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Comment by cperciva | Hacker News Reader

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cperciva

idempotent

I don't think that word means what you think it means.

Not unless you think having sex with goats once is fine but having sex with goats multiple times is bad.

You probably meant "GET requests should be side effect free".

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RFC 2616 HTTP/1.1 has a definition of idempotence with regard to request methods:

http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html

Section 9.1.2

"Methods can also have the property of "idempotence" in that (aside from error or expiration issues) the side-effects of N > 0 identical requests is the same as for a single request."

Idempotent means that the side effects of n > 0 requests are the same as for a single request. The "update status" method is not idempotent, as calling it multiple times would post multiple statuses. I think the parent's statement is sound.

A request that is side effect free is idempotent by definition.

Right, but the concept was created to talk about distributed systems and remote procedure calls where if a failure happened on your end and you didn't know if the procedure call worked, you wouldn't have to check if it worked before retrying. So it doesn't quite fit here. Also, I don't think idempotence is a solution to this since a) This is more of a security issue, and less of a systems design one and b) you might actually have a legitimate use case for multiple posts

...the concept was created to talk about distributed systems and remote procedure calls...

I'm pretty sure the concept's use in mathematics predates that by far.

so, if the first call posts a tweet and the second does not, we shall be satisfied? I stand by the side effect free side :)

(notice that tweet posting is actually kind of idempotent in twitter: if you post the same message twice, it will only appear once)

I'm glad it's not slashdot because this would be an impossible choice between +1 informative and +1 funny

Good point. Was getting terminology mixed up with PUT.

While strictly speaking impotence wouldn't prevent this (trivially speaking you could only allow a single tweet with a given content), in practice idempotence is a sufficient to avoid this.

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