Architecture Patterns

Overview

This guide explains the architectural patterns and design principles used throughout the Grounded Docs MCP Server. Read ARCHITECTURE.md in the source repository for the complete system architecture.

Core Architectural Patterns

Tools Layer Pattern

Business logic resides in the tools layer to enable code reuse across interfaces. Why: CLI commands, MCP endpoints, and web routes should all use the same logic. Structure:

Interfaces (CLI, MCP, Web)
       ↓
   Tools Layer (Business Logic)
       ↓
  Pipeline & Storage Services

Example:

// src/tools/SearchTool.ts
export class SearchTool {
  constructor(private docService: IDocumentManagement) {}
  
  async execute(options: SearchToolOptions): Promise<SearchToolResult> {
    // Validation
    if (!options.library) {
      throw new ValidationError('Library name is required');
    }
    
    // Business logic
    const results = await this.docService.search({
      library: options.library,
      query: options.query,
      limit: options.limit ?? 5
    });
    
    return { results };
  }
}

Usage across interfaces:

// CLI
const tool = new SearchTool(documentService);
const result = await tool.execute({ library: 'react', query: 'hooks' });
console.log(result.results);

// MCP Server
server.setRequestHandler(SearchRequestSchema, async (request) => {
  const tool = new SearchTool(documentService);
  return await tool.execute(request.params);
});

// Web API
app.post('/search', async (req, reply) => {
  const tool = new SearchTool(documentService);
  const result = await tool.execute(req.body);
  return result;
});

Write-Through Architecture

Pipeline jobs serve as the single source of truth, with all state changes immediately persisted to the database. Benefits:

Enables recovery after crashes
Provides real-time progress tracking
Ensures consistency between memory and storage

Implementation:

// src/pipeline/PipelineManager.ts
export class PipelineManager {
  private jobs = new Map<string, PipelineJob>();
  
  async updateJobStatus(jobId: string, status: JobStatus) {
    const job = this.jobs.get(jobId);
    if (!job) return;
    
    // Update in-memory state
    job.status = status;
    
    // Immediately persist to database (write-through)
    await this.versionService.updateVersion(jobId, {
      status,
      updatedAt: new Date().toISOString()
    });
    
    // Emit event for real-time updates
    this.eventBus.emit('job:status', { jobId, status });
  }
}

Functionality-Based Design

Components are selected based on capability requirements, not deployment context. Pattern:

// src/pipeline/PipelineFactory.ts
export class PipelineFactory {
  static create(config: AppConfig): IPipeline {
    // Choose based on functionality needed
    if (config.pipeline.serverUrl) {
      // Need remote worker capability
      return new PipelineClient(config.pipeline.serverUrl);
    }
    
    if (config.pipeline.recoverJobs) {
      // Need job recovery capability
      return new PipelineManager({
        eventBus,
        versionService,
        maxConcurrent: config.pipeline.maxConcurrent,
        recoverJobs: true
      });
    }
    
    // Need immediate execution only
    return new PipelineManager({
      eventBus,
      versionService,
      maxConcurrent: config.pipeline.maxConcurrent,
      recoverJobs: false
    });
  }
}

Protocol Abstraction

Transport layer abstracts stdio vs HTTP differences, enabling identical functionality across access methods. Auto-detection:

// src/index.ts
function detectProtocol(): 'stdio' | 'http' {
  // No TTY = AI tool using stdio
  if (!process.stdin.isTTY) {
    return 'stdio';
  }
  
  // Has TTY = interactive terminal using HTTP
  return 'http';
}

const protocol = args.protocol ?? detectProtocol();

if (protocol === 'stdio') {
  await startStdioServer(config);
} else {
  await startHttpServer(config);
}

System Architecture Modes

Unified Mode (In-Process)

All components run in a single process. When to use:

Development environment
Simple deployments
Single-user scenarios

Event flow:

CLI/Web/MCP → PipelineManager → PipelineWorker
                      ↓
                  EventBus
                      ↓
          Real-time Updates to All

Code:

// Unified mode setup
const eventBus = new EventBus();
const pipeline = new PipelineManager({
  eventBus,
  versionService,
  maxConcurrent: 3,
  recoverJobs: true
});

const appServer = new AppServer({
  pipeline,
  eventBus,
  documentService
});

Distributed Mode (Hub & Spoke)

Separate coordinator and worker processes. When to use:

Production deployments
Scaling workload across containers
Multiple coordinators sharing one worker

Event flow:

Coordinators → PipelineClient → tRPC → Worker
     ↓              ↓                      ↓
Local EventBus ← RemoteEventProxy ← Worker EventBus

Worker setup:

// Worker process
const worker = new PipelineWorker({
  eventBus,
  versionService,
  documentService
});

const trpcRouter = createPipelineRouter(worker);
const server = createTRPCServer(trpcRouter);
server.listen(3001);

Coordinator setup:

// Coordinator process
const client = new PipelineClient('http://worker:3001');
const proxy = new RemoteEventProxy(client, eventBus);

const appServer = new AppServer({
  pipeline: client,
  eventBus,
  documentService
});

Content Processing Architecture

Content processing follows a modular pipeline:

Strategy → Fetcher → Pipeline → Splitter → Embedder → Storage

Scraper Strategies

Handle different source types:

// src/scraper/strategies/WebScraperStrategy.ts
export class WebScraperStrategy extends BaseScraperStrategy {
  async scrape(config: ScrapeConfig): Promise<Document[]> {
    // 1. Fetch content
    const pages = await this.fetchPages(config.url);
    
    // 2. Process through pipeline
    const documents = await this.processPipeline(pages);
    
    // 3. Return processed documents
    return documents;
  }
}

Content Pipelines

Transform content using middleware chains:

// src/scraper/pipelines/HtmlPipeline.ts
export class HtmlPipeline implements ContentPipeline {
  async process(content: string, url: string): Promise<ProcessedContent> {
    // Middleware chain
    let processed = content;
    
    // 1. Clean HTML
    processed = await this.cleanHtml(processed);
    
    // 2. Extract main content
    processed = await this.extractMainContent(processed);
    
    // 3. Convert to markdown
    processed = await this.convertToMarkdown(processed);
    
    return {
      content: processed,
      metadata: { url, contentType: 'text/markdown' }
    };
  }
}

Document Splitters

Two-phase splitting approach: Phase 1: Semantic Splitting

// src/splitter/SemanticMarkdownSplitter.ts
export class SemanticMarkdownSplitter {
  split(document: Document): Chunk[] {
    // Preserve document structure
    const sections = this.parseMarkdownSections(document.content);
    
    return sections.map(section => ({
      content: section.content,
      metadata: {
        heading: section.heading,
        level: section.level,
        path: section.path
      }
    }));
  }
}

Phase 2: Size Optimization

// src/splitter/GreedySplitter.ts
export class GreedySplitter {
  optimize(chunks: Chunk[], targetSize: number): Chunk[] {
    // Combine or split chunks to target size
    return this.greedyOptimization(chunks, targetSize);
  }
}

Event-Driven Architecture

The EventBus decouples producers from consumers:

// src/services/EventBus.ts
export class EventBus {
  private listeners = new Map<string, Set<EventListener>>();
  
  emit(event: string, data: unknown): void {
    const listeners = this.listeners.get(event) ?? new Set();
    for (const listener of listeners) {
      listener(data);
    }
  }
  
  on(event: string, listener: EventListener): void {
    if (!this.listeners.has(event)) {
      this.listeners.set(event, new Set());
    }
    this.listeners.get(event)!.add(listener);
  }
}

Usage:

// Producer (PipelineManager)
this.eventBus.emit('job:status', {
  jobId: job.id,
  status: 'RUNNING',
  progress: 0.5
});

// Consumer (Web UI)
eventBus.on('job:status', (data) => {
  updateJobDisplay(data.jobId, data.status, data.progress);
});

// Consumer (MCP Server)
eventBus.on('job:status', (data) => {
  sendNotification(data);
});

Storage Architecture

Normalized SQLite schema with three core tables:

-- Libraries table
CREATE TABLE libraries (
  id INTEGER PRIMARY KEY,
  library TEXT UNIQUE NOT NULL,
  organization TEXT
);

-- Versions table (job state hub)
CREATE TABLE versions (
  id INTEGER PRIMARY KEY,
  library_id INTEGER NOT NULL,
  version TEXT NOT NULL,
  status TEXT NOT NULL,  -- Job status
  progress REAL,         -- Job progress
  error TEXT,            -- Job error
  config TEXT,           -- Scraper config (for refresh)
  FOREIGN KEY (library_id) REFERENCES libraries(id)
);

-- Documents table
CREATE TABLE documents (
  id INTEGER PRIMARY KEY,
  version_id INTEGER NOT NULL,
  content TEXT NOT NULL,
  embedding BLOB,        -- Vector embedding
  metadata TEXT,         -- JSON metadata
  FOREIGN KEY (version_id) REFERENCES versions(id)
);

Hybrid Search:

// src/store/DocumentRetrieverService.ts
export class DocumentRetrieverService {
  async search(options: SearchOptions): Promise<SearchResult[]> {
    // 1. Vector similarity search
    const vectorResults = await this.vectorSearch(options.query);
    
    // 2. Full-text search (FTS5)
    const ftsResults = await this.fullTextSearch(options.query);
    
    // 3. Combine using Reciprocal Rank Fusion (RRF)
    const combined = this.reciprocalRankFusion(
      vectorResults,
      ftsResults,
      { vectorWeight: 0.7, ftsWeight: 0.3 }
    );
    
    return combined.slice(0, options.limit);
  }
}

Configuration System

Configuration resolves once per process with strict validation:

// src/utils/config.ts
import { z } from 'zod';

const AppConfigSchema = z.object({
  database: z.object({
    path: z.string(),
    migrations: z.boolean().default(true)
  }),
  pipeline: z.object({
    maxConcurrent: z.number().min(1).max(10).default(3),
    serverUrl: z.string().url().optional(),
    recoverJobs: z.boolean().default(true)
  }),
  embeddings: z.object({
    provider: z.enum(['openai', 'azure', 'google', 'aws']),
    model: z.string()
  })
});

export function loadConfig(): AppConfig {
  // 1. Load defaults
  const config = { ...DEFAULT_CONFIG };
  
  // 2. Merge config file
  const fileConfig = loadConfigFile();
  Object.assign(config, fileConfig);
  
  // 3. Merge environment variables
  const envConfig = loadEnvConfig();
  Object.assign(config, envConfig);
  
  // 4. Merge CLI arguments
  const cliConfig = parseCLIArgs();
  Object.assign(config, cliConfig);
  
  // 5. Validate with Zod
  return AppConfigSchema.parse(config);
}

Design Principles

Single Responsibility

Each component has one clear purpose:

PipelineManager: Job queue and coordination
PipelineWorker: Job execution
EventBus: Event distribution
DocumentService: Document CRUD operations

Dependency Injection

Services receive dependencies through constructors:

export class SearchTool {
  constructor(
    private docService: IDocumentManagement,
    private logger: Logger = logger
  ) {}
}

// Easy to test with mocks
const tool = new SearchTool(mockDocService, mockLogger);

Interface Segregation

Use focused interfaces:

// Good: Focused interface
interface IDocumentSearch {
  search(options: SearchOptions): Promise<SearchResult[]>;
}

interface IDocumentManagement {
  listLibraries(): Promise<Library[]>;
  addVersion(library: string, version: string): Promise<void>;
  removeVersion(library: string, version: string): Promise<void>;
}

// Bad: God interface
interface IDocumentService {
  search(...): Promise<SearchResult[]>;
  listLibraries(...): Promise<Library[]>;
  addVersion(...): Promise<void>;
  removeVersion(...): Promise<void>;
  // ... 20 more methods
}

Composition Over Inheritance

Prefer composition:

// Good: Composition
class AppServer {
  constructor(
    private pipeline: IPipeline,
    private eventBus: EventBus,
    private docService: IDocumentManagement
  ) {}
}

// Bad: Deep inheritance
class AppServer extends BaseServer {
  // Tightly coupled to parent
}

Extension Points

To add new functionality:

New Content Source

Create strategy in src/scraper/strategies/
Implement BaseScraperStrategy interface
Register in strategy factory

// src/scraper/strategies/CustomScraperStrategy.ts
export class CustomScraperStrategy extends BaseScraperStrategy {
  async scrape(config: ScrapeConfig): Promise<Document[]> {
    // Custom scraping logic
  }
}

New Tool

Create tool in src/tools/
Add validation and business logic
Expose in CLI, MCP, and Web interfaces

// src/tools/CustomTool.ts
export class CustomTool {
  constructor(private service: IService) {}
  
  async execute(options: CustomToolOptions): Promise<CustomToolResult> {
    // Tool implementation
  }
}

New Embedding Provider

Add provider configuration
Implement LangChain embeddings interface
Register in embeddings factory

Best Practices

Layered Architecture

Interfaces delegate to Tools
Tools implement business logic
Services handle data operations
Keep layers separate

Event-Driven Updates

Use EventBus for decoupling
Emit events at state boundaries
Subscribe for real-time updates
Don’t poll for status

Write-Through State

Update memory and DB together
Enable crash recovery
Maintain single source of truth
Emit events after persistence

Configuration

Use Zod for validation
Load once per process
Support environment variables
Provide sensible defaults

Reading the Source

Key files to understand the architecture:

ARCHITECTURE.md - Complete system architecture
src/app/AppServer.ts - Service composition
src/pipeline/PipelineFactory.ts - Mode selection
src/tools/ - Business logic layer
src/store/ - Data persistence

Development

Overview

Core Architectural Patterns

Tools Layer Pattern

Write-Through Architecture

Functionality-Based Design

Protocol Abstraction

System Architecture Modes

Unified Mode (In-Process)

Distributed Mode (Hub & Spoke)

Content Processing Architecture

Scraper Strategies

Content Pipelines

Document Splitters

Event-Driven Architecture

Storage Architecture

Configuration System

Design Principles

Single Responsibility

Dependency Injection

Interface Segregation

Composition Over Inheritance

Extension Points

New Content Source

New Tool

New Embedding Provider

Best Practices

Reading the Source

Next Steps

Getting Started

Code Style Guide

Build docs developers (and LLMs) love

Development

​Overview

​Core Architectural Patterns

​Tools Layer Pattern

​Write-Through Architecture

​Functionality-Based Design

​Protocol Abstraction

​System Architecture Modes

​Unified Mode (In-Process)

​Distributed Mode (Hub & Spoke)

​Content Processing Architecture

​Scraper Strategies

​Content Pipelines

​Document Splitters

​Event-Driven Architecture

​Storage Architecture

​Configuration System

​Design Principles

​Single Responsibility

​Dependency Injection

​Interface Segregation

​Composition Over Inheritance

​Extension Points

​New Content Source

​New Tool

​New Embedding Provider

​Best Practices

​Reading the Source

​Next Steps

Getting Started

Code Style Guide

Build docs developers (and LLMs) love

Overview

Core Architectural Patterns

Tools Layer Pattern

Write-Through Architecture

Functionality-Based Design

Protocol Abstraction

System Architecture Modes

Unified Mode (In-Process)

Distributed Mode (Hub & Spoke)

Content Processing Architecture

Scraper Strategies

Content Pipelines

Document Splitters

Event-Driven Architecture

Storage Architecture

Configuration System

Design Principles

Single Responsibility

Dependency Injection

Interface Segregation

Composition Over Inheritance

Extension Points

New Content Source

New Tool

New Embedding Provider

Best Practices

Reading the Source

Next Steps