A DNS TTL is a lifetime attached to cached data, not a refresh job scheduled for every second. A cache can reuse a valid answer while it retains that entry. After expiry, a subsequent request usually requires fresh resolution unless another configured behavior applies. An idle record need not produce an upstream query merely because its TTL has reached zero. ...
Redis expiration answers when a key should stop being available, not how much memory the server may consume. A deadline belongs to the key, and later commands can preserve, replace, or remove it. Reliable TTL use therefore requires testing update paths as well as the initial insertion. A successful write alone says nothing about the deadline remaining afterward. ...
DNS can return useful information by saying that data is absent. NXDOMAIN reports a name error; NODATA describes a successful response without the requested record type. In a direct query without aliases, these distinguish a nonexistent name from an existing name that lacks the requested data. Caching those results avoids repeating the same unsuccessful upstream lookup for every client request. ...