POST vs PUT: Complete HTTP Method Guide

Brief Answer

POST and PUT differ primarily in how they handle resource URIs and idempotency. POST creates resources where the server determines the URI of the new resource and is not idempotent (repeated requests may create multiple resources), while PUT creates or replaces resources at a specific client-determined URI and is idempotent (repeated requests have the same effect as a single request).

Contents

• Understanding the POST Method
• Understanding the PUT Method
• Key Differences Between POST and PUT
• Idempotency and Safety Considerations
• When to Use POST vs PUT
• Best Practices for API Design
• Conclusion

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Understanding the POST Method

The POST method, as defined in RFC 2616 § 9.5, is a request method used to submit data to a specified resource. When you use POST, the server determines the URI of the newly created resource, and this URI is typically returned in the response.

When a client sends a POST request, the enclosed entity is processed by the resource identified by the Request-URI. The result of this process is a new resource that becomes a subordinate of the specified resource, as stated in the RFC.

Key characteristics of POST include:

• Server-determined URI: The client doesn’t specify the exact URI for the new resource; the server assigns it.
• Non-idempotent: Multiple identical POST requests may result in multiple resources being created.
• Can trigger server-side processes: POST requests can be used to initiate processes that aren’t directly related to resource manipulation.
• No caching: By default, POST responses aren’t cacheable.

POST is commonly used for:

• Creating new resources when the client doesn’t know the exact URI
• Submitting forms to web servers
• Uploading files
• Sending data to be processed by a server-side application

According to the RFC, “The POST method is used to request that the origin server accept the entity enclosed in the request as a new subordinate of the resource identified by the Request-URI in the Request-Line.”

This means that POST is essentially saying “Take this data and do something with it,” without specifying exactly where the result should be stored.

Understanding the PUT Method

The PUT method, as defined in RFC 2616 § 9.6, requests that the enclosed entity be stored under the supplied Request-URI. This is a key difference from POST, as the client specifies the exact URI where the resource should be created or updated.

PUT has two primary use cases:

1. Creating a new resource: If the Request-URI doesn’t point to an existing resource, and that URI can be defined as a new resource by the client, the server can create the resource with that URI.
2. Replacing an existing resource: If the Request-URI refers to an already existing resource, the enclosed entity should be considered as a modified version of the one residing on the origin server.

Key characteristics of PUT include:

• Client-determined URI: The client specifies the exact URI for the resource.
• Idempotent: Multiple identical PUT requests will have the same effect as a single request.
• Safe to retry: Because of idempotency, failed PUT requests can be safely retried without unintended side effects.
• Caching behavior: PUT responses may be cacheable under certain conditions.

PUT is commonly used for:

• Updating existing resources when the client knows the exact URI
• Creating new resources at a client-specified URI
• Bulk updates or replacements of resources
• Synchronization between client and server

The RFC states: “The PUT method requests that the enclosed entity be stored under the supplied Request-URI. If the Request-URI refers to an already existing resource, the enclosed entity SHOULD be considered as a modified version of the one residing on the origin server. If the Request-URI does not point to an existing resource, and that URI is capable of being defined as a new resource by the requesting user agent, the origin server can create the resource with that URI.”

Key Differences Between POST and PUT

The fundamental differences between POST and PUT can be summarized in the following table:

Characteristic	POST	PUT
URI determination	Server determines URI of new resource	Client specifies the exact URI
Idempotency	Not idempotent (repeated requests create multiple resources)	Idempotent (repeated requests have same effect as single request)
Usage for creation	Creates resources without specifying URI	Creates or replaces resources at specific URI
Response typically includes	Location header with URI of new resource	Status code indicating success/failure
Common HTTP status codes	201 Created, 200 OK, 204 No Content	201 Created, 200 OK, 204 No Content
Cacheability	Generally not cacheable	May be cacheable under certain conditions

The most significant difference lies in how the URI of the resource being manipulated is determined:

• With POST, the client sends data to a URI, and the server decides where to store the new resource, returning the location of the created resource in the response (typically via a Location header).

• With PUT, the client specifies the exact URI where the resource should be stored or updated. The server simply stores the data at that URI, creating it if it doesn’t exist or replacing it if it does.

This difference has implications for the design of RESTful APIs and how clients interact with resources.

Idempotency and Safety Considerations

Idempotency is a crucial concept when choosing between POST and PUT methods. An idempotent operation is one that can be applied multiple times without changing the result beyond the initial application.

PUT is Idempotent

When you send a PUT request to a URI with specific data, and you send the same PUT request again with the same data, the result will be the same as if you had sent it only once. The resource at that URI will either be created with that data (if it didn’t exist) or updated to contain that data (if it did exist).

Idempotency of PUT is valuable because:

• Failed requests can be safely retried without unintended side effects
• Network reliability issues can be mitigated
• Caching becomes more effective
• It simplifies client logic

POST is Not Idempotent

Sending the same POST request multiple times may result in multiple resources being created, each with different URIs. This is because each POST request is processed independently, and the server may generate a new URI for each request.

Safety Considerations

In HTTP terminology, a “safe” method is one that doesn’t modify the resource state. Neither POST nor PUT is considered safe because both can modify resources on the server.

However, PUT’s idempotency makes it safer in terms of retrying failed requests. If a network error occurs after a PUT request has been processed by the server but before the client receives the response, retrying the request won’t cause unintended modifications.

When to Use POST vs PUT

Choosing POST Over PUT

Use POST when:

• The client doesn’t know the URI of the resource to be created
• The server needs to determine the URI based on business logic
• You’re creating a new resource as a child of a collection
• The request is meant to trigger a process rather than directly create a resource
• You’re submitting form data
• You’re uploading files where the filename is determined by the server

Example: Creating a new blog post in a blog application. The client sends the post content to /api/posts, and the server determines the specific URL (like /api/posts/123) and returns it in the response.

Choosing PUT Over Use

Use PUT when:

• The client knows the exact URI of the resource
• The client has complete control over the resource representation
• You need idempotent behavior for safe retries
• You’re replacing an entire resource (not updating partial attributes)
• You’re implementing a synchronization mechanism

Example: Updating user profile information at a known URI like /api/users/123. The client knows the exact URI and can safely retry the PUT request if it fails.

Should Both Be Supported for Resource Creation?

In many RESTful APIs, both POST and PUT are used for resource creation, but in different contexts:

• POST is typically used to create resources in a collection, where the server assigns the ID or URI
• PUT is typically used to create a resource at a specific, client-determined URI

For example:

• POST /api/users creates a new user, with the server assigning an ID
• PUT /api/users/123 creates or updates the user with ID 123

Supporting both methods provides flexibility while following REST principles. However, it’s important to maintain consistency in your API design and document the expected behavior clearly.

Best Practices for API Design

When designing APIs that use POST and PUT methods, consider these best practices:

For POST Requests

1. Use meaningful endpoints: For creating resources, use collection endpoints (e.g., /users rather than /create-user).
2. Return proper status codes: Use 201 Created for successful resource creation with a Location header pointing to the new resource.
3. Include the created resource: Consider returning the full representation of the created resource in the response body.
4. Handle validation errors: Return 400 Bad Request for invalid input and 422 Unprocessable Entity for semantic errors.
5. Document the expected structure: Provide clear documentation on the request payload format.

For PUT Requests

1. Use resource-specific endpoints: For creating or updating specific resources, use resource identifiers (e.g., /users/123).
2. Ensure idempotency: Make sure PUT requests with identical payloads produce identical results.
3. Handle partial updates carefully: If supporting partial updates, consider using PATCH instead of PUT.
4. Return appropriate status codes: Use 200 OK or 204 No Content for successful updates.
5. Handle resource not found: Return 404 Not Found when attempting to update a non-existent resource (unless your API allows creation via PUT).

General Best Practices

1. Be consistent: Choose a pattern and stick to it throughout your API.
2. Document clearly: Provide clear documentation on when to use each method.
3. Consider HTTP caching: Implement appropriate cache headers to improve performance.
4. Use versioning: Consider using API versioning to support future changes.
5. Follow REST principles: Adhere to REST constraints when designing your API.

Conclusion

The differences between POST and PUT HTTP methods are fundamental to RESTful API design and have important implications for how clients interact with resources:

1. URI Determination: POST allows the server to determine the URI of the new resource, while PUT requires the client to specify the exact URI.

2. Idempotency: PUT is idempotent, making it safe to retry, while POST is not idempotent and may create multiple resources if repeated.

3. Use Cases: POST is ideal for creating resources within collections or triggering processes, while PUT is suitable for creating or replacing resources at specific, known URIs.

4. Best Practice: Both methods can and should be supported for resource creation in different contexts - POST for collection-based creation and PUT for resource-specific creation.

5. API Design: When designing APIs, it’s important to be consistent, document clearly, and choose the appropriate method based on the specific requirements of each operation.

By understanding these differences and applying them appropriately in your API design, you can create more intuitive, reliable, and efficient systems that follow REST principles and provide a better experience for API consumers.

The choice between POST and PUT ultimately depends on whether the client knows the URI of the resource to be created or updated. If the URI is determined by the server, POST is typically appropriate. If the client knows the exact URI, PUT is the better choice.