🔹 What is a Cursor in Pagination?

A cursor in pagination is a pointer/marker that tells the database or API where to resume fetching the next set of results.

Instead of using page numbers (page=2, page=3), a cursor-based system uses an encoded ID or token from the last item fetched to get the next set of results.

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🔹 Example

Imagine you have a list of users sorted by created_at:

id	name	created_at
1	Alice	2024-09-01 10:00:00
2	Bob	2024-09-01 10:05:00
3	Carl	2024-09-01 10:10:00
4	Dave	2024-09-01 10:15:00
5	Emma	2024-09-01 10:20:00

Offset Pagination (traditional)

• GET /users?page=2&limit=2 → skip first 2, fetch next 2.
• Issues: If new users are inserted between requests, results shift (inconsistent).

Cursor Pagination

• First call: GET /users?limit=2 → returns Alice, Bob.
• Response includes a cursor: "nextCursor": "created_at=2024-09-01 10:05:00".
• Next call: GET /users?limit=2&cursor=2024-09-01 10:05:00 → returns Carl, Dave.

👉 The cursor points to the last seen item, so you always continue from that exact spot, even if new data is inserted.

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🔹 Benefits of Cursor Pagination

✅ More consistent for real-time data (avoids duplicates or missing items if data changes). ✅ Faster for large datasets (no expensive OFFSET n scans). ✅ Works well for infinite scroll (social media feeds, chat apps).

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🔹 Where It’s Used

• Twitter/Instagram feeds (scrolling posts).
• Messaging apps (load older messages when scrolling up).
• APIs (GitHub, GraphQL, Firebase, DynamoDB, etc. use cursor pagination).

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👉 In short: Cursor = bookmark to continue fetching results from where you left off.

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🔹 Traditional Pagination (pageNumber + pageSize)

• Suppose you load messages with page=2&size=20.

• If new messages arrive, page boundaries shift.

  • Example: You loaded page 2 (messages 21–40).
  • But while you’re scrolling, 5 new messages arrive.
  • Now page 2 is different (messages 26–45), so you might see duplicates or miss some messages.

This is why offset/page-based pagination is not reliable for real-time data.

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🔹 Cursor Pagination (used in WhatsApp/Chats)

• Instead of pageNumber, WhatsApp uses a cursor (timestamp or messageId).

• Example:

  • You fetch the latest 20 messages.
  • Server returns them sorted by timestamp and gives you a cursor = oldestMessageTimestamp.
  • Next request → GET /messages?before=2024-09-01T10:05:00&limit=20
  • This fetches the 20 older messages (before that timestamp).

👉 Even if new messages arrive while you’re scrolling, they don’t disturb your “older messages” loading, because you’re going back in time using a cursor, not using a page number.

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🔹 Why This Works for Chats/Feeds

• Messages are naturally ordered by time (newest → oldest).
• Cursor = “last seen timestamp/messageId”.
• Always consistent, regardless of new inserts.

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✅ So yes —

• Feeds/chats (WhatsApp, Instagram, Twitter) → cursor-based pagination.
• Static data (reports, admin dashboards) → pageNumber + pageSize is usually fine.

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