What is the difference between POST and PUT in HTTP?

In HTTP, POST and PUT are both methods for sending data to a server, but they serve different purposes and follow different semantics. Here’s a breakdown of their differences:

1. Purpose

POST	PUT
Used to create a new resource.	Used to update an existing resource (or create it if it doesn’t exist).
The server determines the URI for the new resource.	The client specifies the URI for the resource.

2. Idempotency

POST	PUT
Not idempotent: Multiple identical POST requests may create multiple resources.	Idempotent: Multiple identical PUT requests have the same effect as a single request.

3. URI

POST	PUT
Sent to a URI that represents a collection (e.g., /users).	Sent to a URI that represents a specific resource (e.g., /users/123).

4. Examples

POST (Create a new user):

POST /users HTTP/1.1
Content-Type: application/json

{
  "name": "Alice",
  "email": "alice@example.com"
}

• Result: Server creates a new user with a unique ID (e.g., /users/456).

PUT (Update user with ID 123):

PUT /users/123 HTTP/1.1
Content-Type: application/json

{
  "name": "Bob",
  "email": "bob@example.com"
}

• Result: Server updates or creates the resource at /users/123.

5. Use Cases

POST	PUT
Submitting form data.	Updating a user’s profile.
Uploading a file to a server.	Replacing an existing document.
Triggering a process (e.g., payment).	Creating a resource at a client-defined URI.

6. Response Codes

POST	PUT
201 Created (on success).	200 OK (update) or 201 Created (new resource).
404 Not Found (if the URI is invalid).	204 No Content (common for updates).

Key Takeaway

• Use POST when you want the server to handle resource creation and assign a URI.
• Use PUT when you want to create/update a resource at a specific URI you control.
• Idempotency makes PUT safer for retries in unreliable networks.

By choosing the right method, you align with RESTful API design principles and ensure predictable behavior.