What is the difference between POST and PUT in HTTP?

Overall:

Both PUT and POST can be used for creating.

You have to ask, "what are you performing the action upon?", to distinguish what you should be using. Let's assume you're designing an API for asking questions. If you want to use POST, then you would do that to a list of questions. If you want to use PUT, then you would do that to a particular question.

Great, both can be used, so which one should I use in my RESTful design:

You do not need to support both PUT and POST.

Which you use is up to you. But just remember to use the right one depending on what object you are referencing in the request.

Some considerations:

• Do you name the URL objects you create explicitly, or let the server decide? If you name them then use PUT. If you let the server decide then use POST.
• PUT is defined to assume idempotency, so if you PUT an object twice, it should have no additional effect. This is a nice property, so I would use PUT when possible. Just make sure that the PUT-idempotency actually is implemented correctly in the server.
• You can update or create a resource with PUT with the same object URL
• With POST you can have 2 requests coming in at the same time making modifications to a URL, and they may update different parts of the object.

An example:

I wrote the following as part of another answer on SO regarding this:

POST:

Used to modify and update a resource

POST /questions/<existing_question> HTTP/1.1
Host: www.example.com/

Note that the following is an error:

POST /questions/<new_question> HTTP/1.1
Host: www.example.com/

If the URL is not yet created, you should not be using POST to create it while specifying the name. This should result in a 'resource not found' error because <new_question> does not exist yet. You should PUT the <new_question> resource on the server first.

You could though do something like this to create a resources using POST:

POST /questions HTTP/1.1
Host: www.example.com/

Note that in this case the resource name is not specified, the new objects URL path would be returned to you.

PUT:

Used to create a resource, or overwrite it. While you specify the resources new URL.

For a new resource:

PUT /questions/<new_question> HTTP/1.1
Host: www.example.com/

To overwrite an existing resource:

PUT /questions/<existing_question> HTTP/1.1
Host: www.example.com/

Additionally, and a bit more concisely, RFC 7231 Section 4.3.4 PUT states (emphasis added),

4.3.4. PUT

The PUT method requests that the state of the target resource be created or replaced with the state defined by the representation enclosed in the request message payload.

You can find assertions on the web that say

• POST should be used to create a resource, and PUT should be used to modify one
• PUT should be used to create a resource, and POST should be used to modify one

Neither is quite right.

Better is to choose between PUT and POST based on idempotence of the action.

PUT implies putting a resource - completely replacing whatever is available at the given URL with a different thing. By definition, a PUT is idempotent. Do it as many times as you like, and the result is the same. x=5 is idempotent. You can PUT a resource whether it previously exists, or not (eg, to Create, or to Update)!

POST updates a resource, adds a subsidiary resource, or causes a change. A POST is not idempotent, in the way that x++ is not idempotent.

By this argument, PUT is for creating when you know the URL of the thing you will create. POST can be used to create when you know the URL of the "factory" or manager for the category of things you want to create.

so:

POST /expense-report

or:

PUT  /expense-report/10929

• POST to a URL creates a child resource at a server defined URL.
• PUT to a URL creates/replaces the resource in its entirety at the client defined URL.
• PATCH to a URL updates part of the resource at that client defined URL.

The relevant specification for PUT and POST is RFC 2616 §9.5ff.

POST creates a child resource, so POST to /items creates a resources that lives under the /items resource. Eg. /items/1. Sending the same post packet twice will create two resources.

PUT is for creating or replacing a resource at a URL known by the client.

Therefore: PUT is only a candidate for CREATE where the client already knows the url before the resource is created. Eg. /blogs/nigel/entry/when_to_use_post_vs_put as the title is used as the resource key

PUT replaces the resource at the known url if it already exists, so sending the same request twice has no effect. In other words, calls to PUT are idempotent.

The RFC reads like this:

The fundamental difference between the POST and PUT requests is reflected in the different meaning of the Request-URI. The URI in a POST request identifies the resource that will handle the enclosed entity. That resource might be a data-accepting process, a gateway to some other protocol, or a separate entity that accepts annotations. In contrast, the URI in a PUT request identifies the entity enclosed with the request -- the user agent knows what URI is intended and the server MUST NOT attempt to apply the request to some other resource. If the server desires that the request be applied to a different URI,

Note: PUT has mostly been used to update resources (by replacing them in their entireties), but recently there is movement towards using PATCH for updating existing resources, as PUT specifies that it replaces the whole resource. RFC 5789.

Update 2018: There is a case that can be made to avoid PUT. See "REST without PUT"

With “REST without PUT” technique, the idea is that consumers are forced to post new 'nounified' request resources. As discussed earlier, changing a customer’s mailing address is a POST to a new “ChangeOfAddress” resource, not a PUT of a “Customer” resource with a different mailing address field value.

taken from REST API Design - Resource Modeling by Prakash Subramaniam of Thoughtworks

This forces the API to avoid state transition problems with multiple clients updating a single resource, and matches more nicely with event sourcing and CQRS. When the work is done asynchronously, POSTing the transformation and waiting for it to be applied seems appropriate.

POST means "create new" as in "Here is the input for creating a user, create it for me".

PUT means "insert, replace if already exists" as in "Here is the data for user 5".

You POST to example.com/users since you don't know the URL of the user yet, you want the server to create it.

You PUT to example.com/users/id since you want to replace/create a specific user.

POSTing twice with the same data means create two identical users with different ids. PUTing twice with the same data creates the user the first and updates him to the same state the second time (no changes). Since you end up with the same state after a PUT no matter how many times you perform it, it is said to be "equally potent" every time - idempotent. This is useful for automatically retrying requests. No more 'are you sure you want to resend' when you push the back button on the browser.

A general advice is to use POST when you need the server to be in control of URL generation of your resources. Use PUT otherwise. Prefer PUT over POST.

Summary:

Create:

Can be performed with both PUT or POST in the following way:

PUT

Creates THE new resource with newResourceId as the identifier, under the /resources URI, or collection.

PUT /resources/<newResourceId> HTTP/1.1 

POST

Creates A new resource under the /resources URI, or collection. Usually the identifier is returned by the server.

POST /resources HTTP/1.1

Update:

Can only be performed with PUT in the following way:

PUT

Updates the resource with existingResourceId as the identifier, under the /resources URI, or collection.

PUT /resources/<existingResourceId> HTTP/1.1

Explanation:

When dealing with REST and URI as general, you have generic on the left and specific on the right. The generics are usually called collections and the more specific items can be called resource. Note that a resource can contain a collection.

Examples:

<-- generic -- specific -->

URI: website.com/users/john
website.com  - whole site
users        - collection of users
john         - item of the collection, or a resource

URI:website.com/users/john/posts/23
website.com  - whole site
users        - collection of users
john         - item of the collection, or a resource
posts        - collection of posts from john
23           - post from john with identifier 23, also a resource

When you use POST you are always refering to a collection, so whenever you say:

POST /users HTTP/1.1

you are posting a new user to the users collection.

If you go on and try something like this:

POST /users/john HTTP/1.1

it will work, but semantically you are saying that you want to add a resource to the john collection under the users collection.

Once you are using PUT you are refering to a resource or single item, possibly inside a collection. So when you say:

PUT /users/john HTTP/1.1

you are telling to the server update, or create if it doesn't exist, the john resource under the users collection.

Spec:

Let me highlight some important parts of the spec:

POST

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

Hence, creates a new resource on a collection.

PUT

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."

Hence, create or update based on existence of the resource.

Reference:

• HTTP/1.1 Spec
• Wikipedia - REST
• Uniform Resource Identifiers (URI): Generic Syntax and Semantics