One cache for all instancesOne cache for all instances

Two instances, one balancer

Revalidate on A, then send 10 users.

// next.config.js
module.exports = {
  output: 'standalone',
  cacheMaxMemorySize: 0,
  cacheHandlers: {
    default: require.resolve('./cache-handler.js'),
  },
};

// cache-handler.js
const Redis = require('ioredis');
const redis = new Redis(process.env.REDIS_URL);
const CACHE_PREFIX = 'nextjs:';

module.exports = class CacheHandler {
  async get(key) {
    const data = await redis.get(CACHE_PREFIX + key);
    if (!data) return null;
    const parsed = JSON.parse(data);
    return { value: parsed.value, lastModified: parsed.lastModified };
  }

  async set(key, data, ctx) {
    const cacheData = JSON.stringify({ value: data, lastModified: Date.now() });
    if (ctx?.revalidate) {
      await redis.setex(CACHE_PREFIX + key, ctx.revalidate, cacheData);
    } else {
      await redis.set(CACHE_PREFIX + key, cacheData);
    }
  }

  async revalidateTag(tags) {
    // not in the skill's example: map tags to keys yourself
  }
};

Replay the verification script

Run the script, or step one command at a time.

PORT=3001 node .next/standalone/server.js &
PORT=3002 node .next/standalone/server.js &
curl http://localhost:3001/api/revalidate?path=/posts
curl http://localhost:3001/posts
curl http://localhost:3002/posts

What a second instance breaks

Switch the deployment.

# Dockerfile
COPY --from=builder /app/.next/standalone ./
COPY --from=builder /app/.next/static ./.next/static
COPY --from=builder /app/public ./public
ENV HOSTNAME="0.0.0.0"
CMD ["node", "server.js"]

// app/api/health/route.ts
export async function GET() {
  return Response.json({ status: 'healthy' }, { status: 200 });
}

Why it matters, pros and cons