is needed. Simple `Load`
usage retains its current behavior, including version ownership and validation
timing. Layered loading uses the same version policy. Final-value provenance covers input,
defaults, and recorded domain resolution, and metadata distinguishes explicit
zero/false from absence even when the typed values are equal. Benchmarks
demonstrate that adding inputs does not entail avoidable full-collection copying
or quadratic key lookup. Public contracts and examples clearly distinguish
composition, overlay, any selected replacement operation, and downstream domain
resolution. Equivalent-input tests and the maintained consumer example establish
semantic consistency and reduced integration code, not merely API availability.

## Open Decisions

### 1. Whole-definition replacement

**Recommendation:** include a small fixed replacement operation alongside compose
and recursive overlay. For PromptKit-shaped catalogs, explicitly identify a
string-keyed collection path and replace each supplied entry as a whole; retain
unmentioned entries. Do not infer identities from object fields. An empty supplied
collection contributes no entries; an explicitly supplied empty entry replaces
that entry's lower input. Use the same rule for every consumer, not arbitrary
per-field policies. This meets the concrete identity-precedence need without
silently redefining ordinary overlay. Final typed defaults still apply.

**Alternative:** ship only existing composition/overlay semantics and leave
whole-definition selection downstream. This is smaller and closest to today's
surface, but may force consumers to parse/select definitions and reconstruct
origins outside ConfigKit, undermining the request's purpose. Confirm with the
combined consumer example before claiming this narrower scope meets the need.

### 2. Initial programmatic representation

**Recommendation:** labeled string-keyed mappings with the scalar/list/map value
set already supported by explicit overrides. Missing keys express omission and
nil expresses null. Reuse private conversion machinery, not CLI-specific public
records. This is predictable, directly expressible in Go, and consistent with
current dynamic configuration semantics.

**Alternative:** typed Go values with a separate explicit-presence description.
This can reduce downstream mapping code, but introduces reconciliation of field
bindings, masks, nulls, and typed zero values and a larger public contract. Do not
infer omission from zero values. Add typed construction only if real examples
show that the simple mapping representation imposes substantial repetitive work.

### 3. How consumers obtain filesystem resource context

**Recommendation:** a separate source-context query exposing standard filesystem
access and a source-relative base location through root-owned API types, while
ordinary source records remain descriptive. The returned filesystem is a borrowed
capability, not a cloned or immutable object. This avoids a mandatory downstream
registry and supports origins from imports as well as initial sources. Make
lifetime and nil/no-context behavior explicit before implementation.

**Alternative:** expose opaque context IDs and relative paths; callers retain a
registry mapping IDs to their filesystems. Metadata stays capability-free and
serializable, but every consumer must maintain and correctly reconcile another
registry. This is viable if limiting retained filesystem capabilities proves
more important than the additional integration machinery.

### 4. Selecting the version-owning input for layered loads

**Recommendation:** let callers explicitly designate one of the supplied base
inputs as the version owner when configuring a `VersionRule`. It need not be a
host file or a newly manufactured root document. Other contributions retain
fragment/override roles with respect to that path. Explicit designation avoids
making version ownership depend on incidental list order and preserves the
existing selected-source model without changing version acceptance or timing.
No designation is needed when version checking is not configured.

**Alternative:** make the first base input the version owner when checking is
configured. This reduces API ceremony and naturally extends a main-source-plus-
fragments model, but couples ownership to ordering and can make reordering
otherwise independent contributions consequential. Either choice must preserve
the existing version contract; this decision does not reopen effective-version
or per-document version validation.