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

Filesystem Sources and Source-Backed Underlays

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

Address the filesystem and layered-input needs in the PromptKit request as one coherent extension: native fs.FS sources and explicit source-backed underlays, including programmatic input. ConfigKit's recursive combination model takes precedence over PromptKit's requested whole-definition replacement. 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 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, 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

Extend public loading with explicitly supplied lower-precedence configuration sources, using the same typed definition/result lifecycle as ordinary Load. Support native filesystem, host-file, byte, reader, and programmatic inputs. The central workflow is bundled defaults overridden by application configuration, not an unrestricted sequence of user-selectable merge algorithms. Underlays can form explicitly ordered default tiers; ConfigKit does not discover priorities or know about PromptKit's catalog tiers. Source kind does not confer priority: programmatic data can supply the appropriate supported input role without being misrepresented as CLI flags.

The precedence model, from lowest to highest, is:

Typed application defaults < source underlays < strictly composed main configuration < selected overlay < explicit CLI overrides.

Typed defaults remain the existing Go-value baseline. Source underlays add configuration-backed defaults with imports, filesystem context, and source provenance; they are not a replacement for the typed-defaults facility.

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. Strictly composing independent contributions and intentionally overlaying precedence tiers express 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.

Underlay is a precedence role, not another combination algorithm. Assemble the main document and all its imports by strict composition before applying that result over the underlays using recursive overlay. A conflicting main/import definition still fails, even if it would overwrite an underlay or be overwritten by the selected overlay. Never flatten main and imported documents into a last-wins list.

An underlay source may bring its explicitly enabled imports and source context. Independent contributions within an underlay tier compose strictly; intentional precedence between explicitly ordered tiers uses recursive overlay. Discovery order does not establish tiers or authorize overriding. Syntax, source, and composition failures cannot be hidden by higher-priority input.

There is no whole-definition replacement mode, per-layer recursive/atomic switch, collection-specific identity matching, implicit sequence merge, profile registry, or merge callback in this feature. Existing atomic scalar, sequence, null, and cross-kind replacement under overlay remains unchanged.

Preserve the main configuration's version check after its strict composition and schema construction, before precedence assembly with underlays, selected overlay, 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.

Recorded application resolution follows input assembly and decoding. 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.

Higher-priority mappings retain omitted lower-input children. Empty mappings do not clear those children; sequences, including empty sequences, replace as a unit, and null remains explicit subject to the final schema. Typed defaults continue to fill fields absent from the assembled input under existing rules. No reset/delete operator is added.

For example, a bundled profile containing {model: example-model, timeout: 30} combined with application input {model: another-model} retains timeout: 30 from the bundle. Its model origin points to the application and its timeout origin points to the underlay. PromptKit must validate that final profile rather than assume the application entry discarded all bundled settings. This knowingly differs from its requested identity-based whole-definition replacement. Consumer examples should adopt the shared semantics, not implement a hidden replacement pass after loading. A replacement operation could be considered separately if future concrete needs justify it, but is not an open choice in this roadmap.

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 underlays, overlays, or CLI overrides cannot supply or alter it, even to an equal value. Preserve the current version-path and ancestor protections when integrating the new input role.

Keep the checker at its existing lifecycle point: after base composition and schema construction, before underlay precedence assembly, selected 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 3 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 1 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 underlay tiers, repeated scalar/sequence 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 source-backed underlays and programmatic input, sharing one engine path with ordinary loading rather than building a parallel configuration system.
• Resolve the representation 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, recursively retained underlay fields, unchanged atomic-value replacement, typed defaults, strict main/import conflicts, 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. Show explicitly that a same-name profile recursively combines, rather than replacing its whole lower definition. 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 strict within-tier composition, recursive cross-tier overlay, 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. 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.

2. 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.

3. Selecting the version-owning input for layered loads

Recommendation: the designated main source owns the version when configuring a VersionRule, extending the existing selected-source role rather than letting an underlay win ownership by list position. It need not be a host file or a newly manufactured root document. Its imports remain fragments and underlays cannot declare the application's version. No special version-owner designation is needed when checking is not configured, including loads using only underlays.

Alternative: allow explicit designation of a base input as the version owner separately from its main/underlay role. This supports bundles carrying the application version, but adds another ownership choice and complicates the otherwise clear distinction between defaults and application configuration. Either choice must preserve the existing version contract; this decision does not reopen effective-version or per-document version validation.