Design a URL Shortener (TinyURL/Bit.ly)

1// ============================================
2// URL Shortener — System Design Implementation
3// ============================================
4
5class URLShortener {
6  constructor(db, cache, idGenerator) {
7    this.db = db;
8    this.cache = cache;
9    this.idGen = idGenerator;
10    this.BASE62 = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz';
11  }
12
13  // Create short URL
14  async createShortURL(longURL, customAlias = null, expiresAt = null) {
15    // Check if URL already shortened (deduplication)
16    const existing = await this.db.findByLongURL(longURL);
17    if (existing && !customAlias) return existing.shortCode;
18
19    let shortCode;
20    if (customAlias) {
21      // Custom alias — check availability
22      const taken = await this.db.findByShortCode(customAlias);
23      if (taken) throw new Error('Alias already taken');
24      shortCode = customAlias;
25    } else {
26      // Generate unique short code
27      const id = await this.idGen.nextId();
28      shortCode = this.toBase62(id);
29    }
30
31    const url = {
32      shortCode,
33      longURL,
34      createdAt: new Date(),
35      expiresAt: expiresAt ? new Date(expiresAt) : null,
36      clickCount: 0,
37    };
38
39    await this.db.save(url);
40    await this.cache.set(`url:\${shortCode}`, longURL, 'EX', 86400);
41    return shortCode;
42  }
43
44  // Redirect: shortCode → longURL
45  async resolve(shortCode) {
46    // Check cache first (hot path)
47    let longURL = await this.cache.get(`url:\${shortCode}`);
48    if (longURL) {
49      this.recordClick(shortCode); // Async, don't block redirect
50      return longURL;
51    }
52
53    // Cache miss — query database
54    const url = await this.db.findByShortCode(shortCode);
55    if (!url) return null;
56    if (url.expiresAt && new Date() > url.expiresAt) return null;
57
58    // Populate cache
59    await this.cache.set(`url:\${shortCode}`, url.longURL, 'EX', 86400);
60    this.recordClick(shortCode);
61    return url.longURL;
62  }
63
64  toBase62(num) {
65    let result = '';
66    while (num > 0) {
67      result = this.BASE62[num % 62] + result;
68      num = Math.floor(num / 62);
69    }
70    return result.padStart(7, '0');
71  }
72
73  async recordClick(shortCode) {
74    // Fire-and-forget to analytics queue (don't block redirect)
75    this.cache.incr(`clicks:\${shortCode}`).catch(() => {});
76  }
77}
78
79// Demo
80const shortener = {
81  toBase62(num) {
82    const chars = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz';
83    let result = '';
84    while (num > 0) { result = chars[num % 62] + result; num = Math.floor(num / 62); }
85    return result.padStart(7, '0');
86  }
87};
88console.log('Short code for ID 123456789:', shortener.toBase62(123456789));
89console.log('Total possible 7-char codes:', Math.pow(62, 7).toLocaleString());