Is changing or appending data an idempotent operation? As far as I know idempotent operation is a operation that can be applied many times with the same effect. Also I learnt recently that updating a tuple in a database is also idempotent. I thought that an idempotent operation was like a read-only stateless call (thanks to HTTP world with its famous GET request example). So, is changing data in a file (or database) idempotent? Is appending (extending) data in file/database idempotent? Why?
An operation $O$ is idempotent if $O(O(x)) = O(x)$. For example, if you update record $R$ to value $T$ and then again update record $R$ to value $T$, then it's the same as if you updated only once. But if you append $T$ to record $R$ twice, it's not the same as appending only once.