05 December 2013

Previously I poured my thoughts on REST/JSON protocol differences. I am still researching on how different server and client frameworks work, but as an interlude I’ll comment on the interpretation of the PUT operation in relation to its use on “RESTful” APIs.

I’ve seen a lot of people state that PUT /resource/<id> should create the resource if it does not exist. Like here, here and here and here and and.

This is absolutely wrong. This is a misinterpretation of idempotency. Following this logic to the extreme causes both semantic and practical problems.

### Idempotency

I am making a strong statement here regarding PUT semantics here, so let me first introduce you to the idea of idempotency. I’ll quote from the wikipedia entry on idempotence:

Idempotence (/ˌaɪdɨmˈpoʊtəns/ eye-dəm-poh-təns) is the property of certain operations in mathematics and computer science, that can be applied multiple times without changing the result beyond the initial application.

And here is a small light switch system which has both idempotent and non-idempotent functions:

(define lights-state #f)
(define (lights) lights-state)
(define (lights! on-or-off) ...) ; sets lights on or off
(define (lights-toggle!) (lights! (not (lights))))


If you repeatedly call lights, you’ll get the same value every time. The getter is both safe (no side effects) and idempotent (returns same value on repeated calls). Similarly lights! is not safe (it has a world-changing side effect) but is idempotent:

> (lights! #t) (lights)
> #t
> (lights! #t) (lights)
> #t


(lights-toggle!, of course, is not idempotent.)

Now you are asking me what’s in the lights! function I didn’t show you earlier. I’ll show you now:

(define (lights! on-or-off)
(if (and (boolean? lights-state) (boolean? on-or-off))
(set! lights-state on-or-off))
lights-state)


This is an idempotent function. As long as lights-state stays boolean (guaranteed if only lights! or toggle-lights! are used to change light state) it will change the value of lights-state to match the request.

Now the surprising bit. If lights-state is not a boolean value, lights-state is still an idempotent function and lights and lights! are too!

Now consider a multi-user system (aka real world) where this happens:

Me> (lights! #t)
=> #t
Elsie> (set! lights-state 'explode)
Me> (lights! #t) ; just making sure
=> 'explode


Boom! What happened? Wasn’t lights! supposed to be idempotent? Yes, and it still is. But wait, I thought that idempotency means that any idempotent operation should work the same if repeated later!

Let’s go back to wikipedia entry and scroll a bit down:

A composition of idempotent methods or subroutines, however, is not necessarily idempotent if a later method in the sequence changes a value that an earlier method depends on – idempotence is not closed under composition.

“Not closed under composition”. Technically, when you call a function (method, procedure, script, whatever) in a real-world situation its result is a composition of the current system state, inputs you provided and the function implementation itself. Idempotency guarantees that any changes to the system state by the idempotent operation are such that calling the same operation with the updated system state will result in the same final result as calling with the unaltered system state.

What it does not guarantee is that if you call with some other system state you would get the same results. If anyone else has changed the system state between your calls to the idempotent routine, then the system state has changed and there are no guarantees that the result from your call will be the same. This is exactly what happened, Elsie changed the system state, so even though the lights and lights! functions are still idempotent, my operations from my viewpoint are not since the two calls were composed differently.

At this point you should realize that when standards talk about idempotency or behavior of repeated PUTs they are not guaranteeing you that all your PUTs will give the same response or have the same effect in the system every time under all conditions. What the operation idempotency guarantee can give you is that when the composition of your PUT has not changed (apart from the changes the original PUT made), subsequent PUTs should give you the same result. But only when that assumption holds, otherwise we are not talking about idempotency at all.

### PUT doesn’t have to create resources

The normal life cycle of any object, entity or resource within computer systems is:

1. It does not exist.
2. It is created.
3. Stuff happens to it.
4. It is destroyed.
5. It is no more.

Interpretation of POST and DELETE operations are straightforward if you think of them as steps #2 and #4 respectively. They manage the life cycle of the resource. The resource exists between creation and destruction, and otherwise exists not.

If we take the viewpoint that these are the only operations to manage a resource’s lifecycle — and I urge you to take this viewpoint too — then PUT is valid only during step #3.

That is, PUT should not create a resource.

Now I can already hear an argument in the line of “but using PUT to create new resources is an idempotent operation” and you are right. If you define PUT to create non-existent resource and update an existing resource, then two sequential PUTs will always get the same result (even if the resource didn’t exist in the first place). But this is circular reasoning. You can’t argument that PUT should be a life cycle operation because it can be one while staying idempotent. We can define that PUT is not a life cycle operation and it still stays an idempotent operation (PUT on non-existent resource would result in the same result both times – a failure).

At this point it should be clear that saying that “PUT should create resources because of idempotency” is a false argument because idempotency holds even if this is not the case.

Which way PUT swings is a design choice. A choice.

I want to convince you that it should not create resources.

(Ed: Changed “strawman argument” to “false argument” above. Thanks Frederic for pointing out the semantic difference!)

### PUTs on DELETEd resources

Now I’ll try to convince you why PUT as a life cycle operation is not a good idea from developer’s perspective because it just causes practical implementation problems (and if you are not aware of these, it can create hidden semantic traps in your system).

This is a real-world case (simplified though):

1. User 1 creates a message (POST).
2. User 1 edits the message (GET, PUT).
3. User 2 sees the message and decides to open it for editing (GET).
4. User 1 decides the message is crap and removes it (GET, DELETE).
5. User 2 updates the message (PUT).

If you allow PUT to implicitly create non-existent resources, you get what I’d call semantically inconsistent result. For users, the message exists when it should not. This is entirely consistent from system’s point of view, since the message created at step #5 is not the same message that was deleted at #3.

Unfortunately most of the systems that are written are meant for human consumption and need to work with human expectations. Thus in this case the implicit PUT most definitely was definitely not helping system development at all.

Oh no, wait! Here’s another!

1. User 1 has CREATE permission on messages.
2. User 2 has EDIT permission on messages.
3. User 3 has REMOVE permission on messages.

I think you can already guess where this is going. If there’s implicit create on PUT I have to check for CREATE permission in two different places, both POST and PUT. (This is another real-world scenario where some people can CREATE and EDIT, others can only EDIT and some DELETE but not create or edit. Auditability requirements…)

### What then is PUT?

Simple:

• It is idempotent. (See above on limits.)
• It operates on existing resources.
• It is not a life cycle operation. It cannot create or destroy resources.

Idempotent PUT still stays the very same and very powerful and useful feature as before as it allows you to just repeat the request in case of transient network or server failures. Just please don’t think PUT as a life cycle management operation, because it should not be.