MSDN Documentation

API Design: Pagination

Pagination is a crucial aspect of API design when dealing with large datasets. It allows clients to retrieve data in manageable chunks, improving performance, reducing bandwidth usage, and preventing server overload. This document outlines best practices for implementing pagination in your APIs.

Why Paginate?

• Performance: Fetching thousands or millions of records at once is inefficient and can lead to timeouts or excessive memory usage.
• User Experience: For UIs, displaying a huge list of items at once is impractical. Pagination breaks it down into digestible pages.
• Resource Management: Prevents the server from being overwhelmed by a single, massive request.
• Bandwidth: Reduces the amount of data transferred between the client and server for each request.

Common Pagination Strategies

1. Offset/Limit (or Page/Size)

This is one of the most straightforward methods. The client specifies the number of items to skip (offset) and the maximum number of items to return (limit) per page.

Example Request:

GET /api/v1/users?page=2&size=50

In this example, the client is requesting the second page of results, with 50 items per page. This is equivalent to an offset of 50 (assuming page numbers are 1-based).

Pros:

• Simple to understand and implement.
• Easy for clients to navigate to specific pages.

Cons:

• Performance Issues with Large Datasets: As the offset increases, the database must still scan and discard all the preceding records, which can become very slow.
• Skipping Records: If new records are added or removed between page requests, the client might skip records or see duplicates.

2. Cursor-Based Pagination (Keyset Pagination)

This method uses a pointer (cursor) to indicate the position in the dataset. Instead of an offset, the client requests items "after" a specific cursor. This is generally more performant for large datasets.

The cursor typically represents a unique, ordered identifier (like an ID or timestamp) of the last item seen on the previous page.

Example Request:

GET /api/v1/users?limit=50&after_cursor=user_abc123

Response Example:

{
  "data": [
    { "id": "user_def456", "name": "Jane Doe" },
    // ... 49 more users
  ],
  "meta": {
    "next_cursor": "user_ghi789",
    "has_more": true
  }
}

Pros:

• Performance: Much more efficient for large datasets as it uses database indexing to fetch records.
• Consistency: Less prone to skipping or duplicating records when data is added or removed.

Cons:

• Cannot directly jump to a specific page number (e.g., "go to page 100").
• Requires careful implementation to ensure cursors are unique and consistently ordered.

Common Response Structure

A well-designed paginated response should include metadata about the pagination itself, not just the data.

Example Response (Offset/Limit):

{
  "data": [
    // ... array of items ...
  ],
  "meta": {
    "total_items": 5432,
    "items_per_page": 50,
    "current_page": 2,
    "total_pages": 109,
    "next_page_url": "/api/v1/users?page=3&size=50",
    "prev_page_url": "/api/v1/users?page=1&size=50"
  }
}

Example Response (Cursor-Based):

{
  "data": [
    // ... array of items ...
  ],
  "links": {
    "next": "/api/v1/users?limit=50&after_cursor=user_ghi789",
    "prev": "/api/v1/users?limit=50&before_cursor=user_abc123"
  },
  "meta": {
    "has_more": true,
    "count": 50 // Number of items returned in this response
  }
}

Key Considerations for Pagination Implementation

• Consistency: Always return the same number of items per page, unless the client explicitly requests a different number and it's supported.
• Defaults: Provide sensible default values for page size if the client doesn't specify them.
• Maximum Limit: Implement a maximum allowed page size to prevent clients from requesting excessively large chunks.
• Clear Naming: Use descriptive parameter names (e.g., page, size, limit, after_cursor).
• Sorting: Pagination is often used in conjunction with sorting. Ensure that cursors or ordering logic are stable and predictable based on the sort parameters.
• Empty Responses: Handle cases where there are no items to return gracefully.

By implementing effective pagination, you can build more robust, scalable, and user-friendly APIs.