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<main>
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<h1 id="adr-001-nickel-vs-toml-for-configuration"><a class="header" href="#adr-001-nickel-vs-toml-for-configuration">ADR-001: Nickel vs TOML for Configuration</a></h1>
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<p><strong>Status</strong>: Accepted</p>
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<p><strong>Date</strong>: 2026-01-17</p>
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<p><strong>Deciders</strong>: Architecture Team</p>
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<p><strong>Context</strong>: Configuration Strategy for Knowledge Base System</p>
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<hr />
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<h2 id="context"><a class="header" href="#context">Context</a></h2>
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<p>The KOGRAL requires a flexible, type-safe configuration format that supports:</p>
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<ol>
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<li><strong>Complex nested structures</strong> (graph settings, storage configs, embedding providers)</li>
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<li><strong>Type validation</strong> (prevent runtime errors from config mistakes)</li>
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<li><strong>Composition and inheritance</strong> (shared configs, environment-specific overrides)</li>
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<li><strong>Documentation</strong> (self-documenting schemas)</li>
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<li><strong>Validation before runtime</strong> (catch errors early)</li>
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</ol>
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<p>We evaluated two primary options:</p>
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<h3 id="option-1-toml-traditional-config-format"><a class="header" href="#option-1-toml-traditional-config-format">Option 1: TOML (Traditional Config Format)</a></h3>
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<p><strong>Pros</strong>:</p>
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<ul>
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<li>Widely adopted in Rust ecosystem (<code>Cargo.toml</code>)</li>
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<li>Simple, human-readable syntax</li>
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<li>Native <code>serde</code> support</li>
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<li>IDE support (syntax highlighting, completion)</li>
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</ul>
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<p><strong>Cons</strong>:</p>
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<ul>
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<li>No type system (validation only at runtime)</li>
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<li>Limited composition (no imports, no functions)</li>
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<li>No schema validation (errors discovered during execution)</li>
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<li>Verbose for complex nested structures</li>
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<li>No documentation in config files</li>
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</ul>
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<p><strong>Example TOML</strong>:</p>
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<pre><code class="language-toml">[graph]
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name = "my-project"
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version = "1.0.0"
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[storage]
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primary = "filesystem" # String, not validated as enum
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[storage.secondary]
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enabled = true
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type = "surrealdb" # Typo would fail at runtime
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url = "ws://localhost:8000"
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[embeddings]
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enabled = true
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provider = "openai" # No validation of valid providers
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model = "text-embedding-3-small"
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</code></pre>
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<p><strong>Problems</strong>:</p>
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<ul>
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<li>Typos in enum values (<code>"surrealdb"</code> vs <code>"surealdb"</code>) fail at runtime</li>
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<li>No validation that <code>provider = "openai"</code> requires <code>api_key_env</code></li>
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<li>No documentation of valid options</li>
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<li>No way to compose configs (e.g., base config + environment override)</li>
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</ul>
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<h3 id="option-2-nickel-functional-configuration-language"><a class="header" href="#option-2-nickel-functional-configuration-language">Option 2: Nickel (Functional Configuration Language)</a></h3>
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<p><strong>Pros</strong>:</p>
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<ul>
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<li><strong>Type system</strong> with contracts (validate before runtime)</li>
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<li><strong>Composition</strong> via imports and merging</li>
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<li><strong>Documentation</strong> in schemas (self-documenting)</li>
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<li><strong>Validation</strong> at export time (catch errors early)</li>
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<li><strong>Functions</strong> for conditional logic</li>
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<li><strong>Default values</strong> in schema definitions</li>
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</ul>
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<p><strong>Cons</strong>:</p>
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<ul>
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<li>Less familiar to Rust developers</li>
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<li>Requires separate <code>nickel</code> CLI tool</li>
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<li>Smaller ecosystem</li>
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<li>Steeper learning curve</li>
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</ul>
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<p><strong>Example Nickel</strong>:</p>
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<pre><code class="language-nickel"># schemas/kb-config.ncl
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{
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KbConfig = {
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graph | GraphConfig,
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storage | StorageConfig,
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embeddings | EmbeddingConfig,
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},
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StorageConfig = {
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primary | [| 'filesystem, 'memory |], # Enum validated at export
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secondary | {
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enabled | Bool,
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type | [| 'surrealdb, 'sqlite |], # Typos caught immediately
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url | String,
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} | optional,
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},
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EmbeddingConfig = {
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enabled | Bool,
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provider | [| 'openai, 'claude, 'fastembed |], # Valid providers enforced
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model | String,
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api_key_env | String | doc "Environment variable for API key",
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},
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}
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</code></pre>
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<p><strong>Benefits</strong>:</p>
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<ul>
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<li>Typos in enum values caught at <code>nickel export</code> time</li>
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<li>Schema enforces required fields based on provider</li>
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<li>Documentation embedded in schema</li>
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<li>Config can be composed: <code>import "base.ncl" & { /* overrides */ }</code></li>
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</ul>
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<hr />
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<h2 id="decision"><a class="header" href="#decision">Decision</a></h2>
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<p><strong>We will use Nickel for configuration.</strong></p>
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<p><strong>Implementation</strong>:</p>
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<ol>
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<li>Define schemas in <code>schemas/*.ncl</code> with type contracts</li>
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<li>Users write configs in <code>.kogral/config.ncl</code></li>
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<li>Export to JSON via CLI: <code>nickel export --format json config.ncl</code></li>
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<li>Load JSON in Rust via <code>serde_json</code> into typed structs</li>
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</ol>
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<p><strong>Pattern</strong> (double validation):</p>
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<pre><code>Nickel Config (.ncl)
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↓ [nickel export]
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JSON (validated by Nickel contracts)
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↓ [serde_json::from_str]
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Rust Struct (validated by serde)
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↓
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Runtime (guaranteed valid config)
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</code></pre>
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<p><strong>Bridge Code</strong> (<code>kb-core/src/config/nickel.rs</code>):</p>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>pub fn load_config<P: AsRef<Path>>(path: P) -> Result<KbConfig> {
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// Export Nickel to JSON
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let json = export_nickel_to_json(path)?;
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// Deserialize to Rust struct
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let config: KbConfig = serde_json::from_str(&json)?;
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Ok(config)
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}
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fn export_nickel_to_json<P: AsRef<Path>>(path: P) -> Result<String> {
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let output = Command::new("nickel")
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|
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.arg("export")
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.arg("--format").arg("json")
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.arg(path.as_ref())
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.output()?;
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||
|
|
|
||
|
|
Ok(String::from_utf8(output.stdout)?)
|
||
|
|
}
|
||
|
|
<span class="boring">}</span></code></pre></pre>
|
||
|
|
<hr />
|
||
|
|
<h2 id="consequences"><a class="header" href="#consequences">Consequences</a></h2>
|
||
|
|
<h3 id="positive"><a class="header" href="#positive">Positive</a></h3>
|
||
|
|
<p>✅ <strong>Type Safety</strong>: Config errors caught before runtime</p>
|
||
|
|
<ul>
|
||
|
|
<li>Invalid enum values fail at export: <code>'filesystm</code> → error</li>
|
||
|
|
<li>Missing required fields detected: no <code>graph.name</code> → error</li>
|
||
|
|
<li>Type mismatches prevented: <code>enabled = "yes"</code> → error (expects Bool)</li>
|
||
|
|
</ul>
|
||
|
|
<p>✅ <strong>Self-Documenting</strong>: Schemas serve as documentation</p>
|
||
|
|
<ul>
|
||
|
|
<li><code>| doc "Environment variable for API key"</code> describes fields</li>
|
||
|
|
<li>Enum options visible in schema: <code>[| 'openai, 'claude, 'fastembed |]</code></li>
|
||
|
|
<li>Default values explicit: <code>| default = 'filesystem</code></li>
|
||
|
|
</ul>
|
||
|
|
<p>✅ <strong>Composition</strong>: Config reuse and overrides</p>
|
||
|
|
<pre><code class="language-nickel"># base.ncl
|
||
|
|
{ graph = { version = "1.0.0" } }
|
||
|
|
|
||
|
|
# project.ncl
|
||
|
|
import "base.ncl" & { graph = { name = "my-project" } }
|
||
|
|
</code></pre>
|
||
|
|
<p>✅ <strong>Validation Before Deployment</strong>: Catch errors in CI</p>
|
||
|
|
<pre><code class="language-bash"># CI pipeline
|
||
|
|
nickel typecheck config.ncl
|
||
|
|
nickel export --format json config.ncl > /dev/null
|
||
|
|
</code></pre>
|
||
|
|
<p>✅ <strong>Conditional Logic</strong>: Environment-specific configs</p>
|
||
|
|
<pre><code class="language-nickel">let is_prod = std.string.is_match "prod" (std.env.get "ENV") in
|
||
|
|
{
|
||
|
|
embeddings = {
|
||
|
|
provider = if is_prod then 'openai else 'fastembed,
|
||
|
|
},
|
||
|
|
}
|
||
|
|
</code></pre>
|
||
|
|
<h3 id="negative"><a class="header" href="#negative">Negative</a></h3>
|
||
|
|
<p>❌ <strong>Learning Curve</strong>: Team must learn Nickel syntax</p>
|
||
|
|
<ul>
|
||
|
|
<li><strong>Mitigation</strong>: Provide comprehensive examples in <code>config/</code> directory</li>
|
||
|
|
<li><strong>Mitigation</strong>: Document common patterns in <code>docs/config/</code></li>
|
||
|
|
</ul>
|
||
|
|
<p>❌ <strong>Tool Dependency</strong>: Requires <code>nickel</code> CLI installed</p>
|
||
|
|
<ul>
|
||
|
|
<li><strong>Mitigation</strong>: Document installation in setup guide</li>
|
||
|
|
<li><strong>Mitigation</strong>: Check <code>nickel</code> availability in <code>kb init</code> command</li>
|
||
|
|
</ul>
|
||
|
|
<p>❌ <strong>IDE Support</strong>: Limited compared to TOML</p>
|
||
|
|
<ul>
|
||
|
|
<li><strong>Mitigation</strong>: Use LSP (nickel-lang-lsp) for VSCode/Neovim</li>
|
||
|
|
<li><strong>Mitigation</strong>: Syntax highlighting available for major editors</li>
|
||
|
|
</ul>
|
||
|
|
<p>❌ <strong>Ecosystem Size</strong>: Smaller than TOML</p>
|
||
|
|
<ul>
|
||
|
|
<li><strong>Mitigation</strong>: Nickel actively developed by Tweag</li>
|
||
|
|
<li><strong>Mitigation</strong>: Stable language specification (v1.0+)</li>
|
||
|
|
</ul>
|
||
|
|
<h3 id="neutral"><a class="header" href="#neutral">Neutral</a></h3>
|
||
|
|
<p>⚪ <strong>Two-Stage Loading</strong>: Nickel → JSON → Rust</p>
|
||
|
|
<ul>
|
||
|
|
<li>Not a performance concern (config loaded once at startup)</li>
|
||
|
|
<li>Adds resilience (double validation)</li>
|
||
|
|
<li>Allows runtime config inspection (read JSON directly)</li>
|
||
|
|
</ul>
|
||
|
|
<hr />
|
||
|
|
<h2 id="alternatives-considered"><a class="header" href="#alternatives-considered">Alternatives Considered</a></h2>
|
||
|
|
<h3 id="json-schema"><a class="header" href="#json-schema">JSON Schema</a></h3>
|
||
|
|
<p><strong>Rejected</strong>: Not ergonomic for humans to write</p>
|
||
|
|
<ul>
|
||
|
|
<li>No comments</li>
|
||
|
|
<li>Verbose syntax (<code>{"key": "value"}</code> vs <code>key = value</code>)</li>
|
||
|
|
<li>JSON Schema separate from config (duplication)</li>
|
||
|
|
</ul>
|
||
|
|
<h3 id="yaml"><a class="header" href="#yaml">YAML</a></h3>
|
||
|
|
<p><strong>Rejected</strong>: No type system, ambiguous parsing</p>
|
||
|
|
<ul>
|
||
|
|
<li>Boolean confusion: <code>yes</code>/<code>no</code>/<code>on</code>/<code>off</code>/<code>true</code>/<code>false</code></li>
|
||
|
|
<li>Indentation-sensitive (error-prone)</li>
|
||
|
|
<li>No validation without external tools</li>
|
||
|
|
</ul>
|
||
|
|
<h3 id="dhall"><a class="header" href="#dhall">Dhall</a></h3>
|
||
|
|
<p><strong>Rejected</strong>: More complex than needed</p>
|
||
|
|
<ul>
|
||
|
|
<li>Turing-incomplete by design (limits use cases)</li>
|
||
|
|
<li>Smaller ecosystem than Nickel</li>
|
||
|
|
<li>Steeper learning curve</li>
|
||
|
|
</ul>
|
||
|
|
<h3 id="kcl-kusionstack-configuration-language"><a class="header" href="#kcl-kusionstack-configuration-language">KCL (KusionStack Configuration Language)</a></h3>
|
||
|
|
<p><strong>Rejected</strong>: Kubernetes-focused, less general-purpose</p>
|
||
|
|
<ul>
|
||
|
|
<li>Designed for K8s manifests</li>
|
||
|
|
<li>Less mature than Nickel for general config</li>
|
||
|
|
</ul>
|
||
|
|
<hr />
|
||
|
|
<h2 id="implementation-timeline"><a class="header" href="#implementation-timeline">Implementation Timeline</a></h2>
|
||
|
|
<ol>
|
||
|
|
<li>✅ Define base schemas (<code>schemas/kb-config.ncl</code>)</li>
|
||
|
|
<li>✅ Implement Nickel loader (<code>kb-core/src/config/nickel.rs</code>)</li>
|
||
|
|
<li>✅ Create example configs (<code>config/defaults.ncl</code>, <code>config/production.ncl</code>)</li>
|
||
|
|
<li>✅ Document Nickel usage (<code>docs/config/nickel-schemas.md</code>)</li>
|
||
|
|
<li>⏳ Add LSP recommendations to setup guide</li>
|
||
|
|
<li>⏳ Create Nickel → TOML migration tool (for existing users)</li>
|
||
|
|
</ol>
|
||
|
|
<hr />
|
||
|
|
<h2 id="monitoring"><a class="header" href="#monitoring">Monitoring</a></h2>
|
||
|
|
<p><strong>Success Criteria</strong>:</p>
|
||
|
|
<ul>
|
||
|
|
<li>Config errors caught at export time (not runtime)</li>
|
||
|
|
<li>Users can compose configs for different environments</li>
|
||
|
|
<li>Team comfortable with Nickel syntax within 2 weeks</li>
|
||
|
|
</ul>
|
||
|
|
<p><strong>Metrics</strong>:</p>
|
||
|
|
<ul>
|
||
|
|
<li>Number of config validation errors caught before runtime</li>
|
||
|
|
<li>Time to diagnose config issues (should decrease)</li>
|
||
|
|
<li>User feedback on config complexity</li>
|
||
|
|
</ul>
|
||
|
|
<hr />
|
||
|
|
<h2 id="references"><a class="header" href="#references">References</a></h2>
|
||
|
|
<ul>
|
||
|
|
<li><a href="https://nickel-lang.org/">Nickel Language</a></li>
|
||
|
|
<li><a href="https://nickel-lang.org/user-manual/introduction">Nickel User Manual</a></li>
|
||
|
|
<li><a href="../../crates/kb-core/src/config/README.html">platform-config pattern</a> (reference implementation)</li>
|
||
|
|
<li><a href="https://toml.io/">TOML Specification</a></li>
|
||
|
|
</ul>
|
||
|
|
<hr />
|
||
|
|
<h2 id="revision-history"><a class="header" href="#revision-history">Revision History</a></h2>
|
||
|
|
<div class="table-wrapper"><table><thead><tr><th>Date</th><th>Author</th><th>Change</th></tr></thead><tbody>
|
||
|
|
<tr><td>2026-01-17</td><td>Architecture Team</td><td>Initial decision</td></tr>
|
||
|
|
</tbody></table>
|
||
|
|
</div>
|
||
|
|
<hr />
|
||
|
|
<p><strong>Next ADR</strong>: <a href="002-fastembed-ai-providers.html">ADR-002: FastEmbed via AI Providers</a></p>
|
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