Why Cursor-based pagination in Rest API? - Purpose & Use Cases

The Big Idea

Discover how a simple marker can make endless scrolling smooth and error-free!

The Scenario

Imagine you have a huge list of items, like thousands of products in an online store. You want to show them page by page to users. If you try to load all items at once or jump to a random page by counting items, it gets slow and confusing.

The Problem

Using simple page numbers means the server must count or skip many items each time. This makes the app slow and can show wrong or repeated items if the list changes while you browse. It's like flipping pages in a book that keeps changing its content.

The Solution

Cursor-based pagination uses a unique marker (cursor) from the last item you saw to get the next items. This way, the server quickly finds where to continue without counting or skipping. It keeps the list stable and fast, even if new items are added or removed.

Before vs After

✗ Before

GET /items?page=5&limit=10

✓ After

GET /items?cursor=abc123&limit=10

What It Enables

It enables smooth, fast, and reliable browsing through large or changing lists without missing or repeating items.

Real Life Example

When you scroll through your social media feed, cursor-based pagination helps load new posts seamlessly without jumping or repeating posts, even as new content is added constantly.

Key Takeaways

Manual page numbers slow down with big or changing data.

Cursors mark where to continue, making loading faster and stable.

Cursor-based pagination improves user experience on large lists.

Practice

1.

What is the main purpose of cursor-based pagination in REST APIs?

easy

A. To sort data alphabetically before sending

B. To efficiently fetch the next set of data using a marker

C. To cache all data on the client side

D. To send all data in a single response

Solution

1. Step 1: Understand cursor-based pagination concept

   Cursor-based pagination uses a marker (cursor) to fetch the next set of data instead of page numbers.

2. Step 2: Identify the main purpose

   This method helps efficiently retrieve data in chunks and avoid missing or repeating items when data changes.

3. Final Answer:

   To efficiently fetch the next set of data using a marker -> Option B

4. Quick Check:

   Cursor-based pagination = fetch next data with marker [OK]

Hint: Cursor marks position to get next data chunk [OK]

Common Mistakes:

• Confusing cursor with page number
• Thinking it sends all data at once
• Assuming it sorts data alphabetically

2.

Which of the following is the correct way to include a cursor in a REST API request URL?

GET /items?____=abc123

easy

A. cursor

B. limit

C. page

D. offset

Solution

1. Step 1: Identify the query parameter for cursor

   Cursor-based pagination uses a parameter named cursor to mark the position for the next data fetch.

2. Step 2: Match the parameter in the URL

   The URL should include ?cursor=abc123 to pass the cursor value.

3. Final Answer:

   cursor -> Option A

4. Quick Check:

   Cursor parameter in URL = cursor [OK]

Hint: Cursor parameter is usually named 'cursor' [OK]

Common Mistakes:

• Using 'page' or 'offset' which are for offset pagination
• Confusing 'limit' with cursor
• Leaving out the cursor parameter

3.

Given this API response for cursor-based pagination:

{
  "data": ["item4", "item5"],
  "next_cursor": "xyz789"
}

What should the client do to get the next page of data?

medium

A. Send a request with ?cursor=xyz789

B. Send a request with ?page=2

C. Send a request with ?offset=2

D. Send a request with ?limit=2

Solution

1. Step 1: Read the response for next cursor

   The response includes a next_cursor value "xyz789" which marks the next data position.

2. Step 2: Use the cursor in the next request

   The client should send a request with ?cursor=xyz789 to get the next page.

3. Final Answer:

   Send a request with ?cursor=xyz789 -> Option A

4. Quick Check:

   Use next_cursor value as cursor parameter [OK]

Hint: Use next_cursor value as cursor in next request [OK]

Common Mistakes:

• Using page or offset parameters instead of cursor
• Ignoring next_cursor and repeating last request
• Sending limit without cursor

4.

Consider this code snippet for fetching paginated data using cursor-based pagination:

cursor = None
while True:
    response = api.get_items(cursor=cursor)
    process(response.data)
    cursor = response.next_cursor
    if not cursor:
        break

What is the likely bug in this code?

medium

A. It processes data before fetching

B. It does not check if next_cursor exists before assigning

C. It never updates the cursor value

D. It may cause an infinite loop if next_cursor is empty string

Solution

1. Step 1: Analyze the code structure

   The code fetches data, processes it, then assigns cursor = response.next_cursor without checking if next_cursor is empty string.

2. Step 2: Potential infinite loop

   If next_cursor is an empty string, the condition if not cursor will be true and break the loop, so no infinite loop occurs. But if the check is incorrect or cursor is None, it breaks. However, the main issue is if next_cursor is missing, it raises an error.

3. Step 3: Proper handling

   Should check if next_cursor exists and is truthy before assigning and breaking.

4. Final Answer:

   It may cause an infinite loop if next_cursor is empty string -> Option D

5. Quick Check:

   Check for empty or missing cursor carefully [OK]

Hint: Check for empty or missing cursor carefully [OK]

Common Mistakes:

• Assuming next_cursor always exists
• Not breaking loop on missing cursor
• Updating cursor after break

5.

You have an API that returns data with cursor-based pagination. The API returns the following responses in sequence:

1. {"data": ["a", "b"], "next_cursor": "c1"}
2. {"data": ["c", "d"], "next_cursor": "c2"}
3. {"data": ["e"], "next_cursor": null}

You want to collect all items without duplicates even if the API sometimes returns overlapping data due to data changes. Which approach is best?

hard

A. Collect all items in a list and remove duplicates after fetching all pages

B. Fetch only the first page to avoid duplicates

C. Use a set to store items as you fetch each page to avoid duplicates immediately

D. Ignore duplicates and trust API never repeats data

Solution

1. Step 1: Understand overlapping data issue

   Cursor-based pagination can return overlapping items if data changes during pagination, causing duplicates.

2. Step 2: Choose a method to avoid duplicates

   Using a set to store items as they are fetched avoids duplicates immediately and efficiently.

3. Step 3: Evaluate other options

   Removing duplicates after fetching all pages (list) is less efficient. Ignoring duplicates or fetching only first page is incorrect.

4. Final Answer:

   Use a set to store items as you fetch each page to avoid duplicates immediately -> Option C

5. Quick Check:

   Use set to avoid duplicates during fetch [OK]

Hint: Use set to track unique items during pagination [OK]

Common Mistakes:

• Assuming API never returns duplicates
• Removing duplicates only after all data fetched
• Fetching only first page to avoid duplicates