---
title: Two-Phase Loading
description: Navigation construction splits into two distinct phases: configuration resolution and navigation building. Building navigation requires two fundamentally...
url: https://docs-v3-preview.elastic.dev/elastic/docs-builder/pull/2862/development/navigation/two-phase-loading
---

# Two-Phase Loading
Navigation construction splits into two distinct phases: configuration resolution and navigation building.> **Overview:** For a high-level understanding, see [Functional Principles #1](/elastic/docs-builder/pull/2862/development/navigation/functional-principles#1.-two-phase-loading). This document provides detailed implementation information.

## Why Two Phases?

Building navigation requires two fundamentally different operations:
1. **Loading configuration** - Parse YAML, check files exist, resolve paths
2. **Building structure** - Create tree, set relationships, calculate URLs

These operations have different concerns:

| Aspect            | Configuration           | Navigation               |
|-------------------|-------------------------|--------------------------|
| **Input**         | File system + YAML      | Resolved paths           |
| **Validation**    | Files exist, YAML valid | Tree structure valid     |
| **Errors**        | Missing files, bad YAML | Empty TOCs, broken links |
| **Changes when**  | YAML format changes     | Tree logic changes       |
| **Testing needs** | Mock file system        | Mock config objects      |

Mixing them creates coupling. Configuration parsing shouldn't know about tree structure. Tree building shouldn't touch the file system.
**Concrete benefits:**
**Error messages are clearer:**
```
Phase 1: File 'api/missing.md' not found at /docs/api/missing.md
Phase 2: Folder 'setup' has children defined but none could be created
```

You immediately know which layer failed.
**Testing is simpler:**
```csharp
// Phase 1 test: Does path resolution work?
[Fact] void ResolvesNestedPaths() { /* mock file system */ }

// Phase 2 test: Does tree structure work?
[Fact] void CreatesNavigationTree() { /* mock config, no files */ }
```

Each phase tests one thing.
**Configuration reuses:**
```csharp
// Parse once
var config = DocumentationSetFile.LoadAndResolve(yaml, fileSystem);

// Build multiple ways
var isolated = new DocumentationSetNavigation(config, isolatedContext, factory);
var assembled = new SiteNavigation(siteConfig, context, [isolated], prefix);
```

Same configuration, different navigation structures.
**Separation of concerns:**
- Change YAML format → only Phase 1 changes
- Change URL calculation → only Phase 2 changes
- Swap YAML for JSON → only Phase 1 changes
- Add new node type → only Phase 2 changes


## Phase 1: Configuration Resolution

**Package:** `Elastic.Documentation.Configuration`
**Goal:** Parse YAML → Resolve paths → Validate existence
```
Raw YAML + File System → Fully resolved configuration
```

**What it does:**
1. Parse YAML files (`docset.yml`, `toc.yml`, `navigation.yml`)
2. Resolve relative paths to absolute paths from docset root
3. Validate files exist on disk
4. Load nested `toc.yml` files recursively
5. Emit configuration errors

**Example:**
```csharp
// In: Raw YAML
toc:
  - toc: api
    # api/toc.yml contains:
    toc:
      - file: rest.md  # Relative to api/toc.yml

// Out: Fully resolved
FileRef {
    PathRelativeToDocumentationSet = "api/rest.md" 
}
```

**Key point:** All paths become relative to docset root. No more file I/O needed.

## Phase 2: Navigation Construction

**Package:** `Elastic.Documentation.Navigation`
**Goal:** Build tree → Calculate URLs → Set relationships
```
Resolved Configuration → Navigation tree with URLs
```

**What it does:**
1. Create node objects from configuration
2. Set parent-child relationships
3. Set up home providers (for URL calculation)
4. Calculate navigation indexes
5. Emit navigation errors

**Example:**
```csharp
// In: Resolved configuration
FileRef { PathRelativeToDocumentationSet = "api/rest.md" }

// Out: Navigation with URL
FileNavigationLeaf {
    Url = "/api/rest/",
    Parent = TableOfContentsNavigation,
    NavigationRoot = DocumentationSetNavigation
}
```

**Key point:** URLs calculated dynamically from HomeProvider. No stored paths.

## The Flow

```
┌─────────────────────────────────────┐
│ Phase 1: Configuration              │
├─────────────────────────────────────┤
│ YAML files + File system            │
│           ↓                         │
│ Parse & validate                    │
│           ↓                         │
│ Resolve all paths                   │
│           ↓                         │
│ DocumentationSetFile                │
│ (all paths relative to docset root) │
└─────────────────────────────────────┘
                ↓
┌─────────────────────────────────────┐
│ Phase 2: Navigation                 │
├─────────────────────────────────────┤
│ Resolved configuration              │
│           ↓                         │
│ Build tree                          │
│           ↓                         │
│ Set relationships                   │
│           ↓                         │
│ Set up URL providers                │
│           ↓                         │
│ DocumentationSetNavigation          │
│ (complete tree with URLs)           │
└─────────────────────────────────────┘
```


## Path Resolution Example

**Before Phase 1:**
```yaml
# docset.yml
toc:
  - toc: api

# api/toc.yml
toc:
  - file: rest.md      # ← Relative to api/
  - file: graphql.md   # ← Relative to api/
```

**After Phase 1:**
```csharp
IsolatedTableOfContentsRef {
    PathRelativeToDocumentationSet = "api",
    Children = [
        FileRef { PathRelativeToDocumentationSet = "api/rest.md" },    
        FileRef { PathRelativeToDocumentationSet = "api/graphql.md" }  
    ]
}
```

All paths now relative to docset root. Phase 2 can build without touching filesystem.

## Error Attribution

Clear errors because phases are separate:
**Phase 1 errors (configuration):**
```
Error: File 'api/missing.md' not found at /docs/api/missing.md
Error: TableOfContents 'api' cannot have children in docset.yml
```

→ Fix your YAML or add the file.
**Phase 2 errors (navigation):**
```
Error: Documentation set has no table of contents defined
Error: Folder 'setup' has children defined but none could be created
```

→ Fix your navigation structure.

## Testing Benefits

**Phase 1 tests:**
```csharp
[Fact]
public void LoadAndResolve_ResolvesNestedPaths()
{
    var yaml = "toc:\n  - toc: api";
    var fs = new MockFileSystem();
    fs.AddFile("/docs/api/toc.yml", "toc:\n  - file: rest.md");

    var docset = DocumentationSetFile.LoadAndResolve(
        collector, yaml, fs.NewDirInfo("/docs")
    );

    var fileRef = docset.TableOfContents[0].Children[0] as FileRef;
    Assert.Equal("api/rest.md", fileRef.PathRelativeToDocumentationSet);
}
```

Tests YAML parsing and path resolution.
**Phase 2 tests:**
```csharp
[Fact]
public void Constructor_CreatesNavigationTree()
{
    // Pre-resolved configuration (no file I/O!)
    var docset = new DocumentationSetFile {
        TableOfContents = [
            new FileRef { PathRelativeToDocumentationSet = "index.md" }
        ]
    };

    var nav = new DocumentationSetNavigation<IDocumentationFile>(
        docset, context, factory
    );

    Assert.Equal("/", nav.Index.Url);
}
```

Tests tree construction without file system.

## Reusability

Same configuration works for both build modes:
```csharp
// Phase 1: Build configuration once
var docset = DocumentationSetFile.LoadAndResolve(
    collector, yaml, fileSystem.NewDirInfo("/docs")
);

// Phase 2a: Isolated build
var isolatedNav = new DocumentationSetNavigation<IDocumentationFile>(
    docset, 
    isolatedContext,
    factory
);
// URLs: /api/rest/

// Phase 2b: Assembler build
var siteNav = new SiteNavigation(
    siteConfig,
    assemblerContext,
    [isolatedNav], 
    sitePrefix: null
);
// Re-home: /api/rest/ → /elasticsearch/api/rest/
```


## Assembler Extension

Assembler adds two more phases:
```
Phase 1a: Load individual docset configs
    ↓
Phase 2a: Build isolated navigations
    ↓
Phase 1b: Load site navigation config
    ↓
Phase 2b: Assemble + re-home
```

Each docset goes through Phases 1 & 2 independently, then site navigation assembles them.

## Key Invariants

**After Phase 1:**
- ✅ All paths relative to docset root
- ✅ All files validated to exist
- ✅ All nested TOCs loaded
- ✅ Configuration structure validated

**After Phase 2:**
- ✅ Complete navigation tree
- ✅ All relationships set (parent/child/root)
- ✅ All home providers configured
- ✅ All URLs calculable


## Summary


| Aspect       | Phase 1            | Phase 2          |
|--------------|--------------------|------------------|
| **Package**  | `Configuration`    | `Navigation`     |
| **Input**    | YAML + File system | Resolved config  |
| **Output**   | Resolved config    | Navigation tree  |
| **Errors**   | Config/file issues | Structure issues |
| **File I/O** | Yes                | No               |
| **Testing**  | Mock file system   | Mock config      |
| **Reusable** | Yes (both builds)  | Build-specific   |

**The key insight:** Configuration is about files and YAML. Navigation is about tree structure and URLs. Keep them separate.