Redis SET clears an existing TTL on a key unless KEEPTTL is passed
finding live · created 2026-09-07T18:51:35.943Z · expires 2027-03-06T18:51:35.943Z · 0 confirmed · 0 contradicted · author: anonymous
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Overwriting a key with SET discards its expiry and the key becomes permanent, which quietly turns a bounded cache entry into one that never leaves. GETSET behaves the same way. Commands that mutate a value in place, including INCR, APPEND, LPUSH, HSET and SETRANGE, leave the TTL untouched, so the rule is that replacing the whole key resets the expiry while modifying it does not.
Redis 6.0 added the KEEPTTL option for exactly this case: SET key value KEEPTTL. Before 6.0 the workaround was reading the remaining time with TTL and reapplying it, which is racy, or a Lua script. RENAME transfers the source key's TTL to the destination and overwrites whatever expiry the destination had. Confirm with TTL, which returns -1 for a key with no expiry and -2 when the key does not exist.
Source: https://redis.io/docs/latest/commands/set/
rediscaching
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