Where we left off

Every key set in Lesson 4 lives forever, until something explicitly deletes it. That's wrong for a cache: a cached answer that's never allowed to go stale is just a bug waiting to serve outdated information. This lesson adds a lifespan to a key.

SETEX: set and expire in one call

r.setex("cache:capital_of_france", 60, "Paris")

SETEX key seconds value is SET plus an expiry, in one atomic call: the key is set, and Redis guarantees it disappears on its own after seconds pass, no cron job or cleanup script required. This is the shape a response cache takes throughout this course: cache the answer, give it a TTL (time-to-live) that matches how long it's safe to reuse.

EXPIRE: adding a TTL after the fact

r.set("session:abc123", "active")
r.expire("session:abc123", 300)

EXPIRE key seconds attaches a TTL to a key that's already set, useful when the value and its lifespan come from different places in your code (Lesson 8's checkpoint sets a session hash, then separately decides how long it should live).

Checking and clearing a TTL

r.ttl("session:abc123")   # seconds remaining, or -1 (no TTL), or -2 (gone)
r.persist("session:abc123")  # removes the TTL, key lives forever again

TTL is useful for debugging (is this key about to expire when I didn't expect it to?) and for building on top of, Lesson 10's rate limiter reads it directly to know how long until a counter resets.

Why this matters for an LLM response cache

An LLM call costs money and time. If the same prompt is likely to repeat (a common question, a repeated tool lookup), caching its answer under a TTL means a repeat within that window is a single GET instead of a full model call, while still guaranteeing the cache can't serve a stale answer forever. Lesson 9 builds this into a real cache keyed by a hash of the prompt; this lesson is the TTL mechanics underneath it.

Checkpoint

  • SETEX key seconds value: set and add a TTL in one atomic call, the shape a response cache uses.
  • EXPIRE key seconds: attach a TTL to an already-set key.
  • TTL key: seconds remaining, -1 if no TTL, -2 if the key is gone (expired or never existed).

If anything here still feels unclear, ask before moving to Lesson 6.