当我在本地尝试时，它工作得很完美。但是Heroku在服务器部署后给出了这个错误。有人能帮忙吗？

{
  "message": "Operation `posts.find()` buffering timed out after 10000ms"
}

index.js

const express = require("express");
const cors = require("cors");
const mongoose = require("mongoose");
const bodyParser = require("body-parser");
require("dotenv").config();

const app = express();
const postRouter = require("./routes/post");
const port = process.env.PORT || 5000;

app.use(cors());
app.use(bodyParser.json({ limit: "30mb", extended: true }));
app.use(bodyParser.urlencoded({ limit: "30mb", extended: true }));

app.get("/", (req, res) => {
  res.send("it works");
});
//mongoose connection
const URL = process.env.CONNECTDB_URL;

mongoose.connect(URL, {
  useNewUrlParser: true,
  useCreateIndex: true,
  useUnifiedTopology: true,
});
const connection = mongoose.connection;
connection.once("open", () => {
  console.log("MongoDB database connection established successfully");
});

//router

app.use("/posts", postRouter);

//listening
app.listen(port, () => {
  console.log(`Server is running on port: ${port}`);
});

controller文件在这里。我正在尝试获取所有帖子。本地没有任何错误

const getPosts = async (req, res) => {
  try {
    const posts = await Post.find({});
    res.status(200).json(posts);
  } catch (error) {
    res.status(404).json({
      message: error.message,
    });
  }
};