System Design Interview Framework

1// ============================================
2// Interview Preparation — Practice Template
3// ============================================
4
5// ---------- Back-of-Envelope Estimation Template ----------
6
7function estimateSystem(params) {
8  const {
9    dau,              // Daily Active Users
10    actionsPerUser,   // Actions per user per day
11    avgPayloadBytes,  // Average request/response size
12    readWriteRatio,   // Read:Write ratio (e.g., 10 means 10:1)
13    retentionYears,   // How long to keep data
14    cacheHitRatio,    // Expected cache hit rate (0.8 = 80%)
15  } = params;
16
17  const SECONDS_PER_DAY = 86400;
18
19  // Traffic estimation
20  const totalActions = dau * actionsPerUser;
21  const totalQPS = Math.ceil(totalActions / SECONDS_PER_DAY);
22  const writeQPS = Math.ceil(totalQPS / (1 + readWriteRatio));
23  const readQPS = totalQPS - writeQPS;
24  const peakQPS = totalQPS * 3; // Assume 3x peak
25
26  // Storage estimation
27  const dailyStorage = totalActions * avgPayloadBytes;
28  const yearlyStorage = dailyStorage * 365;
29  const totalStorage = yearlyStorage * retentionYears;
30
31  // Bandwidth
32  const incomingBandwidth = writeQPS * avgPayloadBytes;
33  const outgoingBandwidth = readQPS * avgPayloadBytes;
34
35  // Cache estimation (80/20 rule)
36  const dailyUniqueRequests = totalActions * 0.2; // 20% unique
37  const cacheMemory = dailyUniqueRequests * avgPayloadBytes;
38
39  return {
40    traffic: {
41      totalQPS,
42      readQPS,
43      writeQPS,
44      peakQPS,
45      note: `\${dau.toLocaleString()} DAU × \${actionsPerUser} actions = \${totalActions.toLocaleString()} actions/day`,
46    },
47    storage: {
48      daily: formatBytes(dailyStorage),
49      yearly: formatBytes(yearlyStorage),
50      total: formatBytes(totalStorage),
51      note: `\${retentionYears} years retention`,
52    },
53    bandwidth: {
54      incoming: formatBytes(incomingBandwidth) + '/sec',
55      outgoing: formatBytes(outgoingBandwidth) + '/sec',
56    },
57    cache: {
58      size: formatBytes(cacheMemory),
59      hitRate: `\${cacheHitRatio * 100}%`,
60      note: '80/20 rule — cache top 20% of requests',
61    },
62  };
63}
64
65function formatBytes(bytes) {
66  const units = ['B', 'KB', 'MB', 'GB', 'TB', 'PB'];
67  let i = 0;
68  let val = bytes;
69  while (val >= 1000 && i < units.length - 1) { val /= 1000; i++; }
70  return `\${val.toFixed(1)} \${units[i]}`;
71}
72
73// ---------- Example: Twitter Estimation ----------
74console.log('\n=== Twitter Estimation ===');
75console.log(estimateSystem({
76  dau: 300_000_000,
77  actionsPerUser: 5,      // 5 actions (post, read, like)
78  avgPayloadBytes: 500,   // 500 bytes per tweet
79  readWriteRatio: 100,    // 100 reads per 1 write
80  retentionYears: 10,
81  cacheHitRatio: 0.80,
82}));
83
84// ---------- Example: URL Shortener Estimation ----------
85console.log('\n=== URL Shortener Estimation ===');
86console.log(estimateSystem({
87  dau: 10_000_000,
88  actionsPerUser: 1,      // 1 URL shortened per day
89  avgPayloadBytes: 200,   // URL + metadata
90  readWriteRatio: 10,     // 10 redirects per 1 creation
91  retentionYears: 10,
92  cacheHitRatio: 0.90,
93}));
94
95// ---------- Interview Checklist ----------
96const interviewChecklist = {
97  'Step 1: Requirements (5-7 min)': [
98    'Clarify functional requirements (core features)',
99    'Clarify non-functional requirements (scale, latency, consistency)',
100    'Identify users and use cases',
101    'Agree on scope with interviewer',
102  ],
103  'Step 2: Estimation (3-5 min)': [
104    'Calculate QPS (read and write)',
105    'Calculate storage requirements',
106    'Calculate bandwidth',
107    'Calculate cache size',
108  ],
109  'Step 3: High-Level Design (15-20 min)': [
110    'Define API endpoints',
111    'Design data model (schema)',
112    'Draw architecture diagram',
113    'Walk through the main data flow',
114    'Explain key design decisions',
115  ],
116  'Step 4: Deep Dive (15-20 min)': [
117    'Address scaling challenges',
118    'Discuss failure scenarios',
119    'Explain consistency vs availability choices',
120    'Mention monitoring and observability',
121    'Discuss trade-offs of your design',
122  ],
123};
124
125console.log('\nInterview Checklist:', interviewChecklist);