// WRONG - BEGIN TRANSACTION not supported in Workers
await db.exec('BEGIN TRANSACTION');

// CORRECT - Use batch() for atomic operations
const results = await db.batch([
  db.prepare('INSERT INTO users (name) VALUES (?)').bind('Alice'),
  db.prepare('INSERT INTO logs (action) VALUES (?)').bind('user_created'),
]);

// WRONG - Will exceed execution limits
await db.exec('DELETE FROM logs WHERE created_at < ?', oldDate);

// CORRECT - Batch in chunks
while (true) {
  const result = await db.prepare(
    'DELETE FROM logs WHERE id IN (SELECT id FROM logs WHERE created_at < ? LIMIT 1000)'
  ).bind(oldDate).run();
  if (result.changes === 0) break;
}

// JavaScript numbers are 53-bit precision
// Storing 9007199254740993 may return 9007199254740992
// Use TEXT for large integers if precision matters

[[d1_databases]]
binding = "DB"
database_name = "my-database"
database_id = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"

undefined

// migrations/0001_initial.sql
CREATE TABLE IF NOT EXISTS users (
  id INTEGER PRIMARY KEY AUTOINCREMENT,
  email TEXT UNIQUE NOT NULL,
  created_at TEXT DEFAULT CURRENT_TIMESTAMP
);

CREATE INDEX IF NOT EXISTS idx_users_email ON users(email);

undefined

**Multi-tenant pattern:**
```javascript
// Create per-tenant database
// D1 allows thousands of databases at no extra cost
const tenantDb = env[`DB_${tenantId}`];

[[r2_buckets]]
binding = "BUCKET"
bucket_name = "my-bucket"

undefined

export default {
  async fetch(request, env) {
    const url = new URL(request.url);
    const key = url.pathname.slice(1);

    switch (request.method) {
      case 'PUT': {
        // Upload object
        await env.BUCKET.put(key, request.body, {
          httpMetadata: {
            contentType: request.headers.get('content-type'),
          },
          customMetadata: {
            uploadedBy: 'user-123',
          },
        });
        return new Response('Uploaded', { status: 201 });
      }

      case 'GET': {
        // Download object
        const object = await env.BUCKET.get(key);
        if (!object) {
          return new Response('Not Found', { status: 404 });
        }
        return new Response(object.body, {
          headers: {
            'content-type': object.httpMetadata?.contentType || 'application/octet-stream',
            'etag': object.etag,
          },
        });
      }

      case 'DELETE': {
        await env.BUCKET.delete(key);
        return new Response('Deleted', { status: 200 });
      }

      case 'HEAD': {
        const object = await env.BUCKET.head(key);
        if (!object) {
          return new Response(null, { status: 404 });
        }
        return new Response(null, {
          headers: {
            'content-length': object.size.toString(),
            'etag': object.etag,
          },
        });
      }
    }
  },
};

// WRONG - Loads entire file into memory (128MB Worker limit)
const object = await env.BUCKET.get(key);
const data = await object.text();

// CORRECT - Stream for large files
const object = await env.BUCKET.get(key);
return new Response(object.body); // Stream directly

// WRONG - Body already consumed
const data = await request.text();
await env.BUCKET.put(key, request.body); // Fails!

// CORRECT - Clone request first
const clone = request.clone();
const data = await request.text();
await env.BUCKET.put(key, clone.body);

// Paginate through all objects
let cursor;
const allKeys = [];

do {
  const listed = await env.BUCKET.list({ cursor, limit: 1000 });
  allKeys.push(...listed.objects.map(o => o.key));
  cursor = listed.truncated ? listed.cursor : null;
} while (cursor);

import { AwsClient } from 'aws4fetch';

const r2 = new AwsClient({
  accessKeyId: env.R2_ACCESS_KEY,
  secretAccessKey: env.R2_SECRET_KEY,
});

// Generate presigned upload URL
const signedUrl = await r2.sign(
  new Request(`https://${env.R2_BUCKET}.r2.cloudflarestorage.com/${key}`, {
    method: 'PUT',
  }),
  { aws: { signQuery: true } }
);

[[kv_namespaces]]
binding = "CACHE"
id = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"

// Write (with optional TTL)
await env.CACHE.put('user:123', JSON.stringify(userData), {
  expirationTtl: 3600, // 1 hour
});

// Read
const data = await env.CACHE.get('user:123', { type: 'json' });

// Delete
await env.CACHE.delete('user:123');

// List keys with prefix
const keys = await env.CACHE.list({ prefix: 'user:' });

undefined

```bash

---

export default {
  async fetch(request, env) {
    const url = new URL(request.url);

    // KV: Check cache first
    const cached = await env.CACHE.get(url.pathname);
    if (cached) return new Response(cached);

    // D1: Query database
    const { results } = await env.DB.prepare(
      'SELECT * FROM posts WHERE slug = ?'
    ).bind(url.pathname).all();

    if (!results.length) {
      return new Response('Not Found', { status: 404 });
    }

    const post = results[0];

    // R2: Get associated image
    const image = post.image_key
      ? await env.BUCKET.get(post.image_key)
      : null;

    // Cache the response
    const html = renderPost(post, image);
    await env.CACHE.put(url.pathname, html, { expirationTtl: 300 });

    return new Response(html, {
      headers: { 'content-type': 'text/html' },
    });
  },
};