RESTful API design for devices in IOT Protocols - Time & Space Complexity

Time Complexity: RESTful API design for devices

O(n)

Understanding Time Complexity

When devices communicate using RESTful APIs, the time it takes to handle requests matters a lot.

We want to know how the processing time grows as more devices or requests come in.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

GET /devices
  fetch all devices from database
  for each device:
    fetch device status
  return list of devices with status
    

This code fetches all devices and then gets the status for each device before sending the response.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through each device to fetch its status.
• How many times: Once for every device in the list.

How Execution Grows With Input

As the number of devices grows, the time to fetch all statuses grows too.

Input Size (n)	Approx. Operations
10	10 status fetches
100	100 status fetches
1000	1000 status fetches

Pattern observation: The work grows directly with the number of devices.

Final Time Complexity

Time Complexity: O(n)

This means if you double the devices, the time to process doubles too.

Common Mistake

[X] Wrong: "Fetching all devices and their statuses is always fast regardless of device count."

[OK] Correct: Each device adds extra work, so more devices mean more time needed.

Interview Connect

Understanding how request time grows with device count shows you can design APIs that scale well.

Self-Check

"What if the API fetched all device statuses in one batch call instead of one by one? How would the time complexity change?"

Practice

1. Which HTTP method is typically used to retrieve data from a device in a RESTful API?

easy

A. DELETE

B. GET

C. PUT

D. POST

Solution

1. Step 1: Understand HTTP methods in REST

   GET is used to request data without changing it, ideal for reading device info.

2. Step 2: Match method to action

   Since retrieving data means reading, GET fits best over POST, PUT, or DELETE.

3. Final Answer:

   GET -> Option B

4. Quick Check:

   Retrieve data = GET [OK]

Hint: GET always fetches data without changing it [OK]

Common Mistakes:

• Confusing POST with GET for data retrieval
• Using DELETE to get data
• Thinking PUT retrieves data

2. Which of the following is the correct RESTful URL to update the settings of a device with ID 123?

easy

A. /update/device/123

B. /device/123/update

C. /devices/settings/123

D. /devices/123/settings

Solution

1. Step 1: Identify RESTful URL structure

   RESTful URLs use nouns and hierarchy: resource then ID then sub-resource.

2. Step 2: Check options for correct pattern

   /devices/123/settings correctly shows device resource, ID, then settings sub-resource.

3. Final Answer:

   /devices/123/settings -> Option D

4. Quick Check:

   Resource/ID/sub-resource = /devices/123/settings [OK]

Hint: Use resource/ID/sub-resource format for REST URLs [OK]

Common Mistakes:

• Placing ID after sub-resource
• Using verbs like 'update' in URL
• Mixing singular and plural resource names

3. Given this RESTful API call: DELETE /devices/456, what is the expected result?

medium

A. Remove device 456 from the system

B. Update device 456 information

C. Retrieve device 456 details

D. Create a new device with ID 456

Solution

1. Step 1: Understand DELETE method purpose

   DELETE removes the specified resource from the server.

2. Step 2: Apply to device 456

   DELETE /devices/456 means remove device with ID 456 from the system.

3. Final Answer:

   Remove device 456 from the system -> Option A

4. Quick Check:

   DELETE method = remove resource [OK]

Hint: DELETE removes resources, not fetch or update [OK]

Common Mistakes:

• Thinking DELETE retrieves data
• Confusing DELETE with POST or PUT
• Assuming DELETE creates resources

4. You wrote this API call to update a device: POST /devices/789 but it creates a new device instead. What is the likely mistake?

medium

A. Using POST instead of PUT for update

B. Missing device ID in URL

C. Using GET method instead of POST

D. Incorrect device ID format

Solution

1. Step 1: Understand POST vs PUT

   POST usually creates new resources; PUT updates existing ones.

2. Step 2: Analyze the API call

   POST /devices/789 creates new device; to update, use PUT /devices/789.

3. Final Answer:

   Using POST instead of PUT for update -> Option A

4. Quick Check:

   Update needs PUT, not POST [OK]

Hint: Use PUT to update, POST to create [OK]

Common Mistakes:

• Using POST for updates
• Confusing GET with POST
• Ignoring HTTP method meaning

5. You want to design a RESTful API to control multiple devices' power state at once. Which design is best?

hard

A. PUT /device/{id}/power for each device separately

B. GET /devices/power to fetch all power states

C. POST /devices/power with JSON list of device IDs and states

D. DELETE /devices/power to reset all devices

Solution

1. Step 1: Identify batch update needs

   To control many devices at once, send a batch request with data.

2. Step 2: Match HTTP method and URL

   POST /devices/power with JSON list allows updating multiple devices in one call.

3. Step 3: Evaluate other options

   GET fetches data, PUT is per device, DELETE resets devices--none fit batch control well.

4. Final Answer:

   POST /devices/power with JSON list of device IDs and states -> Option C

5. Quick Check:

   Batch update = POST with data list [OK]

Hint: Use POST with data list for batch device updates [OK]

Common Mistakes:

• Using GET to change device states
• Trying PUT for batch updates
• Using DELETE to control power