Reliability Engineering & Disaster Recovery

1// ============================================
2// Reliability Engineering — Patterns
3// ============================================
4
5// ---------- Canary Deployment ----------
6class CanaryDeployment {
7  constructor(loadBalancer) {
8    this.lb = loadBalancer;
9    this.canaryPercentage = 5; // Start with 5%
10    this.metrics = { canary: { errors: 0, requests: 0 }, stable: { errors: 0, requests: 0 } };
11  }
12
13  routeRequest(req) {
14    const isCanary = Math.random() * 100 < this.canaryPercentage;
15    return isCanary ? 'canary' : 'stable';
16  }
17
18  recordResult(version, success) {
19    this.metrics[version].requests++;
20    if (!success) this.metrics[version].errors++;
21
22    // Auto-promote or rollback
23    if (this.metrics.canary.requests >= 1000) {
24      const canaryErrorRate = this.metrics.canary.errors / this.metrics.canary.requests;
25      const stableErrorRate = this.metrics.stable.errors / this.metrics.stable.requests;
26
27      if (canaryErrorRate > stableErrorRate * 2) {
28        console.log('🚨 Canary error rate too high — ROLLING BACK');
29        this.canaryPercentage = 0;
30      } else {
31        this.canaryPercentage = Math.min(100, this.canaryPercentage * 2);
32        console.log(`✅ Canary healthy — increasing to \${this.canaryPercentage}%`);
33      }
34    }
35  }
36}
37
38// ---------- Feature Flags ----------
39class FeatureFlags {
40  constructor() {
41    this.flags = new Map();
42  }
43
44  setFlag(name, config) {
45    this.flags.set(name, {
46      enabled: config.enabled || false,
47      percentage: config.percentage || 0,
48      allowlist: config.allowlist || [],
49      rules: config.rules || [],
50    });
51  }
52
53  isEnabled(flagName, userId = null, context = {}) {
54    const flag = this.flags.get(flagName);
55    if (!flag) return false;
56    if (!flag.enabled) return false;
57
58    // Check allowlist (specific users)
59    if (userId && flag.allowlist.includes(userId)) return true;
60
61    // Check percentage rollout
62    if (flag.percentage > 0 && userId) {
63      const hash = this.stableHash(userId + flagName);
64      return (hash % 100) < flag.percentage;
65    }
66
67    return flag.enabled;
68  }
69
70  stableHash(str) {
71    let hash = 0;
72    for (let i = 0; i < str.length; i++) {
73      hash = ((hash << 5) - hash + str.charCodeAt(i)) | 0;
74    }
75    return Math.abs(hash);
76  }
77}
78
79// Demo — Feature flags
80const ff = new FeatureFlags();
81ff.setFlag('new_checkout', {
82  enabled: true,
83  percentage: 10, // 10% of users
84  allowlist: ['user_internal1', 'user_beta'],
85});
86
87console.log('Internal user:', ff.isEnabled('new_checkout', 'user_internal1')); // true (allowlist)
88console.log('Random user:', ff.isEnabled('new_checkout', 'user_random'));      // ~10% chance
89
90// Canary demo
91const canary = new CanaryDeployment(null);
92for (let i = 0; i < 100; i++) {
93  const version = canary.routeRequest({});
94  canary.recordResult(version, Math.random() > 0.02);
95}
96console.log('Canary metrics:', canary.metrics);