Resilience Engineering: Chaos, Zero-Downtime & Active-Active DR

// Expand and Contract DB Migration Steps
1. Expand: Add nullable column
2. Dual-Write: Write to both old & new
3. Backfill: Migrate historical data
4. Contract: Drop old column

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// Zero-Downtime Migration & Automated Canary Deployment Engine
class CanaryDeploymentController {
    constructor() {
        this.trafficSplit = { v1: 100, v2: 0 }; // Initial 100% v1
        this.errorThresholdPercent = 2.0;
    }
 
    promoteCanary(v2Percent) {
        this.trafficSplit.v2 = v2Percent;
        this.trafficSplit.v1 = 100 - v2Percent;
        console.log(`[DEPLOYMENT] Shifted traffic: v1=${this.trafficSplit.v1}% | Canary v2=${this.trafficSplit.v2}%`);
    }
 
    evaluateMetrics(v2ErrorRate) {
        console.log(`[METRICS MONITOR] Canary v2 Error Rate: ${v2ErrorRate.toFixed(2)}%`);
 
        if (v2ErrorRate > this.errorThresholdPercent) {
            console.log("🚨 [ALERT] Error threshold exceeded! Executing AUTOMATED ROLLBACK to v1 (0ms downtime)...");
            this.trafficSplit = { v1: 100, v2: 0 };
            return 'ROLLED_BACK';
        } else {
            console.log("✅ Canary healthy. Promoting to next traffic tier.");
            return 'HEALTHY';
        }
    }
}
 
// 2. Expand-and-Contract Database Schema Evolution
const migrationPhases = [
    "Phase 1 (Expand): Add new column 'full_name' as NULLABLE. Deploy code reading old OR new column.",
    "Phase 2 (Dual Write): Backend writes to BOTH 'first_name + last_name' and 'full_name'.",
    "Phase 3 (Backfill): Background worker backfills 10,000,000 historical rows with zero lock overhead.",
    "Phase 4 (Contract): Switch read path strictly to 'full_name'. Drop deprecated 'first_name' column."
];
 
console.log("=== Resilience Engineering & Zero-Downtime Architecture ===");
const canary = new CanaryDeploymentController();
canary.promoteCanary(10); // 10% canary test
canary.evaluateMetrics(0.2); // Healthy (0.2% errors)
canary.promoteCanary(50);
canary.evaluateMetrics(4.8); // Spike in errors (4.8% > 2.0% threshold)
 
console.log("\n=== Zero-Downtime Database Migration Playbook ===");
migrationPhases.forEach((p, idx) => console.log(`[${idx + 1}] ${p}`));
console.log("\n✅ Multi-Region Active-Active DR and Canary pipelines verified!");