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PUT Versus POST

PUT versus POST: one of those conversations you try not to have. It leads to broken friendships, rainy picnics, and sad-looking kittens. People are passionate about REST, and this is one of the really sensitive topics.

First, you can read the technical descriptions in the rfc2616 document I mentioned earlier. It’s actually pretty cool stuff. But this is more than theory: you’ll need to know when to choose PUT and when to choose POST, so listen up!

Safe Methods

Each HTTP method is said to be “safe” or “unsafe”. An HTTP method is “safe” if using it doesn’t modify anything on the server. Ok, yes, logs will be written and analytics collected, but “safe” methods should never modify any real data. GET and HEAD are “safe” methods.

Making a request with unsafe methods - like POST, PUT and DELETE - does change data. Actually, if you make a request with an unsafe method it may not change anything. For example, if I try to update a programmer’s avatarNumber to the value it already has, nothing happens.

The point is that if a client uses an unsafe method, it knows that this method may have consequences. But if it uses a safe method, that request won’t ever have consequences. You could of course write an API that violates this. But that’s dishonest - like showing a picture of ice cream and then giving people broccoli. I like brocolli, but don’t be a jerk.

Being “safe” affects caching. Safe requests can be cached by a client, but unsafe requests can’t be. But caching is a whole different topic!

Idempotent

Within the unsafe methods, we have to talk about the famous term: “idempotency”. A request is idempotent if the side effects of making the request 1 time is the same as making the request 2, 3, 4, or 1072 times. PUT and DELETE are idempotent, POST is not.

For example, if we make the PUT request from our test once, it updates the avatarNumber to 2. If we make it again, the avatarNumber will still be 2. If we make the PUT request 1 time or 10 times, the server always results in the same state.

Now think about the POST request in our test, and imagine for a second that the nickname doesn’t have to be unique, because that detail clouds things here unnecessarily.

If we make the request once, it would create a programmer. If we make it again, it would create another programmer. So making the request 12 times is not the same as making it just once. This is not idempotent.

Now you can see why it seems right to say that POST creates resources and PUT updates them.

POST or PUT? The 2 Rules of PUT

Other than PATCH, which is an edge case we’ll discuss next, if you’re building an endpoint that will modify data, it should use a POST or PUT method.

Deciding between POST and PUT is easy: use PUT if and only if the endpoint will follow these 2 rules:

  1. The endpoint must be idempotent: so safe to redo the request over and over again;
  2. The URI must be the address to the resource being updated.

When we use PUT, we’re saying that we want the resource that we’re sending in our request to be stored at the given URI. We’re literally “putting” the resource at this address.

This is what we’re doing when we PUT to /api/programmers/CowboyCoder. This results in an update because CowboyCoder already exists. But imagine if we changed our controller code so that if CowboyCoder didn’t exist, it would be created. Yes, that should still be a PUT: we’re putting the resource at this URI. Because of this, PUT is usually thought of as an update, but it could be used to create resources. You may never choose to use PUT this way, but technically it’s ok.

Heck, we could even support making a PUT request to /api/programmers. But if we did - and we followed the rules of PUT - we’d expect the client to pass us a collection of programmers, and we’d replace the existing collection with this new one.

POST and Non-Idempotency

One last thing. POST is not idempotent, so making a POST request more than one time may have additional side effects, like creating a second, third and fourth programmer. But the key word here is may. Just because an endpoint uses POST doesn’t mean that it must have side effects on every request. It just might have side effects.

When choosing between PUT and POST, don’t just say “this request is idempotent, it must be PUT!”. Instead, look at the above 2 rules for put. If it fails one of those, use POST: even if the endpoint is idempotent.

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