Resource-based design thinking in Rest API - Time & Space Complexity

Time Complexity: Resource-based design thinking

O(n)

Understanding Time Complexity

When designing REST APIs using resource-based thinking, it's important to understand how the number of resources affects the work the server does.

We want to know how the time to handle requests grows as the number of resources increases.

Scenario Under Consideration

Analyze the time complexity of the following REST API endpoint that lists all resources.

GET /api/resources

// Server code example:
function getAllResources() {
  const resources = database.getAll();
  return resources.map(resource => format(resource));
}

This code fetches all resources from the database and formats each one before sending the response.

Identify Repeating Operations

Look for repeated actions in the code.

• Primary operation: Looping over all resources to format each one.
• How many times: Once for each resource in the database.

How Execution Grows With Input

As the number of resources grows, the server does more work formatting each one.

Input Size (n)	Approx. Operations
10	10 formatting operations
100	100 formatting operations
1000	1000 formatting operations

Pattern observation: The work grows directly with the number of resources.

Final Time Complexity

Time Complexity: O(n)

This means the time to handle the request grows in a straight line as the number of resources increases.

Common Mistake

[X] Wrong: "Fetching all resources is always fast because databases are optimized."

[OK] Correct: Even if the database is fast, the server still processes each resource one by one, so more resources mean more work and longer time.

Interview Connect

Understanding how your API's work grows with data size shows you can design efficient and scalable services, a key skill in real projects.

Self-Check

"What if we added pagination to this API? How would the time complexity change?"

Practice

1. What is the main idea behind resource-based design thinking in REST APIs?

easy

A. Focusing only on database tables

B. Using only POST requests for all actions

C. Designing APIs without any structure

D. Organizing APIs around things like users or products

Solution

1. Step 1: Understand the concept of resource-based design

   Resource-based design means APIs are built around resources, which are things like users, products, or orders.

2. Step 2: Compare options to the concept

   Only Organizing APIs around things like users or products correctly describes organizing APIs around things (resources). Other options describe incorrect or unrelated ideas.

3. Final Answer:

   Organizing APIs around things like users or products -> Option D

4. Quick Check:

   Resource-based design = Organizing APIs around resources [OK]

Hint: Think of APIs as managing 'things' like users or products [OK]

Common Mistakes:

• Confusing resource-based design with database design
• Thinking all requests must be POST
• Ignoring the concept of resources

2. Which HTTP method is typically used to update an existing resource in a REST API?

easy

A. PUT

B. POST

C. GET

D. DELETE

Solution

1. Step 1: Recall HTTP methods and their common uses

   GET retrieves data, POST creates new data, PUT updates existing data, DELETE removes data.

2. Step 2: Identify the method for updating

   PUT is the standard method used to update an existing resource in REST APIs.

3. Final Answer:

   PUT -> Option A

4. Quick Check:

   Update resource = PUT [OK]

Hint: PUT means replace or update a resource fully [OK]

Common Mistakes:

• Using POST to update instead of create
• Confusing GET with update
• Using DELETE for updating

3. Given this REST API call: DELETE /products/123, what is the expected result?

medium

A. Retrieve product with ID 123

B. Create a new product with ID 123

C. Delete product with ID 123

D. Update product with ID 123

Solution

1. Step 1: Analyze the HTTP method and URL

   DELETE method means remove. The URL targets the product resource with ID 123.

2. Step 2: Determine the action

   DELETE /products/123 means delete the product with ID 123 from the system.

3. Final Answer:

   Delete product with ID 123 -> Option C

4. Quick Check:

   DELETE + resource ID = delete that resource [OK]

Hint: DELETE + resource ID means remove that resource [OK]

Common Mistakes:

• Thinking DELETE retrieves data
• Confusing DELETE with POST or PUT
• Ignoring the resource ID in URL

4. You see this API request: POST /users/45. What is wrong with this request in resource-based design?

medium

A. The URL should be /users without ID for creating

B. POST cannot be used to create resources

C. The HTTP method should be PUT

D. POST must include a resource ID in the URL

Solution

1. Step 1: Understand POST usage in REST

   POST is used to create new resources. The URL should point to the collection, not a specific resource ID.

2. Step 2: Analyze the request URL

   Including an ID (like /users/45) with POST is incorrect because the server assigns IDs when creating new resources.

3. Final Answer:

   The URL should be /users without ID for creating -> Option A

4. Quick Check:

   POST to collection URL without ID = correct [OK]

Hint: POST creates; URL should be collection without ID [OK]

Common Mistakes:

• Using POST with resource ID in URL
• Thinking POST updates existing resource
• Confusing POST and PUT URLs

5. You want to design an API to manage orders and their items. Which URL and HTTP method combination best follows resource-based design to update the quantity of item 7 in order 42?

hard

A. POST /orders/42/items with item ID and quantity in body

B. PUT /orders/42/items/7 with updated quantity in body

C. DELETE /orders/42/items/7 to update quantity

D. GET /orders/42/items/7 with quantity in query

Solution

1. Step 1: Identify the resource and action

   The resource is the specific item (ID 7) inside order 42. Updating quantity means modifying this resource.

2. Step 2: Choose correct HTTP method and URL

   PUT is used to update existing resources. The URL should point to the specific item resource: /orders/42/items/7.

3. Final Answer:

   PUT /orders/42/items/7 with updated quantity in body -> Option B

4. Quick Check:

   Update nested resource = PUT to specific item URL [OK]

Hint: Use PUT on specific nested resource URL to update it [OK]

Common Mistakes:

• Using POST to update existing item
• Using DELETE to update data
• Using GET with query to change data