409 Conflict in Rest API - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the HTTP status code returned?

Consider a REST API endpoint that updates a user profile. If two clients try to update the same user data simultaneously, the server detects a conflict and returns a 409 Conflict status code.

What will be the HTTP status code in the response when this conflict occurs?

Rest API

HTTP/1.1 409 Conflict
Content-Type: application/json

{"error": "Conflict detected: resource has been modified."}

A 409 Conflict

B 404 Not Found

C 200 OK

D 500 Internal Server Error

💡 Hint

Think about which status code means the request conflicts with the current state of the resource.

The 409 Conflict status code indicates that the request could not be completed due to a conflict with the current state of the target resource. This is commonly used when multiple clients try to update the same resource simultaneously.

🧠 Conceptual

intermediate

Why use 409 Conflict instead of 400 Bad Request?

When a client sends a request that conflicts with the current state of a resource, why is it better to respond with a 409 Conflict status code rather than a 400 Bad Request?

A Because 400 Bad Request indicates the server is down.

B Because 400 Bad Request means the client sent malformed syntax, not a conflict.

C Because 409 Conflict means the server accepted the request without issues.

D Because 400 Bad Request is only for authentication errors.

💡 Hint

Think about the difference between syntax errors and resource state conflicts.

400 Bad Request means the request syntax is invalid or malformed. 409 Conflict means the request is syntactically valid but cannot be processed because it conflicts with the resource's current state.

❓ Predict Output

advanced

What is the output of this conflict handling code?

Given the following Python Flask code snippet handling a PUT request to update a resource, what will be the HTTP status code returned if the resource version does not match?

Rest API

from flask import Flask, request, jsonify
app = Flask(__name__)

resources = {"item1": {"version": 1, "data": "old"}}

@app.route('/resource/<id>', methods=['PUT'])
def update_resource(id):
    if id not in resources:
        return jsonify({"error": "Not found"}), 404
    client_version = request.json.get('version')
    if client_version != resources[id]['version']:
        return jsonify({"error": "Version conflict"}), 409
    resources[id]['data'] = request.json.get('data')
    resources[id]['version'] += 1
    return jsonify(resources[id]), 200

# Assume a PUT request with JSON {"version": 0, "data": "new"} to /resource/item1

A 200 OK with updated resource data

B 404 Not Found

C 409 Conflict with error message 'Version conflict'

D 500 Internal Server Error

💡 Hint

Check the version comparison logic in the code.

The code checks if the client's version matches the current resource version. If not, it returns 409 Conflict with an error message.

🔧 Debug

advanced

Identify the bug causing incorrect status code

In this Node.js Express code, the server should return 409 Conflict when a resource update conflicts, but it always returns 200 OK. What is the bug?

Rest API

const express = require('express');
const app = express();
app.use(express.json());

const resources = { item1: { version: 1, data: 'old' } };

app.put('/resource/:id', (req, res) => {
  const id = req.params.id;
  if (!resources[id]) {
    return res.status(404).json({ error: 'Not found' });
  }
  const clientVersion = req.body.version;
  if (clientVersion !== resources[id].version) {
    return res.status(409).json({ error: 'Version conflict' });
  }
  resources[id].data = req.body.data;
  resources[id].version += 1;
  res.status(200).json(resources[id]);
});

A Missing return statements after res.status calls causing multiple responses

B Incorrect comparison operator used for version check

C resources object is not defined properly

D Middleware for parsing JSON is missing

💡 Hint

Check what happens after sending a response with res.status().json()

Without return statements after sending a response, the function continues and sends another response, causing the last one (200 OK) to be sent always.

🚀 Application

expert

How many items are in the conflict error response list?

A REST API returns a 409 Conflict response with a JSON body listing multiple conflicting fields as follows:

{
  "error": "Conflict detected",
  "conflicts": [
    {"field": "email", "message": "Email already in use"},
    {"field": "username", "message": "Username taken"},
    {"field": "phone", "message": "Phone number conflict"}
  ]
}

How many conflict items are in the "conflicts" list?

A 4

B 2

C 1

D 3

💡 Hint

Count the number of objects inside the "conflicts" array.

The "conflicts" array contains three objects, each describing a conflicting field.