Comment by cryptonector

Consider link shortener services...

It's perfectly fine to have certain side-effects, provided the GET is idempotent, but not others/most. Specifically, it's fine to idempotently have side-effects where it doesn't matter who is causing the effect, the side-effects are desirable, and the side-effect load won't be overwhelming.

In the case of a link shortener one can pretend that the side effect did not happen the first time the service sees some particular link, that the shortening has already occurred (at the beginning of time!). There is definitely a side-effect, since it involves updating a persistent hash table, and it is idempotent (though one could construct a shortener where it's possible to get more than one shortened form for a given URI when racing to shorten it, but this is not a problem for this particular sort of service).