404 Not Found in Rest API - Time & Space Complexity

Time Complexity: 404 Not Found

O(n)

Understanding Time Complexity

When a server responds with a 404 Not Found, it means the requested resource isn't available.

We want to understand how the server's work grows when checking if a resource exists.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

GET /resource/{id}

if resource with id exists in database:
    return 200 OK with resource
else:
    return 404 Not Found

This code checks if a resource exists and returns 404 if it does not.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Searching the database for the resource by id.
• How many times: Once per request, but the search may scan multiple entries depending on database structure.

How Execution Grows With Input

The time to find the resource depends on how many entries the database has.

Input Size (n)	Approx. Operations
10	About 10 checks
100	About 100 checks
1000	About 1000 checks

Pattern observation: The more entries, the longer it takes to find or not find the resource.

Final Time Complexity

Time Complexity: O(n)

This means the time to respond grows linearly with the number of resources to check.

Common Mistake

[X] Wrong: "The server instantly knows if a resource exists without checking anything."

[OK] Correct: The server must look through data to confirm if the resource exists, which takes time depending on data size.

Interview Connect

Understanding how servers find resources helps you explain backend efficiency clearly and confidently.

Self-Check

"What if the database used an index to find resources? How would the time complexity change?"

Practice

1. What does the HTTP status code 404 Not Found indicate in a REST API?

easy

A. The requested resource does not exist on the server.

B. The server has encountered an internal error.

C. The request was successful and data is returned.

D. The client is not authorized to access the resource.

Solution

1. Step 1: Understand HTTP status codes

   HTTP status codes communicate the result of a client's request to the server.

2. Step 2: Identify meaning of 404

   404 means the server cannot find the requested resource or URL.

3. Final Answer:

   The requested resource does not exist on the server. -> Option A

4. Quick Check:

   404 = Resource not found [OK]

Hint: 404 means resource missing, not server error [OK]

Common Mistakes:

• Confusing 404 with 500 internal server error
• Thinking 404 means unauthorized access
• Assuming 404 means request succeeded

2. Which of the following is the correct way to send a 404 Not Found response in a REST API using HTTP headers?

easy

A. HTTP/1.1 403 Forbidden\r\nContent-Type: application/json\r\n\r\n

B. HTTP/1.1 200 OK\r\nContent-Type: application/json\r\n\r\n

C. HTTP/1.1 404 Not Found\r\nContent-Type: application/json\r\n\r\n

D. HTTP/1.1 500 Internal Server Error\r\nContent-Type: text/html\r\n\r\n

Solution

1. Step 1: Identify correct HTTP status line for 404

   The status line must start with HTTP version and '404 Not Found' to indicate missing resource.

2. Step 2: Check headers format

   Headers like Content-Type follow the status line, separated by CRLF, then a blank line before body.

3. Final Answer:

   HTTP/1.1 404 Not Found\r\nContent-Type: application/json\r\n\r\n -> Option C

4. Quick Check:

   Status line with 404 and proper headers = HTTP/1.1 404 Not Found\r\nContent-Type: application/json\r\n\r\n [OK]

Hint: 404 response starts with 'HTTP/1.1 404 Not Found' [OK]

Common Mistakes:

• Using 200 OK status for missing resource
• Mixing 404 with 500 or 403 status codes
• Incorrect header formatting or missing blank line

3. Consider this Python Flask code snippet for a REST API endpoint:

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

@app.route('/items/')
def get_item(item_id):
    items = {1: 'apple', 2: 'banana'}
    if item_id not in items:
        abort(404)
    return jsonify({'item': items[item_id]})

What will be the HTTP status code if a client requests /items/3?

medium

A. 404 Not Found

B. 200 OK

C. 500 Internal Server Error

D. 400 Bad Request

Solution

1. Step 1: Check if item_id 3 exists in items dictionary

   Items only have keys 1 and 2, so 3 is missing.

2. Step 2: Understand abort(404) behavior

   Calling abort(404) sends a 404 Not Found response to the client.

3. Final Answer:

   404 Not Found -> Option A

4. Quick Check:

   Missing item triggers abort(404) = 404 Not Found [OK]

Hint: abort(404) sends 404 status when resource missing [OK]

Common Mistakes:

• Assuming 200 OK even if item missing
• Confusing abort(404) with server error
• Thinking 400 Bad Request applies here

4. You have this Express.js route handler:

app.get('/users/:id', (req, res) => {
  const users = {1: 'Alice', 2: 'Bob'};
  const user = users[req.params.id];
  if (!user) {
    res.status(404);
  }
  res.send(user);
});

What is the problem with this code when a user is not found?

medium

A. It correctly sends a 404 status with no body.

B. It sends a 404 status but still sends undefined as response body.

C. It throws a syntax error due to missing semicolon.

D. It sends a 500 Internal Server Error automatically.

Solution

1. Step 1: Analyze response when user not found

   res.status(404) sets status but does not end response or send body.

2. Step 2: Check what res.send(user) does

   res.status(404) sets the status, but execution continues to res.send(undefined), which sends the string 'undefined' as body with 404 status.

3. Final Answer:

   It sends a 404 status but still sends undefined as response body. -> Option B

4. Quick Check:

   Missing res.send after status causes wrong response [OK]

Hint: Always send response body or end response after setting status [OK]

Common Mistakes:

• Setting status without sending response body
• Assuming status alone ends response
• Not checking for undefined response body

5. You are designing a REST API for a library. When a client requests a book by ID that does not exist, you want to return a 404 Not Found with a helpful JSON message. Which of the following Express.js code snippets correctly implements this behavior?

hard

A. app.get('/books/:id', (req, res) => {
const books = {1: '1984', 2: 'Brave New World'};
const book = books[req.params.id];
if (!book) {
res.sendStatus(404);
}
res.json({book});
});

B. app.get('/books/:id', (req, res) => {
const books = {1: '1984', 2: 'Brave New World'};
const book = books[req.params.id];
if (!book) {
res.json({error: 'Book not found'});
}
res.status(404).json({book});
});

C. app.get('/books/:id', (req, res) => {
const books = {1: '1984', 2: 'Brave New World'};
const book = books[req.params.id];
if (!book) {
res.status(404);
}
res.json({book});
});

D. app.get('/books/:id', (req, res) => {
const books = {1: '1984', 2: 'Brave New World'};
const book = books[req.params.id];
if (!book) {
res.status(404).json({error: 'Book not found'});
return;
}
res.json({book});
});

Solution

1. Step 1: Check how 404 and message are sent

   app.get('/books/:id', (req, res) => { const books = {1: '1984', 2: 'Brave New World'}; const book = books[req.params.id]; if (!book) { res.status(404).json({error: 'Book not found'}); return; } res.json({book}); }); sets status 404 and sends JSON with error message, then returns to avoid sending more.

2. Step 2: Verify other options

   app.get('/books/:id', (req, res) => { const books = {1: '1984', 2: 'Brave New World'}; const book = books[req.params.id]; if (!book) { res.json({error: 'Book not found'}); } res.status(404).json({book}); }); sends JSON without status 404 first, then tries to send 404 with book JSON (wrong order). app.get('/books/:id', (req, res) => { const books = {1: '1984', 2: 'Brave New World'}; const book = books[req.params.id]; if (!book) { res.status(404); } res.json({book}); }); sets status but does not send response properly. app.get('/books/:id', (req, res) => { const books = {1: '1984', 2: 'Brave New World'}; const book = books[req.params.id]; if (!book) { res.sendStatus(404); } res.json({book}); }); sends 404 status but then sends book JSON anyway.

3. Final Answer:

   Sets status(404).json({error: 'Book not found'}); return; to send 404 with helpful JSON message and end response. -> Option D

4. Quick Check:

   Set status, send JSON, and return to end response = app.get('/books/:id', (req, res) => { const books = {1: '1984', 2: 'Brave New World'}; const book = books[req.params.id]; if (!book) { res.status(404).json({error: 'Book not found'}); return; } res.json({book}); }); [OK]

Hint: Set status, send JSON error, and return immediately [OK]

Common Mistakes:

• Not returning after sending 404 response
• Sending response body after status without ending
• Mixing order of status and response methods