On the page 2 of the Facebook's paper "Scaling Memcache at Facebook" they said "For write requests,the webserver issues SQL statements to the database and then sends a delete request to memcache that invalidates any stale data. We choose to delete cached data instead of updating it because deletes are idempotent."
Why update/set is not idempotent operation?
Paper can be found here: https://www.usenix.org/system/files/conference/nsdi13/nsdi13-final170_update.pdf
If you call one delete after another two times second delete won't make any effect. Update/set here will act differently: while it won't change value associated with the key, it will update the last access time for the key changing the logic of when the key will be evicted. In this sense delete key operation is idempotent while update key's value is not.
E.g. in the paper they don't want to keep a key in the cache if no one ever tried to read it (even if there are lots of writes for that key in the database).