HEAD and OPTIONS methods in Rest API - Time & Space Complexity

Time Complexity: HEAD and OPTIONS methods

O(1)

Understanding Time Complexity

We want to understand how the time to handle HEAD and OPTIONS requests changes as the number of resources grows.

How does the server's work grow when it processes these methods?

Scenario Under Consideration

Analyze the time complexity of the following REST API handling code.

// Example pseudocode for OPTIONS method
function handleOptions(request) {
  let allowedMethods = ["GET", "POST", "HEAD", "OPTIONS"];
  return { "Allow": allowedMethods.join(", ") };
}

// Example pseudocode for HEAD method
function handleHead(request, resource) {
  let headers = getHeaders(resource);
  return { headers };
}
    

This code returns allowed methods for OPTIONS and headers only for HEAD without body.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Joining the list of allowed methods for OPTIONS.
• How many times: The join operation runs once per OPTIONS request, over a fixed small list.
• HEAD method: Fetching headers is a direct lookup, no loops over data.

How Execution Grows With Input

The allowed methods list is small and fixed, so the work for OPTIONS stays the same no matter how many resources exist.

Fetching headers for HEAD depends on the resource but usually is a direct access, so it grows very little with resource size.

Input Size (n)	Approx. Operations
10	Few operations (joining 4 methods, fetching headers)
100	Same few operations, no increase
1000	Still same few operations, no increase

Pattern observation: The work stays constant regardless of input size.

Final Time Complexity

Time Complexity: O(1)

This means the time to handle HEAD or OPTIONS requests stays the same no matter how many resources the server has.

Common Mistake

[X] Wrong: "Handling OPTIONS or HEAD requests takes longer as the number of resources grows."

[OK] Correct: These methods usually only check fixed information or headers, so their work does not grow with resource count.

Interview Connect

Understanding that some HTTP methods have constant time handling helps you explain efficient API design and server behavior clearly.

Self-Check

"What if the OPTIONS method dynamically generated allowed methods based on resource state? How would the time complexity change?"

Practice

1. What is the primary purpose of the HEAD method in REST APIs?

easy

A. To create a new resource on the server

B. To delete a resource from the server

C. To update a resource partially

D. To retrieve only the headers of a resource without the body

Solution

1. Step 1: Understand the HEAD method role

   The HEAD method requests headers only, not the full content.

2. Step 2: Compare with other methods

   Unlike GET, it does not return the body, only metadata.

3. Final Answer:

   To retrieve only the headers of a resource without the body -> Option D

4. Quick Check:

   HEAD returns headers only [OK]

Hint: HEAD fetches headers only, no content body [OK]

Common Mistakes:

• Confusing HEAD with GET which returns full content
• Thinking HEAD modifies data on server
• Assuming HEAD returns body content

2. Which of the following is the correct syntax to send an OPTIONS request using curl?

easy

A. curl -X POST https://api.example.com/resource

B. curl -X HEAD https://api.example.com/resource

C. curl -X OPTIONS https://api.example.com/resource

D. curl --get https://api.example.com/resource

Solution

1. Step 1: Identify the method flag in curl

   The -X option specifies the HTTP method to use.

2. Step 2: Match the method to the question

   To send an OPTIONS request, use -X OPTIONS.

3. Final Answer:

   curl -X OPTIONS https://api.example.com/resource -> Option C

4. Quick Check:

   curl -X OPTIONS sends OPTIONS request [OK]

Hint: Use curl -X OPTIONS to send OPTIONS request [OK]

Common Mistakes:

• Using -X HEAD instead of OPTIONS
• Using --get which sends GET request
• Using POST method incorrectly

3. Given the following HTTP response headers from an OPTIONS request:

Allow: GET, POST, OPTIONS, HEAD

What does this header tell the client?

medium

A. The server supports only GET and POST methods

B. The server supports GET, POST, OPTIONS, and HEAD methods on this resource

C. The server denies all methods except OPTIONS

D. The server supports DELETE and PUT methods

Solution

1. Step 1: Read the Allow header

   The Allow header lists HTTP methods the server supports for the resource.

2. Step 2: Interpret the methods listed

   It shows GET, POST, OPTIONS, and HEAD are allowed methods.

3. Final Answer:

   The server supports GET, POST, OPTIONS, and HEAD methods on this resource -> Option B

4. Quick Check:

   Allow header lists supported methods [OK]

Hint: Allow header shows all supported methods [OK]

Common Mistakes:

• Ignoring OPTIONS and HEAD in the list
• Assuming only GET and POST are allowed
• Confusing Allow header with authentication

4. You wrote this code to handle HEAD requests in your REST API server:

if request.method == 'HEAD':
    return get_resource()

But clients receive the full content body. What is the problem?

medium

A. The code returns full content instead of headers only

B. The method name should be lowercase 'head'

C. HEAD requests must be redirected to GET method

D. The server must respond with status 201 for HEAD

Solution

1. Step 1: Analyze the code behavior

   The code calls get_resource() which returns full content including body.

2. Step 2: Understand HEAD response requirements

   HEAD must return headers only, no body content.

3. Final Answer:

   The code returns full content instead of headers only -> Option A

4. Quick Check:

   HEAD returns headers only, not full body [OK]

Hint: HEAD must not return body, only headers [OK]

Common Mistakes:

• Returning full content for HEAD requests
• Using wrong method name case
• Confusing HEAD with GET behavior

5. You want to optimize your REST API by using HEAD and OPTIONS methods. Which approach best improves client efficiency?

hard

A. Use HEAD to check resource existence and OPTIONS to discover allowed methods before sending requests

B. Use OPTIONS to fetch full resource data and HEAD to update resources

C. Use HEAD to delete resources and OPTIONS to create new ones

D. Ignore HEAD and OPTIONS and always use GET and POST

Solution

1. Step 1: Understand HEAD usage

   HEAD checks if a resource exists by fetching headers without body, saving bandwidth.

2. Step 2: Understand OPTIONS usage

   OPTIONS tells clients which HTTP methods are allowed, helping avoid invalid requests.

3. Step 3: Combine for efficiency

   Using both before heavy requests reduces unnecessary data transfer and errors.

4. Final Answer:

   Use HEAD to check resource existence and OPTIONS to discover allowed methods before sending requests -> Option A

5. Quick Check:

   HEAD checks headers, OPTIONS lists methods [OK]

Hint: HEAD checks existence, OPTIONS lists allowed methods [OK]

Common Mistakes:

• Using OPTIONS to fetch full data (wrong)
• Using HEAD to modify resources (wrong)
• Ignoring these methods and wasting bandwidth