First API request and response in Rest API - Time & Space Complexity

Time Complexity: First API request and response

O(n)

Understanding Time Complexity

When we send an API request and get a response, it is important to know how long this process takes as the data grows.

We want to understand how the time to get a response changes when the request asks for more data.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

GET /users

// Server fetches all users from database
// Then sends the list back as response

This code sends a request to get all users and waits for the server to return the full list.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Server reads each user record from the database.
• How many times: Once for each user in the database.

How Execution Grows With Input

Explain the growth pattern intuitively.

Input Size (n)	Approx. Operations
10	Reads 10 user records
100	Reads 100 user records
1000	Reads 1000 user records

Pattern observation: The time grows directly with the number of users requested.

Final Time Complexity

Time Complexity: O(n)

This means the time to get the response grows in a straight line as the number of users increases.

Common Mistake

[X] Wrong: "The response time stays the same no matter how many users there are."

[OK] Correct: Because the server must read each user to include it in the response, more users mean more work and longer time.

Interview Connect

Understanding how request time grows with data size helps you explain API performance clearly and shows you think about real user experience.

Self-Check

"What if the API only returned the first 10 users regardless of total users? How would the time complexity change?"

Practice

1. What does a GET request in an API usually do?

easy

A. It asks the server to send data back.

B. It sends data to the server to be saved.

C. It deletes data from the server.

D. It updates existing data on the server.

Solution

1. Step 1: Understand the GET method

   The GET method is used to request data from a server without changing it.

2. Step 2: Identify the purpose of GET in API

   GET asks the server to send data back to the client.

3. Final Answer:

   It asks the server to send data back. -> Option A

4. Quick Check:

   GET = Request data [OK]

Hint: GET means get data from server, no changes made [OK]

Common Mistakes:

• Confusing GET with POST or DELETE methods
• Thinking GET changes data on server
• Mixing up request and response roles

2. Which of these is the correct way to write a simple GET request URL?

easy

A. POST /api/data HTTP/1.1

B. DELETE /api/data HTTP/1.1

C. GET /api/data HTTP/1.1

D. PUT /api/data HTTP/1.1

Solution

1. Step 1: Identify HTTP methods

   GET is the method used to request data, so the request line must start with GET.

2. Step 2: Check the request format

   The correct format is "GET /path HTTP/1.1" for a GET request.

3. Final Answer:

   GET /api/data HTTP/1.1 -> Option C

4. Quick Check:

   GET request line starts with GET [OK]

Hint: GET requests start with GET keyword in HTTP line [OK]

Common Mistakes:

• Using POST, DELETE, or PUT instead of GET
• Wrong HTTP version or missing method
• Confusing request line with response line

3. Given this API response snippet:

{"status":200,"data":{"name":"Alice","age":30}}

What does the status code 200 mean?

medium

A. The request was unauthorized.

B. The requested resource was not found.

C. There was a server error processing the request.

D. The request was successful and data is returned.

Solution

1. Step 1: Understand HTTP status codes

   Status code 200 means the request succeeded and the server returned the requested data.

2. Step 2: Match status code to meaning

   200 is standard for success; 404 means not found, 500 means server error, 401 means unauthorized.

3. Final Answer:

   The request was successful and data is returned. -> Option D

4. Quick Check:

   Status 200 = Success [OK]

Hint: 200 means OK, request worked fine [OK]

Common Mistakes:

• Confusing 200 with error codes like 404 or 500
• Thinking 200 means no data returned
• Mixing status codes with data content

4. You send a GET request to an API but get no response. Which of these is a likely cause?

medium

A. The server returned status code 200.

B. The API endpoint URL is incorrect or misspelled.

C. You used the GET method instead of POST.

D. Your request included a valid Authorization header.

Solution

1. Step 1: Check the URL correctness

   If the URL is wrong or misspelled, the server won't respond properly or may not be found.

2. Step 2: Evaluate other options

   Using GET instead of POST may cause wrong data but usually gets a response; status 200 means success; valid Authorization helps access.

3. Final Answer:

   The API endpoint URL is incorrect or misspelled. -> Option B

4. Quick Check:

   Wrong URL = No response [OK]

Hint: Check URL spelling first if no response [OK]

Common Mistakes:

• Assuming method error causes no response
• Ignoring URL typos
• Confusing status codes with no response

5. You want to get user info from an API at https://api.example.com/users/123. Which is the best way to check if your request worked?

hard

A. Check if the response status code is 200 and the data contains user details.

B. Check if the request method was POST and the server returned 404.

C. Check if the response status code is 500 and the data is empty.

D. Check if the URL contains the word 'users' and the method is DELETE.

Solution

1. Step 1: Understand success criteria for API requests

   A successful GET request returns status 200 and the expected data.

2. Step 2: Evaluate options for correctness

   POST method and 404 means not found; 500 means server error; DELETE method is for removing data, not getting it.

3. Final Answer:

   Check if the response status code is 200 and the data contains user details. -> Option A

4. Quick Check:

   Status 200 + data = success [OK]

Hint: Success = status 200 + expected data [OK]

Common Mistakes:

• Confusing POST with GET for data retrieval
• Ignoring status codes meaning
• Checking wrong HTTP methods for GET requests