RESTful PUT if not exists?

Question

Let's say I'm creating a RESTful interface, and I want to upload a resource using PUT to /resources/{id}. But I only want to be upload the thing if it hasn't been uploaded before.

I realize that PUT should be idempotent, so if I PUT something twice to the same URL it should succeed both times, right?

I also understand that I could use HEAD on an existing resource, and then use an ETag to in my PUT to ensure that the resource hasn't been modified since I last checked.

But how can I ensure that I only upload a thing if the thing doesn't already exist? That is, how can I make sure I don't step on someone else's thing?

Answer 1

I realize that PUT should be idempotent, so if I PUT something twice to the same URL it should succeed both times right?

Correct.

But how can I ensure that I only upload a thing if the thing doesn't already exist? That is, how can I make sure I don't step on someone else's thing?

Don't use a HEAD call. Make a PUT call using the header If-None-Match: *. This will instruct the server to only perform the operation if no current entity exists, as detailed in RFC 2616 paragraph 3.

Answer 2

See http://greenbytes.de/tech/webdav/rfc7232.html#rfc.section.3.2.p.7:

"If-None-Match can also be used with a value of "*" to prevent an unsafe request method (e.g., PUT) from inadvertently modifying an existing representation of the target resource when the client believes that the resource does not have a current representation (Section 4.2.1 of [RFC7231]). This is a variation on the "lost update" problem that might arise if more than one client attempts to create an initial representation for the target resource."