Cache Eviction and Expiration

Caching is great for speed, but cached data can become stale, meaning it no longer reflects the true source of information. Also, caches have limited memory.

This lesson explores how to manage these issues using expiration and eviction techniques in Redis.

Time To Live (TTL) defines how long a cached item remains valid. Once its TTL expires, Redis automatically removes the key.

• Prevents stale data from being served indefinitely.
• Frees up memory for newer, more relevant data.

The EXPIRE command sets an expiration time, in seconds, on an existing key. After the set time, the key will be automatically deleted.

Try setting a key and then giving it a 10-second expiration:

Instead of two commands (SET then EXPIRE), you can use SETEX to set a key's value and its expiration time atomically (in one go).

This is generally preferred for new keys as it's more efficient.

You can check a key's remaining TTL with the TTL command. It returns the remaining seconds, or -2 if the key doesn't exist, -1 if it exists but has no expiration.

The PERSIST command removes the expiration from a key, making it permanent again.

For more precise control, PEXPIRE and PSETEX work just like their second-based counterparts, but accept expiration times in milliseconds.

This can be useful for very short-lived caches or specific timing requirements.

What happens when your Redis cache runs out of memory? Redis can be configured to automatically evict (remove) keys to free up space.

This behavior is controlled by the maxmemory setting and an eviction policy.

Redis offers several eviction policies configured via maxmemory-policy:

• noeviction: Returns errors on write operations when memory limit is reached.
• allkeys-lru: Evicts least recently used (LRU) keys from all keys.
• volatile-lru: Evicts LRU keys *only* from those with an explicit TTL set.
• allkeys-random: Evicts random keys from all keys.

Sometimes, data changes unexpectedly, or you need to ensure a cache entry is immediately removed. In such cases, you can manually invalidate a key.

The DEL command removes one or more specified keys from Redis immediately.

You have a Redis key named session:user:123. You want it to expire in exactly 30 minutes from now. Which command would achieve this?

In this lesson, you mastered key techniques for managing cached data in Redis:

• Using EXPIRE, SETEX, and PEXPIRE to set time-based expirations.
• Understanding TTL and PERSIST to manage key lifetimes.
• Learning about automatic eviction policies to handle memory limits.
• Implementing manual cache invalidation with DEL for immediate updates.

These skills are crucial for maintaining fresh data and efficient memory usage in your Redis cache!