Caching - Expirations

Every cached key can have a time-to-live (TTL) TTL (time-to-live) A timer on a cached key. When it runs out, the cache deletes the key on its own, which keeps data fresh and frees memory. : a timer after which the cache deletes it automatically. TTLs keep data fresh and stop memory from filling up forever.

How to use it

await cache.set(
  "popular_posts",
  JSON.stringify(posts),
  // EX is Redis convention for "expire after N seconds"
  { EX: 60 * 5 } // expires in 5 minutes
);

await cache.set("otp:user-123", "482910", { EX: 60 }); // gone after a minute

Freshness vs. staleness

A cached value is a copy, so it can go stale Stale A cached value that no longer matches the real data because the source changed after the copy was made. when the real data changes. Two ways to handle it:

• Short TTL: accept that data may be a little out of date, and let it refresh on its own
• Invalidate on write Cache invalidation Deleting a cached key when the underlying data changes, so the next read fetches a fresh copy. : delete the cache key whenever the underlying data changes

Even without a TTL, a cache has limited memory. When it's full, it evicts Eviction The cache removing keys to make room when memory is full, often the least recently used ones. Treat every cached value as something that can vanish at any time. keys to make room (commonly the least recently used). So treat every cached value as something that can disappear at any time.

Don't confuse the three "expirations"

By now you've seen three different things that "expire," and they're easy to mix up. The TTL here is the one that lives in memory:

Exercise