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Rest APIprogramming~5 mins

PATCH for partial updates in Rest API - Time & Space Complexity

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Time Complexity: PATCH for partial updates
O(k)
Understanding Time Complexity

When using PATCH requests to update parts of data, it's important to know how the time to process changes as the data grows.

We want to understand how the server's work changes when updating only some fields.

Scenario Under Consideration

Analyze the time complexity of the following PATCH request handler.


PATCH /users/{id}
Request body: { "email": "new@example.com" }

// Server-side pseudo-code
user = findUserById(id)
for each field in request body:
  user[field] = request body[field]
save user
return updated user

This code updates only the fields sent in the PATCH request for a user.

Identify Repeating Operations

Look for repeated steps that take time depending on input size.

  • Primary operation: Looping over fields in the request body to update user data.
  • How many times: Once per field sent in the PATCH request.
How Execution Grows With Input

The time to update grows with how many fields you send to change.

Input Size (fields to update)Approx. Operations
11 update operation
55 update operations
2020 update operations

Pattern observation: The work grows directly with the number of fields you update.

Final Time Complexity

Time Complexity: O(k)

This means the time grows linearly with the number of fields you want to update, not the total data size.

Common Mistake

[X] Wrong: "PATCH always takes the same time no matter how many fields are updated."

[OK] Correct: The server must process each field you send, so more fields mean more work and longer time.

Interview Connect

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

Self-Check

"What if the PATCH request included nested objects to update? How would that affect the time complexity?"

Practice

(1/5)
1. What is the main purpose of using the PATCH method in REST APIs?
easy
A. To update only specific fields of a resource without sending the entire data
B. To create a new resource on the server
C. To delete a resource completely
D. To retrieve data from the server

Solution

  1. Step 1: Understand HTTP methods

    HTTP methods like GET, POST, PUT, DELETE, and PATCH have different purposes. PATCH is designed for partial updates.

  2. Step 2: Identify PATCH behavior

    PATCH sends only the fields that need to be changed, unlike PUT which requires the full resource data.

  3. Final Answer:

    To update only specific fields of a resource without sending the entire data -> Option A

  4. Quick Check:

    PATCH updates parts only [OK]
Hint: PATCH updates parts, not whole resource [OK]
Common Mistakes:
  • Confusing PATCH with PUT which replaces full resource
  • Thinking PATCH creates new resources
  • Using PATCH to delete resources
2. Which of the following is the correct way to send a PATCH request to update a user's email using JSON?
easy
A. PATCH /users/123 {"email": "new@example.com"}
B. PATCH /users/123 {"username": "newuser"}
C. GET /users/123 {"email": "new@example.com"}
D. DELETE /users/123 {"email": "new@example.com"}

Solution

  1. Step 1: Identify HTTP method and endpoint

    PATCH is used to update part of the resource at /users/123.

  2. Step 2: Check JSON body for correct field

    The JSON body contains only the field to update, here "email" with new value.

  3. Final Answer:

    PATCH /users/123 {"email": "new@example.com"} -> Option A

  4. Quick Check:

    PATCH + partial JSON body = correct syntax [OK]
Hint: PATCH with only fields to update in JSON body [OK]
Common Mistakes:
  • Using GET or DELETE instead of PATCH
  • Sending unrelated fields in PATCH body
  • Trying to update username when email is asked
3. Given this PATCH request body to update a product: 
{"price": 19.99}
What will be the result if the original product data was 
{"name": "Book", "price": 25.00, "stock": 10}
?
medium
A. {"name": "Book", "price": 25.00, "stock": 10}
B. {"price": 19.99}
C. {"name": "Book", "price": 19.99, "stock": 10}
D. {"name": "Book", "stock": 10}

Solution

  1. Step 1: Understand PATCH partial update

    PATCH updates only the fields sent, here "price" changes to 19.99.

  2. Step 2: Combine unchanged fields with updated field

    Fields not in PATCH body remain unchanged, so "name" and "stock" stay the same.

  3. Final Answer:

    {"name": "Book", "price": 19.99, "stock": 10} -> Option C

  4. Quick Check:

    PATCH updates price only, others unchanged [OK]
Hint: PATCH changes only sent fields, keeps others [OK]
Common Mistakes:
  • Replacing entire object instead of partial update
  • Removing fields not in PATCH body
  • Ignoring updated field in result
4. You send this PATCH request to update a user's profile: 
{"email": "new@example.com", "age": "twenty"}
The server expects age as an integer. What is the likely problem?
medium
A. The PATCH request will succeed and update both fields
B. The server will delete the age field from the user profile
C. The server will ignore the age field and update only email
D. The server will reject the request due to invalid data type for age

Solution

  1. Step 1: Check data types in PATCH body

    "age" is sent as string "twenty" but server expects integer.

  2. Step 2: Understand server validation

    Most servers validate data types and reject requests with invalid types.

  3. Final Answer:

    The server will reject the request due to invalid data type for age -> Option D

  4. Quick Check:

    Invalid data type causes rejection [OK]
Hint: Check data types match server expectations [OK]
Common Mistakes:
  • Assuming server auto-converts wrong types
  • Thinking PATCH ignores invalid fields silently
  • Believing PATCH deletes fields on error
5. You have a REST API for managing tasks with fields: title, completed, and priority. You want to mark a task as completed without changing other fields. Which PATCH request body is best?
hard
A. {"title": "", "completed": true, "priority": null}
B. {"completed": true}
C. {"completed": true, "priority": "high"}
D. {"title": "New Task"}

Solution

  1. Step 1: Identify minimal update needed

    Only the "completed" field needs to change, so send only that.

  2. Step 2: Avoid sending unrelated or empty fields

    Sending empty or null fields may overwrite data unintentionally.

  3. Final Answer:

    {"completed": true} -> Option B

  4. Quick Check:

    Send only changed fields in PATCH body [OK]
Hint: Send only fields you want to update in PATCH [OK]
Common Mistakes:
  • Sending all fields even if unchanged
  • Sending empty or null values accidentally
  • Using PUT instead of PATCH for partial update
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