Batch delete patterns in Rest API - Time & Space Complexity

Time Complexity: Batch delete patterns

O(n)

Understanding Time Complexity

When deleting many items at once using a batch delete in a REST API, it is important to understand how the time taken grows as the number of items increases.

We want to know how the work done changes when we delete more items in one request.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

// Example batch delete endpoint
app.delete('/items', async (req, res) => {
  const ids = req.body.ids; // array of item IDs to delete
  for (const id of ids) {
    await database.deleteItemById(id);
  }
  res.send({ message: 'Items deleted' });
});

This code deletes each item one by one by looping through the list of IDs and deleting each from the database.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through the list of IDs and deleting each item individually.
• How many times: Once for each item in the input list.

How Execution Grows With Input

As the number of items to delete grows, the total work grows roughly the same amount.

Input Size (n)	Approx. Operations
10	10 delete calls
100	100 delete calls
1000	1000 delete calls

Pattern observation: The work grows linearly with the number of items to delete.

Final Time Complexity

Time Complexity: O(n)

This means the time to delete items grows directly in proportion to how many items you want to delete.

Common Mistake

[X] Wrong: "Deleting multiple items in one batch always takes the same time as deleting one item."

[OK] Correct: Each item still needs to be deleted, so the total time adds up with more items.

Interview Connect

Understanding how batch operations scale helps you design efficient APIs and explain your choices clearly in interviews.

Self-Check

"What if the batch delete used a single database query to delete all items at once? How would the time complexity change?"

Practice

1.

What is the main advantage of using batch delete in a REST API?

easy

A. It removes multiple items with a single request, saving time.

B. It automatically backs up data before deletion.

C. It encrypts data during deletion.

D. It requires multiple requests for each item.

Solution

1. Step 1: Understand batch delete purpose

   Batch delete is designed to remove many items at once, not one by one.

2. Step 2: Identify the benefit

   Removing multiple items in one request saves time and reduces server load.

3. Final Answer:

   It removes multiple items with a single request, saving time. -> Option A

4. Quick Check:

   Batch delete = single request for many items [OK]

Hint: Batch delete means one request deletes many items [OK]

Common Mistakes:

• Thinking batch delete backs up data
• Confusing batch delete with encryption
• Assuming batch delete needs multiple requests

2.

Which of the following is the correct HTTP method to perform a batch delete in a REST API?

easy

A. GET

B. POST

C. DELETE

D. PUT

Solution

1. Step 1: Recall HTTP methods for REST

   DELETE method is used to remove resources in REST APIs.

2. Step 2: Match method to batch delete

   Batch delete also uses DELETE but with multiple IDs in body or query.

3. Final Answer:

   DELETE -> Option C

4. Quick Check:

   Batch delete uses DELETE method [OK]

Hint: DELETE method removes resources in REST APIs [OK]

Common Mistakes:

• Using POST instead of DELETE for deletion
• Confusing GET with delete operations
• Using PUT which is for updates

3.

Given this REST API call to delete users with IDs 3, 5, and 7:
DELETE /users?ids=3,5,7
What is the expected behavior?

medium

A. Only user with ID 3 is deleted.

B. Users are deleted one by one with separate requests.

C. The server returns an error because multiple IDs are not allowed.

D. Users with IDs 3, 5, and 7 are deleted in one request.

Solution

1. Step 1: Analyze the query parameter usage

   The query parameter 'ids=3,5,7' lists multiple IDs to delete.

2. Step 2: Understand batch delete behavior

   The API deletes all specified users in one DELETE request.

3. Final Answer:

   Users with IDs 3, 5, and 7 are deleted in one request. -> Option D

4. Quick Check:

   Batch delete with multiple IDs = one request deletes all [OK]

Hint: Multiple IDs in query means batch delete in one request [OK]

Common Mistakes:

• Assuming only first ID is deleted
• Thinking server rejects multiple IDs
• Believing multiple requests are sent automatically

4.

Consider this batch delete request:
DELETE /items
with JSON body:
{ "ids": [10, 20, 30] }
But the server responds with 400 Bad Request. What is the likely cause?

medium

A. The JSON format is incorrect.

B. The server expects IDs in the query string, not body.

C. The DELETE method cannot have a request body.

D. The IDs array is empty.

Solution

1. Step 1: Check DELETE method usage

   Some APIs require IDs in query string, not in request body for DELETE.

2. Step 2: Identify server expectation

   Since server returns 400, it likely expects IDs in query parameters, not JSON body.

3. Final Answer:

   The server expects IDs in the query string, not body. -> Option B

4. Quick Check:

   Server expects IDs in query, not body [OK]

Hint: Check if server wants IDs in query or body for DELETE [OK]

Common Mistakes:

• Assuming DELETE cannot have a body always
• Thinking JSON format is wrong without checking
• Ignoring server API documentation

5.

You want to design a batch delete endpoint for a REST API that deletes multiple products by IDs sent in the request body as JSON. Which approach is best practice?

{
  "ids": [101, 102, 103]
}

Choose the correct method and request format.

hard

A. Use DELETE method with JSON body containing IDs array.

B. Use POST method with JSON body containing IDs array.

C. Use DELETE method with IDs in query string separated by commas.

D. Use GET method with IDs in URL path.

Solution

1. Step 1: Understand REST conventions for delete

   DELETE method is standard for removing resources, including batch deletes.

2. Step 2: Check request body usage

   Modern APIs allow DELETE with JSON body to specify multiple IDs for batch delete.

3. Step 3: Evaluate options

   POST is not for deletion, GET is for retrieval, query string can be limited in length.

4. Final Answer:

   Use DELETE method with JSON body containing IDs array. -> Option A

5. Quick Check:

   DELETE + JSON body for batch delete = best practice [OK]

Hint: Use DELETE with JSON body for multiple IDs batch delete [OK]

Common Mistakes:

• Using POST instead of DELETE for deletion
• Putting IDs in query string risking URL length limits
• Using GET which is not for deletion