Batch update patterns in Rest API - Time & Space Complexity

Time Complexity: Batch update patterns

O(n)

Understanding Time Complexity

When updating many items at once using a batch update, it is important to understand how the time needed grows as the number of items increases.

We want to know how the total work changes when we update more records in one go.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

POST /api/items/batch-update
Request Body: { "items": [ {"id":1, "value":"A"}, {"id":2, "value":"B"}, ... ] }

// Server side pseudo-code
for item in request.items:
    update_database(item.id, item.value)
return success
    

This code updates each item in the batch one by one in the database.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Loop over each item to update it in the database.
• How many times: Once for every item in the batch.

How Execution Grows With Input

As the number of items to update grows, the total work grows in a similar way.

Input Size (n)	Approx. Operations
10	10 updates
100	100 updates
1000	1000 updates

Pattern observation: Doubling the number of items roughly doubles the work needed.

Final Time Complexity

Time Complexity: O(n)

This means the time to complete the batch update grows directly in proportion to the number of items.

Common Mistake

[X] Wrong: "Batch updating many items is always constant time because it's one request."

[OK] Correct: Even though it's one request, the server still processes each item one by one, so time grows with the number of items.

Interview Connect

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

Self-Check

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

Practice

1. What is the main advantage of using batch update patterns in REST APIs?

easy

A. They only work with GET requests.

B. They allow updating multiple items with a single request, saving time.

C. They require sending separate requests for each item.

D. They increase the number of requests to the server.

Solution

1. Step 1: Understand batch update purpose

   Batch update patterns let you update many items at once instead of one by one.

2. Step 2: Identify the benefit

   This reduces the number of requests and saves time and server resources.

3. Final Answer:

   They allow updating multiple items with a single request, saving time. -> Option B

4. Quick Check:

   Batch update = single request for many items [OK]

Hint: Batch updates reduce requests by combining many updates [OK]

Common Mistakes:

• Thinking batch updates increase requests
• Confusing batch update with single item update
• Assuming batch updates use GET method

2. Which HTTP method is most appropriate for batch updates in REST APIs?

easy

A. GET

B. POST

C. PATCH

D. DELETE

Solution

1. Step 1: Recall HTTP methods for updates

   PATCH is used to partially update resources, suitable for batch updates.

2. Step 2: Eliminate other methods

   GET retrieves data, POST creates new data, DELETE removes data, so they are not best for batch updates.

3. Final Answer:

   PATCH -> Option C

4. Quick Check:

   Batch update uses PATCH method [OK]

Hint: Use PATCH for partial or batch updates [OK]

Common Mistakes:

• Using GET for updates
• Confusing POST with update method
• Using DELETE instead of PATCH

3. Given this JSON payload sent in a PATCH request for batch update:

[{"id":1,"status":"done"},{"id":2,"status":"pending"}]

What does this payload represent?

medium

A. Retrieve items with id 1 and 2.

B. Create two new items with given ids and statuses.

C. Delete items with id 1 and 2.

D. Update status of items with id 1 and 2 to 'done' and 'pending' respectively.

Solution

1. Step 1: Analyze JSON payload

   The payload is an array of objects each with 'id' and 'status' fields, indicating updates.

2. Step 2: Understand PATCH semantics

   PATCH updates existing items partially, so this changes status fields for items with id 1 and 2.

3. Final Answer:

   Update status of items with id 1 and 2 to 'done' and 'pending' respectively. -> Option D

4. Quick Check:

   PATCH with array updates multiple items [OK]

Hint: PATCH with array updates multiple items by id [OK]

Common Mistakes:

• Thinking PATCH creates new items
• Confusing update with delete
• Assuming it retrieves data

4. You send this PATCH request payload to update multiple items:

[{"id":1,"status":"done"}, {"status":"pending"}]

What is the likely problem?

medium

A. Missing 'id' in second object causes update failure or error.

B. The payload is valid and will update all items.

C. Using PATCH instead of POST causes syntax error.

D. The array should be empty for batch updates.

Solution

1. Step 1: Check each object in payload

   The second object lacks 'id', so the server cannot identify which item to update.

2. Step 2: Understand update requirements

   Each update must specify the item to update, usually by 'id'. Missing it causes failure or error.

3. Final Answer:

   Missing 'id' in second object causes update failure or error. -> Option A

4. Quick Check:

   Each update needs an identifier like 'id' [OK]

Hint: Every update object must include an identifier like 'id' [OK]

Common Mistakes:

• Assuming missing 'id' is allowed
• Confusing PATCH with POST errors
• Thinking empty arrays are required

5. You want to batch update user roles but only for users currently active. Which pattern best fits this requirement?

hard

A. Send a PATCH request with an array of user objects filtered to active users only.

B. Send separate PATCH requests for all users regardless of status.

C. Use GET request to update roles for active users.

D. Send a DELETE request with active users to update roles.

Solution

1. Step 1: Identify filtering need

   Only active users should be updated, so filter users before sending batch update.

2. Step 2: Choose correct method and payload

   PATCH with an array of filtered user objects updates roles efficiently in one request.

3. Final Answer:

   Send a PATCH request with an array of user objects filtered to active users only. -> Option A

4. Quick Check:

   Filter then batch update with PATCH [OK]

Hint: Filter data first, then batch update with PATCH [OK]

Common Mistakes:

• Sending updates for all users without filtering
• Using GET or DELETE for updates
• Sending multiple separate requests instead of batch