Vercel Reliability Patterns

---
name: vercel-reliability-patterns
description: "'Implement reliability patterns for Vercel deployments including circuit breakers, retry logic, and graceful degradation. Use when building fault-tolerant serverless functions, implementing retry strategies, or adding resilience to production Vercel services. Trigger with phrases like \"vercel reliability\", \"vercel circuit breaker\", \"vercel resilience\", \"vercel fallback\", \"vercel graceful degradation\". '"
allowed-tools: "Read, Write, Edit"
category: devops-and-infra
source_repo: jeremylongshore/claude-code-plugins-plus-skills
source_path: "skills/.curated/vercel-reliability-patterns/SKILL.md"
source_url: https://github.com/jeremylongshore/claude-code-plugins-plus-skills/blob/HEAD/skills/.curated/vercel-reliability-patterns/SKILL.md
---

# 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

```typescript
// 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

```typescript
// 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

```typescript
// 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

```typescript
// 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

```typescript
// 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

```bash
# 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](https://vercel.com/docs/instant-rollback)
- [Vercel KV](https://vercel.com/docs/storage/vercel-kv)
- [Edge Config](https://vercel.com/docs/edge-config)
- [Circuit Breaker Pattern](https://martinfowler.com/bliki/CircuitBreaker.html)

## Next Steps

For policy guardrails, see `vercel-policy-guardrails`.

---

**Source:** [`jeremylongshore/claude-code-plugins-plus-skills`](https://github.com/jeremylongshore/claude-code-plugins-plus-skills) → `skills/.curated/vercel-reliability-patterns/SKILL.md`

**Also appears in:** `jeremylongshore/claude-code-plugins-plus-skills/plugins/saas-packs/vercel-pack/skills/vercel-reliability-patterns/SKILL.md`