formbricks-formbricks/apps/web/app/middleware/rateLimit.ts

import { LRUCache } from "lru-cache";
import { REDIS_HTTP_URL } from "@formbricks/lib/constants";

type Options = {
  interval: number;
  allowedPerInterval: number;
};

const inMemoryRateLimiter = (options: Options) => {
  const tokenCache = new LRUCache<string, number>({
    max: 1000,
    ttl: options.interval * 1000, // converts to expected input of milliseconds
  });

  return async (token: string) => {
    const currentUsage = tokenCache.get(token) || 0;
    if (currentUsage >= options.allowedPerInterval) {
      throw new Error("Rate limit exceeded");
    }
    tokenCache.set(token, currentUsage + 1);
  };
};

const redisRateLimiter = (options: Options) => async (token: string) => {
  try {
    const tokenCountResponse = await fetch(`${REDIS_HTTP_URL}/INCR/${token}`);
    if (!tokenCountResponse.ok) {
      console.error("Failed to increment token count in Redis", tokenCountResponse);
      return;
    }

    const { INCR } = await tokenCountResponse.json();
    if (INCR === 1) {
      await fetch(`${REDIS_HTTP_URL}/EXPIRE/${token}/${options.interval}`);
    } else if (INCR > options.allowedPerInterval) {
      throw new Error();
    }
  } catch (e) {
    throw new Error("Rate limit exceeded for IP: " + token);
  }
};

export const rateLimit = (options: Options) => {
  if (REDIS_HTTP_URL) {
    return redisRateLimiter(options);
  } else {
    return inMemoryRateLimiter(options);
  }
};