Resource identifiers in URLs in Rest API - Time & Space Complexity

Time Complexity: Resource identifiers in URLs

O(1)

Understanding Time Complexity

When using resource identifiers in URLs, it's important to see how the system handles requests as the number of resources grows.

We want to know how the time to find or access a resource changes when there are more resources.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

GET /users/{userId}

// Server receives a request with a userId in the URL
// It looks up the user in a database or data store by that ID
// Then returns the user data as a response

This code handles a request to get a user by their unique ID from a collection of users.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Searching for the user by ID in the data store.
• How many times: Depends on the data structure; could be one direct lookup or a search through all users.

How Execution Grows With Input

As the number of users grows, the time to find one user depends on how the data is stored.

Input Size (n)	Approx. Operations
10	10 or 1 (depending on lookup method)
100	100 or 1
1000	1000 or 1

Pattern observation: If the system searches through all users, operations grow with n; if it uses direct lookup, operations stay about the same.

Final Time Complexity

Time Complexity: O(1)

This means finding a user by their ID takes about the same time no matter how many users there are, assuming a direct lookup method.

Common Mistake

[X] Wrong: "Finding a user by ID always takes longer as the number of users grows."

[OK] Correct: Many systems use direct lookup methods like hash tables or indexed databases, so the time stays constant regardless of size.

Interview Connect

Understanding how resource identifiers affect lookup time helps you explain efficient API design and data access in real projects.

Self-Check

"What if the user data was stored in a simple list without indexing? How would the time complexity change?"

Practice

1. What is the main purpose of resource identifiers in REST API URLs?

easy

A. To uniquely identify a specific resource

B. To specify the HTTP method used

C. To define the server address

D. To set the response format

Solution

1. Step 1: Understand resource identifiers

   Resource identifiers are unique parts of a URL that point to a specific item or resource.

2. Step 2: Identify their role in REST APIs

   They help the server know exactly which resource the client wants to access or modify.

3. Final Answer:

   To uniquely identify a specific resource -> Option A

4. Quick Check:

   Resource identifiers = unique resource ID [OK]

Hint: Resource IDs point to one item in the URL [OK]

Common Mistakes:

• Confusing resource identifiers with HTTP methods
• Thinking resource identifiers define server location
• Mixing resource identifiers with response formats

2. Which of the following is the correct way to include a resource identifier for a user with ID 42 in a REST API URL?

easy

A. /users#42

B. /users?id=42

C. /users/42/details

D. /users/42

Solution

1. Step 1: Identify the standard REST URL format

   Resource identifiers usually come directly after the resource name separated by a slash.

2. Step 2: Check each option

   /users/42 uses "/users/42" which correctly places the ID after the resource name. /users?id=42 uses a query parameter, which is less common for resource identification. /users/42/details adds extra path which may refer to a sub-resource. /users#42 uses a fragment which is not for resource IDs.

3. Final Answer:

   /users/42 -> Option D

4. Quick Check:

   Resource ID follows resource name with slash [OK]

Hint: Resource ID follows resource name with a slash / [OK]

Common Mistakes:

• Using query parameters instead of path segments
• Adding extra path parts that change meaning
• Using URL fragments (#) for resource IDs

3. Given the URL /products/123/reviews/456, which resource does the identifier 456 represent?

medium

A. Product with ID 456

B. Review with ID 456 for product 123

C. Product with ID 123

D. All reviews for product 456

Solution

1. Step 1: Break down the URL path

   The URL has two resource names: "products" and "reviews" with IDs 123 and 456 respectively.

2. Step 2: Understand the hierarchy

   "reviews/456" comes after "products/123", so 456 is the review ID related to product 123.

3. Final Answer:

   Review with ID 456 for product 123 -> Option B

4. Quick Check:

   Last ID is for last resource in URL [OK]

Hint: Last ID belongs to the last resource named [OK]

Common Mistakes:

• Mixing IDs and thinking 456 is a product ID
• Ignoring resource hierarchy in URL
• Assuming IDs apply to all resources

4. Identify the error in this REST API URL for accessing order 789: /orders/ 789

medium

A. The resource identifier should be before the resource name

B. There should be no slash between resource name and ID

C. The ID should be part of the path, not separate

D. The URL is correct as is

Solution

1. Step 1: Analyze the URL structure

   The URL shows /orders/ and then 789 separately, which suggests the ID is not part of the path.

2. Step 2: Understand correct resource ID placement

   The ID should be directly after the resource name in the same path segment, like /orders/789.

3. Final Answer:

   The ID should be part of the path, not separate -> Option C

4. Quick Check:

   Resource ID must be in the path segment [OK]

Hint: Resource ID must be in the same path segment [OK]

Common Mistakes:

• Placing ID outside the URL path
• Removing the slash between resource and ID
• Thinking resource ID comes before resource name

5. You want to design a REST API URL to access a comment with ID 555 on a post with ID 101. Which URL correctly uses resource identifiers to reflect this nested relationship?

hard

A. /posts/101/comments/555

B. /comments/555/posts/101

C. /posts/comments/555/101

D. /posts/555/comments/101

Solution

1. Step 1: Understand resource hierarchy

   The comment belongs to a post, so the post resource should come first in the URL.

2. Step 2: Check each URL option

   /posts/101/comments/555 correctly places the post ID first, then the comments resource with its ID. Other options mix the order or IDs incorrectly.

3. Final Answer:

   /posts/101/comments/555 -> Option A

4. Quick Check:

   Parent resource ID before child resource ID [OK]

Hint: Parent resource ID comes before child resource ID in URL [OK]

Common Mistakes:

• Reversing parent and child resource order
• Mixing IDs between resources
• Placing IDs in wrong path segments