Use of PUT vs PATCH methods in REST API real life scenarios
TLDR - Dumbed Down Version
PUT => Set all new attributes for an existing resource.
PATCH => Partially update an existing resource (not all attributes required).
Though Dan Lowe's excellent answer very thoroughly answered the OP's question about the difference between PUT and PATCH, its answer to the question of why PATCH is not idempotent is not quite correct.
To show why PATCH isn't idempotent, it helps to start with the definition of idempotence (from Wikipedia):
The term idempotent is used more comprehensively to describe an operation that will produce the same results if executed once or multiple times [...] An idempotent function is one that has the property f(f(x)) = f(x) for any value x.
In more accessible language, an idempotent PATCH could be defined as: After PATCHing a resource with a patch document, all subsequent PATCH calls to the same resource with the same patch document will not change the resource.
Conversely, a non-idempotent operation is one where f(f(x)) != f(x), which for PATCH could be stated as: After PATCHing a resource with a patch document, subsequent PATCH calls to the same resource with the same patch document do change the resource.
To illustrate a non-idempotent PATCH, suppose there is a /users resource, and suppose that calling GET /users returns a list of users, currently:
[{ "id": 1, "username": "firstuser", "email": "[email protected]" }]
Rather than PATCHing /users/{id}, as in the OP's example, suppose the server allows PATCHing /users. Let's issue this PATCH request:
PATCH /users
[{ "op": "add", "username": "newuser", "email": "[email protected]" }]
Our patch document instructs the server to add a new user called newuser to the list of users. After calling this the first time, GET /users would return:
[{ "id": 1, "username": "firstuser", "email": "[email protected]" },
{ "id": 2, "username": "newuser", "email": "[email protected]" }]
Now, if we issue the exact same PATCH request as above, what happens? (For the sake of this example, let's assume that the /users resource allows duplicate usernames.) The "op" is "add", so a new user is added to the list, and a subsequent GET /users returns:
[{ "id": 1, "username": "firstuser", "email": "[email protected]" },
{ "id": 2, "username": "newuser", "email": "[email protected]" },
{ "id": 3, "username": "newuser", "email": "[email protected]" }]
The /users resource has changed again, even though we issued the exact same PATCH against the exact same endpoint. If our PATCH is f(x), f(f(x)) is not the same as f(x), and therefore, this particular PATCH is not idempotent.
Although PATCH isn't guaranteed to be idempotent, there's nothing in the PATCH specification to prevent you from making all PATCH operations on your particular server idempotent. RFC 5789 even anticipates advantages from idempotent PATCH requests:
A PATCH request can be issued in such a way as to be idempotent, which also helps prevent bad outcomes from collisions between two PATCH requests on the same resource in a similar time frame.
In Dan's example, his PATCH operation is, in fact, idempotent. In that example, the /users/1 entity changed between our PATCH requests, but not because of our PATCH requests; it was actually the Post Office's different patch document that caused the zip code to change. The Post Office's different PATCH is a different operation; if our PATCH is f(x), the Post Office's PATCH is g(x). Idempotence states that f(f(f(x))) = f(x), but makes no guarantes about f(g(f(x))).
I was curious about this as well and found a few interesting articles. I may not answer your question to its full extent, but this at least provides some more information.
http://restful-api-design.readthedocs.org/en/latest/methods.html
The HTTP RFC specifies that PUT must take a full new resource representation as the request entity. This means that if for example only certain attributes are provided, those should be remove (i.e. set to null).
Given that, then a PUT should send the entire object. For instance,
/users/1
PUT {id: 1, username: 'skwee357', email: '[email protected]'}
This would effectively update the email. The reason PUT may not be too effective is that your only really modifying one field and including the username is kind of useless. The next example shows the difference.
/users/1
PUT {id: 1, email: '[email protected]'}
Now, if the PUT was designed according the spec, then the PUT would set the username to null and you would get the following back.
{id: 1, username: null, email: '[email protected]'}
When you use a PATCH, you only update the field you specify and leave the rest alone as in your example.
The following take on the PATCH is a little different than I have never seen before.
http://williamdurand.fr/2014/02/14/please-do-not-patch-like-an-idiot/
The difference between the PUT and PATCH requests is reflected in the way the server processes the enclosed entity to modify the resource identified by the Request-URI. In a PUT request, the enclosed entity is considered to be a modified version of the resource stored on the origin server, and the client is requesting that the stored version be replaced. With PATCH, however, the enclosed entity contains a set of instructions describing how a resource currently residing on the origin server should be modified to produce a new version. The PATCH method affects the resource identified by the Request-URI, and it also MAY have side effects on other resources; i.e., new resources may be created, or existing ones modified, by the application of a PATCH.
PATCH /users/123
[
{ "op": "replace", "path": "/email", "value": "[email protected]" }
]
You are more or less treating the PATCH as a way to update a field. So instead of sending over the partial object, you're sending over the operation. i.e. Replace email with value.
The article ends with this.
It is worth mentioning that PATCH is not really designed for truly REST APIs, as Fielding's dissertation does not define any way to partially modify resources. But, Roy Fielding himself said that PATCH was something [he] created for the initial HTTP/1.1 proposal because partial PUT is never RESTful. Sure you are not transferring a complete representation, but REST does not require representations to be complete anyway.
Now, I don't know if I particularly agree with the article as many commentators point out. Sending over a partial representation can easily be a description of the changes.
For me, I am mixed on using PATCH. For the most part, I will treat PUT as a PATCH since the only real difference I have noticed so far is that PUT "should" set missing values to null. It may not be the 'most correct' way to do it, but good luck coding perfect.
NOTE: When I first spent time reading about REST, idempotence was a confusing concept to try to get right. I still didn't get it quite right in my original answer, as further comments (and Jason Hoetger's answer) have shown. For a while, I have resisted updating this answer extensively, to avoid effectively plagiarizing Jason, but I'm editing it now because, well, I was asked to (in the comments).
After reading my answer, I suggest you also read Jason Hoetger's excellent answer to this question, and I will try to make my answer better without simply stealing from Jason.
Why is PUT idempotent?
As you noted in your RFC 2616 citation, PUT is considered idempotent. When you PUT a resource, these two assumptions are in play:
You are referring to an entity, not to a collection.
The entity you are supplying is complete (the entire entity).
Let's look at one of your examples.
{ "username": "skwee357", "email": "[email protected]" }
If you POST this document to /users, as you suggest, then you might get back an entity such as
## /users/1
{
"username": "skwee357",
"email": "[email protected]"
}
If you want to modify this entity later, you choose between PUT and PATCH. A PUT might look like this:
PUT /users/1
{
"username": "skwee357",
"email": "[email protected]" // new email address
}
You can accomplish the same using PATCH. That might look like this:
PATCH /users/1
{
"email": "[email protected]" // new email address
}
You'll notice a difference right away between these two. The PUT included all of the parameters on this user, but PATCH only included the one that was being modified (email).
When using PUT, it is assumed that you are sending the complete entity, and that complete entity replaces any existing entity at that URI. In the above example, the PUT and PATCH accomplish the same goal: they both change this user's email address. But PUT handles it by replacing the entire entity, while PATCH only updates the fields that were supplied, leaving the others alone.
Since PUT requests include the entire entity, if you issue the same request repeatedly, it should always have the same outcome (the data you sent is now the entire data of the entity). Therefore PUT is idempotent.
Using PUT wrong
What happens if you use the above PATCH data in a PUT request?
GET /users/1
{
"username": "skwee357",
"email": "[email protected]"
}
PUT /users/1
{
"email": "[email protected]" // new email address
}
GET /users/1
{
"email": "[email protected]" // new email address... and nothing else!
}
(I'm assuming for the purposes of this question that the server doesn't have any specific required fields, and would allow this to happen... that may not be the case in reality.)
Since we used PUT, but only supplied email, now that's the only thing in this entity. This has resulted in data loss.
This example is here for illustrative purposes -- don't ever actually do this (unless your intent is to drop the omitted fields, of course... then you are using PUT as it should be used). This PUT request is technically idempotent, but that doesn't mean it isn't a terrible, broken idea.
How can PATCH be idempotent?
In the above example, PATCH was idempotent. You made a change, but if you made the same change again and again, it would always give back the same result: you changed the email address to the new value.
GET /users/1
{
"username": "skwee357",
"email": "[email protected]"
}
PATCH /users/1
{
"email": "[email protected]" // new email address
}
GET /users/1
{
"username": "skwee357",
"email": "[email protected]" // email address was changed
}
PATCH /users/1
{
"email": "[email protected]" // new email address... again
}
GET /users/1
{
"username": "skwee357",
"email": "[email protected]" // nothing changed since last GET
}
My original example, fixed for accuracy
I originally had examples that I thought were showing non-idempotency, but they were misleading / incorrect. I am going to keep the examples, but use them to illustrate a different thing: that multiple PATCH documents against the same entity, modifying different attributes, do not make the PATCHes non-idempotent.
Let's say that at some past time, a user was added. This is the state that you are starting from.
{
"id": 1,
"name": "Sam Kwee",
"email": "[email protected]",
"address": "123 Mockingbird Lane",
"city": "New York",
"state": "NY",
"zip": "10001"
}
After a PATCH, you have a modified entity:
PATCH /users/1
{"email": "[email protected]"}
{
"id": 1,
"name": "Sam Kwee",
"email": "[email protected]", // the email changed, yay!
"address": "123 Mockingbird Lane",
"city": "New York",
"state": "NY",
"zip": "10001"
}
If you then repeatedly apply your PATCH, you will continue to get the same result: the email was changed to the new value. A goes in, A comes out, therefore this is idempotent.
An hour later, after you have gone to make some coffee and take a break, someone else comes along with their own PATCH. It seems the Post Office has been making some changes.
PATCH /users/1
{"zip": "12345"}
{
"id": 1,
"name": "Sam Kwee",
"email": "[email protected]", // still the new email you set
"address": "123 Mockingbird Lane",
"city": "New York",
"state": "NY",
"zip": "12345" // and this change as well
}
Since this PATCH from the post office doesn't concern itself with email, only zip code, if it is repeatedly applied, it will also get the same result: the zip code is set to the new value. A goes in, A comes out, therefore this is also idempotent.
The next day, you decide to send your PATCH again.
PATCH /users/1
{"email": "[email protected]"}
{
"id": 1,
"name": "Sam Kwee",
"email": "[email protected]",
"address": "123 Mockingbird Lane",
"city": "New York",
"state": "NY",
"zip": "12345"
}
Your patch has the same effect it had yesterday: it set the email address. A went in, A came out, therefore this is idempotent as well.
What I got wrong in my original answer
I want to draw an important distinction (something I got wrong in my original answer). Many servers will respond to your REST requests by sending back the new entity state, with your modifications (if any). So, when you get this response back, it is different from the one you got back yesterday, because the zip code is not the one you received last time. However, your request was not concerned with the zip code, only with the email. So your PATCH document is still idempotent - the email you sent in PATCH is now the email address on the entity.
So when is PATCH not idempotent, then?
For a full treatment of this question, I again refer you to Jason Hoetger's answer which already fully answers that.