Smart Placement Patterns

Backend Worker with Database Access

export default {
  async fetch(request: Request, env: Env): Promise<Response> {
    const user = await env.DATABASE.prepare('SELECT * FROM users WHERE id = ?').bind(userId).first();
    const orders = await env.DATABASE.prepare('SELECT * FROM orders WHERE user_id = ?').bind(userId).all();
    return Response.json({ user, orders });
  }
};

{ "placement": { "mode": "smart" }, "d1_databases": [{ "binding": "DATABASE", "database_id": "xxx" }] }

Frontend + Backend Split (Service Bindings)

Frontend: Runs at edge for fast user response Backend: Smart Placement runs close to database

// Frontend Worker - routes requests to backend
interface Env {
  BACKEND: Fetcher;  // Service Binding to backend Worker
}

export default {
  async fetch(request: Request, env: Env): Promise<Response> {
    if (new URL(request.url).pathname.startsWith('/api/')) {
      return env.BACKEND.fetch(request);  // Forward to backend
    }
    return new Response('Frontend content');
  }
};

// Backend Worker - database operations
interface BackendEnv {
  DATABASE: D1Database;
}

export default {
  async fetch(request: Request, env: BackendEnv): Promise<Response> {
    const data = await env.DATABASE.prepare('SELECT * FROM table').all();
    return Response.json(data);
  }
};

CRITICAL: Use fetch-based Service Bindings (shown above). If using RPC with WorkerEntrypoint, Smart Placement will NOT optimize those method calls - only fetch handlers are affected.

RPC vs Fetch - CRITICAL: Smart Placement ONLY works with fetch-based bindings, NOT RPC.

// ❌ RPC - Smart Placement has NO EFFECT on backend RPC methods
export class BackendRPC extends WorkerEntrypoint {
  async getData() {
    // ALWAYS runs at edge, Smart Placement ignored
    return await this.env.DATABASE.prepare('SELECT * FROM table').all();
  }
}

// ✅ Fetch - Smart Placement WORKS
export default {
  async fetch(request: Request, env: Env): Promise<Response> {
    // Runs close to DATABASE when Smart Placement enabled
    const data = await env.DATABASE.prepare('SELECT * FROM table').all();
    return Response.json(data);
  }
};

External API Integration

export default {
  async fetch(request: Request, env: Env): Promise<Response> {
    const apiUrl = 'https://api.partner.com';
    const headers = { 'Authorization': `Bearer ${env.API_KEY}` };
    
    const [profile, transactions] = await Promise.all([
      fetch(`${apiUrl}/profile`, { headers }),
      fetch(`${apiUrl}/transactions`, { headers })
    ]);
    
    return Response.json({ 
      profile: await profile.json(), 
      transactions: await transactions.json()
    });
  }
};

SSR / API Gateway Pattern

// Frontend (edge) - auth/routing close to user
export default {
  async fetch(request: Request, env: Env) {
    if (!request.headers.get('Authorization')) {
      return new Response('Unauthorized', { status: 401 });
    }
    const data = await env.BACKEND.fetch(request);
    return new Response(renderPage(await data.json()), { 
      headers: { 'Content-Type': 'text/html' } 
    });
  }
};

// Backend (Smart Placement) - DB operations close to data
export default {
  async fetch(request: Request, env: Env) {
    const data = await env.DATABASE.prepare('SELECT * FROM pages WHERE id = ?').bind(pageId).first();
    return Response.json(data);
  }
};

Durable Objects with Smart Placement

Key principle: Smart Placement does NOT control WHERE Durable Objects run. DOs always run in their designated region (based on jurisdiction or smart location hints).

What Smart Placement DOES affect: The location of the coordinator Worker's fetch handler that makes calls to multiple DOs.

Pattern: Enable Smart Placement on coordinator Worker that aggregates data from multiple DOs:

// Worker with Smart Placement - aggregates data from multiple DOs
export default {
  async fetch(request: Request, env: Env): Promise<Response> {
    const userId = new URL(request.url).searchParams.get('user');
    
    // Get DO stubs
    const userDO = env.USER_DO.get(env.USER_DO.idFromName(userId));
    const analyticsID = env.ANALYTICS_DO.idFromName(`analytics-${userId}`);
    const analyticsDO = env.ANALYTICS_DO.get(analyticsID);
    
    // Fetch from multiple DOs
    const [userData, analyticsData] = await Promise.all([
      userDO.fetch(new Request('https://do/profile')),
      analyticsDO.fetch(new Request('https://do/stats'))
    ]);
    
    return Response.json({
      user: await userData.json(),
      analytics: await analyticsData.json()
    });
  }
};

// wrangler.jsonc
{
  "placement": { "mode": "smart" },
  "durable_objects": {
    "bindings": [
      { "name": "USER_DO", "class_name": "UserDO" },
      { "name": "ANALYTICS_DO", "class_name": "AnalyticsDO" }
    ]
  }
}

When this helps:

Worker's fetch handler runs closer to DO regions, reducing network latency for multiple DO calls
Most beneficial when DOs are geographically concentrated or in specific jurisdictions
Helps when coordinator makes many sequential or parallel DO calls

When this DOESN'T help:

DOs are globally distributed (no single optimal Worker location)
Worker only calls a single DO
DO calls are infrequent or cached

Best Practices

Split full-stack apps: frontend at edge, backend with Smart Placement
Use fetch-based Service Bindings (not RPC)
Enable for backend logic: APIs, data aggregation, DB operations
Don't enable for: static content, edge logic, RPC methods, Pages with run_worker_first
Wait 15+ min for analysis, verify placement_status = SUCCESS

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