Strangler Fig Pattern: Legacy schrittweise migrieren

// ❌ Big bang migration — all or nothing, high risk
// Deploy new system → switch DNS → hope it works
// Rollback: switch DNS back → data inconsistency

// ✅ Strangler fig facade — incremental, reversible
import express from "express";
import { createProxyMiddleware } from "http-proxy-middleware";
 
const app = express();
 
// Migration configuration — what's been migrated
const migrationConfig = {
  routes: new Map<string, "legacy" | "new">([
    // Already migrated
    ["/api/users", "new"],
    ["/api/users/*", "new"],
    ["/api/auth/*", "new"],
 
    // In progress — using feature flags
    ["/api/orders", "legacy"],
    ["/api/orders/*", "legacy"],
 
    // Not yet started
    ["/api/reports/*", "legacy"],
    ["/api/inventory/*", "legacy"],
  ]),
};
 
// Legacy system proxy
const legacyProxy = createProxyMiddleware({
  target: process.env.LEGACY_URL,
  changeOrigin: true,
  logLevel: "warn",
  onError: (err, req, res) => {
    console.error(
      `Legacy proxy error: ${err.message}`
    );
    (res as express.Response).status(502).json({
      error: "Legacy service unavailable",
    });
  },
});
 
// New system proxy
const newProxy = createProxyMiddleware({
  target: process.env.NEW_SERVICE_URL,
  changeOrigin: true,
  logLevel: "warn",
});
 
// Route decision middleware
function routeDecision(
  req: express.Request,
  res: express.Response,
  next: express.NextFunction
) {
  const path = req.path;
 
  // Check exact match first, then wildcard
  let target: "legacy" | "new" = "legacy";
 
  for (const [pattern, dest] of migrationConfig.routes) {
    if (pattern.endsWith("/*")) {
      const prefix = pattern.slice(0, -2);
      if (path.startsWith(prefix)) {
        target = dest;
        break;
      }
    } else if (path === pattern) {
      target = dest;
      break;
    }
  }
 
  // Tag for observability
  res.set("X-Routed-To", target);
 
  if (target === "new") {
    newProxy(req, res, next);
  } else {
    legacyProxy(req, res, next);
  }
}
 
app.use("/api", routeDecision);

interface MigrationFlag {
  route: string;
  newServicePercentage: number;
  enabledUserIds?: string[];
  excludedUserIds?: string[];
}
 
const migrationFlags: MigrationFlag[] = [
  {
    route: "/api/orders",
    newServicePercentage: 25,
    enabledUserIds: ["internal-test-user-1"],
  },
  {
    route: "/api/orders/*",
    newServicePercentage: 25,
  },
];
 
function shouldRouteToNew(
  path: string,
  userId?: string
): boolean {
  const flag = migrationFlags.find((f) => {
    if (f.route.endsWith("/*")) {
      return path.startsWith(f.route.slice(0, -2));
    }
    return path === f.route;
  });
 
  if (!flag) return false;
 
  // Explicit user overrides
  if (
    userId &&
    flag.excludedUserIds?.includes(userId)
  ) {
    return false;
  }
  if (userId && flag.enabledUserIds?.includes(userId)) {
    return true;
  }
 
  // Percentage-based rollout
  // Use consistent hashing so users get consistent routing
  if (userId) {
    const hash = simpleHash(userId);
    return hash % 100 < flag.newServicePercentage;
  }
 
  return (
    Math.random() * 100 < flag.newServicePercentage
  );
}
 
function simpleHash(str: string): number {
  let hash = 0;
  for (let i = 0; i < str.length; i++) {
    const char = str.charCodeAt(i);
    hash = (hash * 31 + char) | 0;
  }
  return Math.abs(hash);
}

// Strategy: Dual writes during migration
class OrderService {
  constructor(
    private legacyDb: LegacyDatabase,
    private newDb: NewDatabase,
    private syncEnabled: boolean
  ) {}
 
  async createOrder(order: OrderInput): Promise<Order> {
    // Primary write to the system of record
    const result = await this.newDb.createOrder(order);
 
    // Sync back to legacy for routes still using it
    if (this.syncEnabled) {
      try {
        await this.legacyDb.syncOrder(
          this.toLegacyFormat(result)
        );
      } catch (error) {
        // Log sync failure but don't fail the request
        console.error(
          "Legacy sync failed:",
          error
        );
        await this.queueForRetry(result);
      }
    }
 
    return result;
  }
 
  private toLegacyFormat(order: Order): LegacyOrder {
    return {
      order_id: order.id,
      customer_id: order.userId,
      order_total: order.total.toString(),
      order_status: this.mapStatus(order.status),
      created_date: order.createdAt
        .toISOString()
        .split("T")[0],
    };
  }
 
  private mapStatus(
    status: Order["status"]
  ): string {
    const statusMap: Record<string, string> = {
      pending: "P",
      confirmed: "C",
      shipped: "S",
      delivered: "D",
    };
    return statusMap[status] ?? "P";
  }
 
  private async queueForRetry(
    order: Order
  ): Promise<void> {
    // Push to a retry queue for eventual consistency
    await messageQueue.publish("legacy-sync-retry", {
      type: "order",
      data: order,
      attempts: 0,
      maxAttempts: 5,
    });
  }
}

async function shadowTest(
  req: express.Request
): Promise<{
  match: boolean;
  legacyResponse: unknown;
  newResponse: unknown;
  differences: string[];
}> {
  // Send to both systems in parallel
  const [legacyResult, newResult] = await Promise.all([
    forwardToLegacy(req),
    forwardToNew(req),
  ]);
 
  const differences = compareResponses(
    legacyResult,
    newResult
  );
 
  // Log results for analysis
  if (differences.length > 0) {
    console.warn(
      `Shadow test mismatch on ${req.path}:`,
      differences
    );
  }
 
  return {
    match: differences.length === 0,
    legacyResponse: legacyResult,
    newResponse: newResult,
    differences,
  };
}
 
function compareResponses(
  legacy: unknown,
  modern: unknown
): string[] {
  const diffs: string[] = [];
 
  if (typeof legacy !== typeof modern) {
    diffs.push(
      `Type mismatch: ${typeof legacy} vs ${typeof modern}`
    );
    return diffs;
  }
 
  if (
    typeof legacy === "object" &&
    legacy !== null &&
    modern !== null
  ) {
    const legacyObj = legacy as Record<string, unknown>;
    const modernObj = modern as Record<string, unknown>;
 
    // Check known field mappings
    const fieldMappings: [string, string][] = [
      ["order_id", "id"],
      ["customer_id", "userId"],
      ["order_total", "total"],
    ];
 
    for (const [legacyField, newField] of fieldMappings) {
      const legacyVal = String(
        legacyObj[legacyField] ?? ""
      );
      const newVal = String(
        modernObj[newField] ?? ""
      );
 
      if (legacyVal !== newVal) {
        diffs.push(
          `${legacyField}/${newField}: ` +
            `"${legacyVal}" vs "${newVal}"`
        );
      }
    }
  }
 
  return diffs;
}
 
// Shadow test middleware — only in staging
if (process.env.ENABLE_SHADOW_TESTING === "true") {
  app.use("/api/orders/*", async (req, res, next) => {
    if (req.method === "GET") {
      const result = await shadowTest(req);
      // Store results in metrics system
      metrics.recordShadowTest(
        req.path,
        result.match,
        result.differences
      );
    }
    next();
  });
}

interface MigrationProgress {
  totalRoutes: number;
  migratedRoutes: number;
  inProgressRoutes: number;
  remainingRoutes: number;
  trafficPercentage: {
    legacy: number;
    new: number;
  };
}
 
function getMigrationProgress(): MigrationProgress {
  const routes = [...migrationConfig.routes.entries()];
  const migrated = routes.filter(
    ([, target]) => target === "new"
  ).length;
  const inProgress = migrationFlags.filter(
    (f) =>
      f.newServicePercentage > 0 &&
      f.newServicePercentage < 100
  ).length;
 
  return {
    totalRoutes: routes.length,
    migratedRoutes: migrated,
    inProgressRoutes: inProgress,
    remainingRoutes:
      routes.length - migrated - inProgress,
    trafficPercentage: {
      legacy: 0, // Calculate from actual traffic metrics
      new: 0,
    },
  };
}
 
// Decommission checklist
const decommissionChecklist = [
  "All routes serving from new system",
  "Zero traffic to legacy for 30 days",
  "All data migrated and verified",
  "Legacy sync disabled",
  "Rollback plan documented (just in case)",
  "Legacy database archived",
  "Legacy infrastructure teardown scheduled",
];