Offset-based pagination in Rest API - Step-by-Step Execution

Concept Flow - Offset-based pagination

Client sends request with offset & limit

↓

Server receives offset & limit

↓

Server queries data starting at offset

↓

Server returns limited data slice

↓

Client displays current page data

↓

Client requests next page with updated offset

↓

Repeat process

The client requests data with an offset and limit. The server returns a slice of data starting at the offset. The client uses this to show pages and requests more by changing the offset.

Execution Sample

Rest API

GET /items?offset=10&limit=5

Server:
  data = all_items[10:15]
  return data

Client asks for 5 items starting from the 11th item (offset 10). Server returns items 10 to 14.

Execution Table

Step	Request URL	Offset	Limit	Data Slice Returned	Client Action
1	/items?offset=0&limit=5	0	5	items[0..4]	Display first 5 items
2	/items?offset=5&limit=5	5	5	items[5..9]	Display next 5 items
3	/items?offset=10&limit=5	10	5	items[10..14]	Display next 5 items
4	/items?offset=15&limit=5	15	5	items[15..19]	Display next 5 items
5	/items?offset=20&limit=5	20	5	items[20..24]	Display next 5 items
6	/items?offset=25&limit=5	25	5	items[25..29]	Display next 5 items
7	/items?offset=30&limit=5	30	5	items[30..34]	Display next 5 items
8	/items?offset=35&limit=5	35	5	items[35..39]	Display next 5 items
9	/items?offset=40&limit=5	40	5	items[40..44]	Display next 5 items
10	/items?offset=45&limit=5	45	5	items[45..49]	Display next 5 items
11	/items?offset=50&limit=5	50	5	items[50..54]	Display next 5 items
12	/items?offset=55&limit=5	55	5	items[55..59]	Display next 5 items
13	/items?offset=60&limit=5	60	5	items[60..64]	Display next 5 items
14	/items?offset=65&limit=5	65	5	items[65..69]	Display next 5 items
15	/items?offset=70&limit=5	70	5	items[70..74]	Display next 5 items
16	/items?offset=75&limit=5	75	5	items[75..79]	Display next 5 items
17	/items?offset=80&limit=5	80	5	items[80..84]	Display next 5 items
18	/items?offset=85&limit=5	85	5	items[85..89]	Display next 5 items
19	/items?offset=90&limit=5	90	5	items[90..94]	Display next 5 items
20	/items?offset=95&limit=5	95	5	items[95..99]	Display last 5 items
21	/items?offset=100&limit=5	100	5	[]	No more items, stop
22	N/A	N/A	N/A	N/A	End of pagination

💡 When offset reaches 100, no more items are returned, so pagination stops.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	After 7	After 8	After 9	After 10	Final
offset	0	5	10	15	20	25	30	35	40	45	50	100
limit	5	5	5	5	5	5	5	5	5	5	5	5
data slice	items[0..4]	items[5..9]	items[10..14]	items[15..19]	items[20..24]	items[25..29]	items[30..34]	items[35..39]	items[40..44]	items[45..49]	items[50..54]	[]

Key Moments - 3 Insights

Why does the server return an empty list when offset is 100?

Because the data only has 100 items (0 to 99). When offset is 100, there are no items starting at that position, so the server returns an empty list as shown in execution_table row 21.

What happens if the client requests a limit larger than available items?

The server returns only the remaining items up to the end of the data. For example, if offset is 95 and limit is 10, only items 95 to 99 are returned, fewer than 10 items, as seen in execution_table row 20.

How does the client know when to stop requesting more pages?

When the server returns an empty list or fewer items than the limit, the client knows there are no more pages. This is shown in execution_table row 21 where data slice is empty.

Visual Quiz - 3 Questions

Look at the execution_table at step 3. What is the offset value used in the request?

A 5

B 15

C 10

D 0

💡 Hint

Check the 'Offset' column in execution_table row 3.

At which step does the server return an empty data slice indicating no more items?

A Step 20

B Step 21

C Step 19

D Step 22

💡 Hint

Look at the 'Data Slice Returned' column for an empty list in execution_table.

If the client changes limit from 5 to 10, how will the data slice change at step 1?

A items[0..9]

B items[0..4]

C items[5..14]

D items[10..19]

💡 Hint

Data slice is from offset to offset+limit-1, check variable_tracker for limit effect.

Concept Snapshot

Offset-based pagination uses two numbers: offset and limit.
Offset tells where to start in the list.
Limit tells how many items to return.
Client sends offset and limit in request.
Server returns that slice of data.
Client updates offset to get next page.

Full Transcript

Offset-based pagination is a way to get data in small pieces or pages. The client asks the server for data starting at a certain position called offset, and asks for a certain number of items called limit. The server returns that slice of data. The client shows this data and when the user wants more, it asks again with a bigger offset. This repeats until no more data is left. For example, if the client asks for offset 10 and limit 5, the server returns items 10 to 14. When the offset reaches the end of the data, the server returns an empty list, telling the client to stop. This method is simple and works well for many APIs.

Practice

1. What does offset represent in offset-based pagination?

easy

A. The maximum number of pages available

B. The total number of items to return in the response

C. The current page number being requested

D. The number of items to skip before starting to collect results

Solution

1. Step 1: Understand the role of offset

   Offset tells the system how many items to skip before starting to return data.

2. Step 2: Differentiate offset from limit and page

   Limit controls how many items to return; page number is a different pagination method.

3. Final Answer:

   The number of items to skip before starting to collect results -> Option D

4. Quick Check:

   Offset = items skipped before results [OK]

Hint: Offset means how many items to skip before fetching [OK]

Common Mistakes:

• Confusing offset with limit
• Thinking offset is the page number
• Assuming offset is total items count

2. Which of the following is the correct way to request the second page of results with 10 items per page using offset-based pagination?

easy

A. GET /items?start=10&count=10

B. GET /items?offset=10&limit=10

C. GET /items?page=2&limit=10

D. GET /items?offset=2&limit=10

Solution

1. Step 1: Calculate offset for page 2 with 10 items per page

   Offset = (page number - 1) * limit = (2 - 1) * 10 = 10.

2. Step 2: Identify correct query parameters

   Offset and limit are standard; page parameter is not used in offset-based pagination.

3. Final Answer:

   GET /items?offset=10&limit=10 -> Option B

4. Quick Check:

   Offset = 10 for page 2 with 10 items [OK]

Hint: Offset = (page - 1) x limit for correct pagination [OK]

Common Mistakes:

• Using page instead of offset
• Setting offset to page number directly
• Using non-standard parameter names

3. Given the API call GET /products?offset=5&limit=3 and the product list ["A", "B", "C", "D", "E", "F", "G", "H"], what will be the returned products?

medium

A. ["D", "E", "F"]

B. ["E", "F", "G"]

C. ["F", "G", "H"]

D. ["G", "H"]

Solution

1. Step 1: Identify starting index using offset

   Offset 5 means skip first 5 items: A(0), B(1), C(2), D(3), E(4) skipped; start at index 5.

2. Step 2: Select limit number of items from offset

   Limit is 3, so select items at indices 5, 6, 7: F, G, H.

3. Final Answer:

   ["F", "G", "H"] -> Option C

4. Quick Check:

   Offset 5 + limit 3 = F, G, H [OK]

Hint: Start at offset index, take limit items [OK]

Common Mistakes:

• Starting at offset - 1 index
• Including offset item in skipped items
• Returning fewer or more items than limit

4. You have this API call: GET /users?offset=20&limit=10. The API returns an empty list even though there are 25 users total. What is the likely problem?

medium

A. Offset is too large, skipping all remaining users

B. Limit is too small to return any users

C. Offset and limit parameters are swapped

D. API does not support offset-based pagination

Solution

1. Step 1: Calculate remaining items after offset

   Offset 20 skips first 20 users; only 5 users remain (25 - 20 = 5).

2. Step 2: Understand why empty list is returned

   API returns empty list likely because it expects at least 10 items (limit), but only 5 remain; some APIs may return empty if offset exceeds total count.

3. Final Answer:

   Offset is too large, skipping all remaining users -> Option A

4. Quick Check:

   Offset > total users - limit causes empty results [OK]

Hint: Check if offset skips beyond total items [OK]

Common Mistakes:

• Assuming limit controls start position
• Swapping offset and limit values
• Ignoring total item count in pagination

5. You want to implement offset-based pagination for a large dataset but want to avoid performance issues with very large offsets. Which approach is best to improve performance?

hard

A. Use keyset pagination by filtering with a unique indexed column instead of offset

B. Increase the limit value to reduce the number of pages

C. Cache all pages in memory to avoid database queries

D. Use offset with very large values and rely on database optimization

Solution

1. Step 1: Understand offset performance issues

   Large offsets cause the database to scan many rows before returning results, slowing queries.

2. Step 2: Identify better pagination method

   Keyset pagination uses a unique indexed column (like ID) to fetch next pages efficiently without scanning skipped rows.

3. Step 3: Evaluate other options

   Increasing limit or caching is not scalable; relying on database optimization alone is insufficient.

4. Final Answer:

   Use keyset pagination by filtering with a unique indexed column instead of offset -> Option A

5. Quick Check:

   Keyset pagination avoids large offset performance issues [OK]

Hint: Use keyset pagination to avoid slow large offsets [OK]

Common Mistakes:

• Relying on large offset values for deep pages
• Increasing limit without considering user experience
• Assuming caching solves pagination performance