Understanding REST: is GET fundamentally incompatible with any "number of views" counter?

Question

I'm trying to understand REST. Under REST a GET must not trigger something transactional on the server (this is a definition everybody agrees upon, it is fundamental to REST).

So imagine you've got a website like stackoverflow.com (I say like so if I got the underlying details of SO wrong it doesn't change anything to my question), where everytime someone reads a question, using a GET, there's also some display showing "This question has been read 256 times".

Now someone else reads that question. The counter now is at 257. The GET is transactional because the number of views got incremented and is now incremented again. The "number of views" is incremented in the DB, there's no arguing about that (for example on SO the number of time any question has been viewed is always displayed).

So, is a REST GET fundamentally incompatible with any kind of "number of views" like functionality in a website?

So should it want to be "RESTFUL", should the SO main page either stop display plain HTML links that are accessed using GETs or stop displaying the "this question has been viewed x times"?

Because incrementing a counter in a DB is transactional and hence "unrestful"?

EDIT just so that people Googling this can get some pointers:

From http://www.xfront.com/REST-Web-Services.html :

4. All resources accessible via HTTP GET should be side-effect free. That is, the request should just return a representation of the resource. Invoking the resource should not result in modifying the resource.

Now to me if the representation contains the "number of views", it is part of the resource [and in SO the "number of views" a question has is a very important information] and accessing it definitely modifies the resource.

This is in sharp contrast with, say, a true RESTFUL HTTP GET like the one you can make on an Amazon S3 resource, where your GET is guaranteed not to modify the resource you get back.

But then I'm still very confused.

Answer 1

What matters is that from a client point of view GET is safe (has no side effects) by definition and that a client therefore can safely call GET any number of times without considering any side effect that might have.

What a server does is the server's responsibility. In the case of the view counter the server has to make the decision if it considers the update of the counter a side effect. Usually it won't because the counter is part of the semantic of the resource in the first place.

However, the server might decide NOT to increment the counter for certain requests, such as a GET by a crawler.

Jan

Answer 2

IMO avoiding a statistics update in a GET request because "someone said so" is being dogmatic about ReST. Do what is pragmatic. If that involves updating a counter when responding to a GET request, so be it.

To elaborate further, what is really important (and the reason the advice is there) is that the resource a consumer is accessing is not updated or altered in any manner when the consumers intent is to read it. However, updating other data, in particular stuff like logs and statistics, is not a problem. In short, reading a resource should not have side-effects on the resource being read.

EDIT: To answer your case of a self-incrementing counter, ask yourself what the context you apply is. Clearly, if you define a resource called counterThatIncrementsItselfWhenBeingRead, then it either:

• Breaks ReSTfulness since a read-incrementing counter is a self-contradictory resource if the only rule is that GET can never have side-effects, or
• Is just fine given a different context, where you for instance take a very short resource lifespan into account, and choose to view the increment as something that happens after you have read the resource (or more generally at the resource owner's discression)

Regardless of the resolution you choose to apply, the issue is really about what the expected behavior is. IMO, a counter that increments itself when being read should be incrementing itself when being read. I still access a representation of a resource, albeit one with a very short lifespan, which I know will be changed an instant after I have read it. There's nothing non-ReSTful about that.