vercel-reliability-patterns

Vercel Reliability Patterns

Overview

Build fault-tolerant Vercel deployments with circuit breakers, retry logic, graceful degradation, and instant rollback integration. Addresses reliability at two levels: function-level resilience (protecting against dependency failures) and deployment-level resilience (protecting against bad deploys).

Prerequisites

• Vercel project deployed to production
• Understanding of failure modes in serverless
• External dependencies (databases, APIs) identified

Instructions

Step 1: Circuit Breaker for External Dependencies

// lib/circuit-breaker.ts
type CircuitState = 'CLOSED' | 'OPEN' | 'HALF_OPEN';

class CircuitBreaker {
  private state: CircuitState = 'CLOSED';
  private failures = 0;
  private lastFailure = 0;
  private readonly threshold: number;
  private readonly resetTimeMs: number;

  constructor(threshold = 5, resetTimeMs = 30000) {
    this.threshold = threshold;
    this.resetTimeMs = resetTimeMs;
  }

  async call<T>(fn: () => Promise<T>, fallback: () => T): Promise<T> {
    if (this.state === 'OPEN') {
      if (Date.now() - this.lastFailure > this.resetTimeMs) {
        this.state = 'HALF_OPEN';
      } else {
        console.warn('Circuit OPEN — returning fallback');
        return fallback();
      }
    }

    try {
      const result = await fn();
      this.onSuccess();
      return result;
    } catch (error) {
      this.onFailure();
      console.error('Circuit breaker caught error:', error);
      return fallback();
    }
  }

  private onSuccess(): void {
    this.failures = 0;
    this.state = 'CLOSED';
  }

  private onFailure(): void {
    this.failures++;
    this.lastFailure = Date.now();
    if (this.failures >= this.threshold) {
      this.state = 'OPEN';
      console.warn(`Circuit OPENED after ${this.failures} failures`);
    }
  }
}

// Usage in a serverless function:
const dbCircuit = new CircuitBreaker(3, 30000);

export default async function handler(req, res) {
  const users = await dbCircuit.call(
    () => db.user.findMany({ take: 10 }),
    () => [] // Fallback: empty array when DB is down
  );
  res.json({ users, degraded: users.length === 0 });
}

Important for serverless: Circuit breaker state lives in a single function instance. Different instances have independent circuits. For global circuit state, use Vercel KV or Edge Config.

Step 2: Retry with Exponential Backoff

// lib/retry.ts
interface RetryOptions {
  maxRetries?: number;
  baseDelayMs?: number;
  maxDelayMs?: number;
  retryOn?: (error: unknown) => boolean;
}

async function withRetry<T>(
  fn: () => Promise<T>,
  options: RetryOptions = {}
): Promise<T> {
  const { maxRetries = 3, baseDelayMs = 200, maxDelayMs = 5000, retryOn } = options;

  for (let attempt = 0; attempt <= maxRetries; attempt++) {
    try {
      return await fn();
    } catch (error) {
      if (attempt === maxRetries) throw error;
      if (retryOn && !retryOn(error)) throw error;

      const delay = Math.min(
        baseDelayMs * Math.pow(2, attempt) + Math.random() * 200,
        maxDelayMs
      );
      await new Promise(r => setTimeout(r, delay));
    }
  }
  throw new Error('Unreachable');
}

// Usage:
const data = await withRetry(
  () => fetch('https://api.example.com/data').then(r => {
    if (!r.ok) throw new Error(`HTTP ${r.status}`);
    return r.json();
  }),
  {
    maxRetries: 3,
    retryOn: (err) => {
      // Only retry on network errors and 5xx, not 4xx
      if (err instanceof TypeError) return true; // network error
      return err.message?.includes('5');
    },
  }
);

Step 3: Graceful Degradation with Stale Cache

// api/products.ts — serve stale data when primary source is down
import { get, set } from '@vercel/kv';

export default async function handler(req, res) {
  const cacheKey = 'products:latest';

  try {
    // Try primary data source
    const freshData = await fetchProductsFromDB();

    // Update cache with fresh data
    await set(cacheKey, JSON.stringify(freshData), { ex: 3600 });

    res.setHeader('x-data-source', 'live');
    res.json(freshData);
  } catch (error) {
    // Primary failed — serve stale cache
    const cachedData = await get(cacheKey);

    if (cachedData) {
      console.warn('Serving stale cache — primary source unavailable');
      res.setHeader('x-data-source', 'cache-stale');
      res.json(JSON.parse(cachedData as string));
    } else {
      // No cache available — return degraded response
      res.setHeader('x-data-source', 'degraded');
      res.status(503).json({
        error: 'Service temporarily unavailable',
        degraded: true,
      });
    }
  }
}

Step 4: Idempotency Keys for Mutations

// api/orders/route.ts — idempotent order creation
import { NextRequest, NextResponse } from 'next/server';
import { db } from '@/lib/db';

export async function POST(request: NextRequest) {
  const idempotencyKey = request.headers.get('idempotency-key');
  if (!idempotencyKey) {
    return NextResponse.json(
      { error: 'idempotency-key header required' },
      { status: 400 }
    );
  }

  // Check if this request was already processed
  const existing = await db.idempotencyRecord.findUnique({
    where: { key: idempotencyKey },
  });

  if (existing) {
    // Return the cached response — same status and body
    return NextResponse.json(JSON.parse(existing.responseBody), {
      status: existing.responseStatus,
      headers: { 'x-idempotent-replay': 'true' },
    });
  }

  // Process the order
  const body = await request.json();
  const order = await db.order.create({ data: body });

  // Cache the response for idempotency
  const responseBody = JSON.stringify({ order });
  await db.idempotencyRecord.create({
    data: { key: idempotencyKey, responseStatus: 201, responseBody },
  });

  return NextResponse.json({ order }, { status: 201 });
}

Step 5: Health Check with Dependency Status

// api/health/route.ts
export const dynamic = 'force-dynamic';

interface HealthCheck {
  name: string;
  check: () => Promise<boolean>;
}

const checks: HealthCheck[] = [
  {
    name: 'database',
    check: async () => {
      await db.$queryRaw`SELECT 1`;
      return true;
    },
  },
  {
    name: 'cache',
    check: async () => {
      await kv.ping();
      return true;
    },
  },
  {
    name: 'external-api',
    check: async () => {
      const r = await fetch('https://api.example.com/health', { signal: AbortSignal.timeout(3000) });
      return r.ok;
    },
  },
];

export async function GET() {
  const results: Record<string, 'ok' | 'error'> = {};

  await Promise.all(
    checks.map(async ({ name, check }) => {
      try {
        await check();
        results[name] = 'ok';
      } catch {
        results[name] = 'error';
      }
    })
  );

  const healthy = Object.values(results).every(v => v === 'ok');
  return Response.json(
    { status: healthy ? 'healthy' : 'degraded', checks: results },
    { status: healthy ? 200 : 503 }
  );
}

Step 6: Deployment-Level Resilience

# Instant rollback on health check failure (CI integration)
DEPLOY_URL=$(vercel --prod)
HEALTH=$(curl -s -o /dev/null -w "%{http_code}" "$DEPLOY_URL/api/health")

if [ "$HEALTH" != "200" ]; then
  echo "Health check failed ($HEALTH) — rolling back"
  vercel rollback
  exit 1
fi
echo "Deployment healthy"

Reliability Patterns Summary

Pattern	Protects Against	Vercel Implementation
Circuit breaker	Dependency degradation	In-function state or Edge Config
Retry + backoff	Transient failures	withRetry wrapper
Stale cache	Primary source outage	Vercel KV with TTL
Idempotency	Duplicate mutations	Database record per request
Health checks	Bad deployments	/api/health + rollback automation
Instant rollback	Deployment regression	vercel rollback in CI

Output

• Circuit breaker protecting all external dependency calls
• Retry logic with exponential backoff for transient failures
• Graceful degradation serving stale data when primary fails
• Idempotency preventing duplicate mutations
• Automated health check + rollback pipeline

Error Handling

Error	Cause	Solution
Circuit opens too aggressively	Threshold too low	Increase failure threshold (e.g., 5 → 10)
Retry causes duplicate side effects	No idempotency	Add idempotency-key to mutation endpoints
Stale cache expired	TTL too short or never populated	Increase TTL, seed cache on deploy
Health check false positive	Timeout too short	Increase AbortSignal timeout to 5s
Rollback reverts good deployment	Flaky health check	Add retry to health check before rollback

Resources

• Vercel Instant Rollback
• Vercel KV
• Edge Config
• Circuit Breaker Pattern

Next Steps

For policy guardrails, see vercel-policy-guardrails.