CODEWITHSUNDEEP
javascriptnode.jsreactjstypescriptexpress
Andrija Gajic
Andrija Gajic

Reputation: 382

Express server stops after 5 GET requests

This code works like it should work, but after fifth GET request it does what it should do on the backend(stores the data in db) but it's not logging anything on the server and no changes on frontend(reactjs)

const express = require('express');
const router = express.Router();
const mongoose = require('mongoose');
const User = require('./login').User;

mongoose.connect('mongodb://localhost:27017/animationsdb');

router.get('/', async(req, res) => {
    await User.findOne({ username: req.query.username }, (err, result) => {
        if (result) {
            // when user goes to his profile we send him the list of animations he liked
            // list is stored in array at db, field likedAnimations
            res.send({ animationList: result.likedAnimations });
            console.log("Lajkovane animacije:", result.likedAnimations);
        } else {
            console.log("no result found");
            res.sendStatus(404)
        }
    });
});

router.put('/', async(req, res) => {
    console.log("username:", req.body.username);
    console.log("link:", req.body.link);

    // if animation is already liked, then dislike it
    // if it's not liked, then store it in db
    const user = await User.findOne({ username: req.body.username });
    if (user.likedAnimations.indexOf(req.body.link) === -1) {
        user.likedAnimations.push(req.body.link);
    } else {
        user.likedAnimations = arrayRemove(user.likedAnimations, user.likedAnimations[user.likedAnimations.indexOf(req.body.link)]);
    }
    user.save();
});

function arrayRemove(arr, value) {
    return arr.filter((item) => {
        return item != value;
    });
}

module.exports = router;

For first five requests I get this output:

Liked animations: ["/animations/animated-button.html"]
GET /animation-list/?username=marko 200 5.152 ms - 54
Liked animations: ["/animations/animated-button.html"]
GET /animation-list/?username=marko 304 3.915 ms - -

After that I don't get any output on server console and no changes on front end untill I refresh the page, even though db operations still work and data is saved.

Upvotes: 0

Views: 742

Answers (1)

jfriend00
jfriend00

Reputation: 707218

It appears you have a couple issues going on. First, this request handler is not properly coded to handle errors and thus it leaves requests as pending and does not send a response and the connection will stay as pending until the client eventually times it out. Second, you likely have some sort of database concurrency usage error that is the root issue here. Third, you're not using await properly with your database. You either use await or you pass a callback to your database, not both. You need to fix all three of these.

To address the first and third issues:

router.get('/', async(req, res) => {
    try {
        let result = await User.findOne({ username: req.query.username };
        if (result) {
            console.log("Liked animations:", result.likedAnimations);
            res.send({ animationList: result.likedAnimations });
        } else {
            console.log("no database result found");
            res.sendStatus(404);
        }
   } catch(e) {
       console.log(e);
       res.sendStatus(500);
   }
});

For the second issue, the particular database error you mention appears to be some sort of concurrency/locking issue internal to the database and is triggered by the sequence of database operations your code executes. You can read more about that error in the discussion here. Since the code you show us only shows a single read operation, we would need to see a much larger context of relevant code including the code related to this operation that writes to the database in order to be able to offer any ideas on how to fix the root cause of this issue.

We can't see the whole flow here, but you need to use atomic update operations in your database. Your PUT handler you show is an immediate race condition. In multi-client databases, you don't get a value, modify it and then write it back. That's an opportunity for a race condition because someone else could modify the value while you're sitting their holding it. When you then modify your held value, you overwrite the change that the other client just made. That's a race condition. Instead, you use an atomic operation that updates the operation directly in one database call or you use transactions to make a multi-step operation into a safe operation.

I'd suggest you read this article on atomic operations in mongodb. And, probably you want to use something like .findAndModify() so you can find and change an item in the database in one atomic operation. If you search for "atomic operations in mongodb", there are many other articles on the topic.

Upvotes: 1

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