[meejah]

...and in fact, there's a popular use-case called "using memcached" that does have side-effects. That is, doing a GET on something absolutely changes the server "state" (in the sense that memcached might have a key added to it) but that should be invisible to the user.

Of course, it's not actually-invisible: their next request might be a little bit faster.

[dragonwriter]

Its probably better to think of GET as not affecting the state of the resources within the API rather than not affecting the state of the server.