Strings, Counters, EXPIRE, and TTL

What you'll walk away with

Explain the core ideas behind Strings, Counters, EXPIRE, and TTL
Run the sample Redis command or code and verify its output
Apply the technique correctly to the Tutorial Platform and production scenarios

Build the mental model

Strings store text, numbers, or binary bytes. `INCR` provides an atomic counter across concurrent clients, while `SET ... NX/XX EX/PX` combines conditional writes and expiry in one command. A plain replacement SET can remove an existing TTL, so use `KEEPTTL` or an explicit expiry policy.

Connect it to a real scenario

Use `INCR` for page views, `SET ... EX 600 NX` for an email-verification code, and a JSON string with TTL for a catalog fragment. Applications must distinguish TTL results `-1` (no expiry) and `-2` (missing key).

INCR is an atomic counter
Use SET NX/XX with EX for one-step state changes
Distinguish TTL -1 from -2

Try the working example

shell

INCR metrics:tutorial:42:views
SET verify:email:user:42 834921 EX 600 NX
GET verify:email:user:42
TTL verify:email:user:42
SET tutorial:42:card '{"title":"Redis"}' EX 300
GET tutorial:42:card

You should see

You create an atomic counter, an expiring verification code, and a five-minute cache entry.

5-minute try-it

Implement a download counter, a one-time idempotency key, and a 30-second API response cache.

One important caution

If a process crashes between INCR and EXPIRE, an immortal key can remain. Use a transaction or script when the pair must be atomic.

Redis — Strings — Redis

Easy traps

If a process crashes between INCR and EXPIRE, an immortal key can remain. Use a transaction or script when the pair must be atomic.
Validate sample commands on a local or test instance with recoverable data before applying them to production Redis.

Exercise

Implement a download counter, a one-time idempotency key, and a 30-second API response cache.

You'll know it worked when: You create an atomic counter, an expiring verification code, and a five-minute cache entry.