Redis 缓存基本操作

Welcome! In this lesson, we'll dive into the fundamental operations for managing data in your Redis cache. Think of it as learning the basic "read, write, update, and delete" for your cached information.

Mastering these commands is key to effectively using Redis to speed up your applications and reduce database load.

The most basic operation is storing data. In Redis, you use the SET command to store a key-value pair.

• Key: A unique identifier for your data (e.g., user:123:name).
• Value: The actual data you want to store (e.g., "Alice").

It's like putting a label on a box and storing something inside.

SET myapp:homepage "Welcome to our site!"

This command stores "Welcome to our site!" under the key myapp:homepage.

Once data is stored, you'll need to retrieve it. The GET command fetches the value associated with a specific key.

If the key exists, Redis returns its value. If the key doesn't exist or has expired, Redis returns nil (nothing).

GET myapp:homepage

This would return "Welcome to our site!" if the key exists.

Cached data shouldn't live forever! Time To Live (TTL) defines how long an item should remain in the cache before it's automatically removed. This prevents stale data and saves memory.

• TTL is typically measured in seconds.
• Once the TTL expires, the key is automatically deleted by Redis.

It's like setting an alarm for your cached data.

You can set a TTL right when you store the data using SETEX or PSETEX.

• SETEX key seconds value: Sets an expiration in whole seconds.
• PSETEX key milliseconds value: Sets an expiration in milliseconds (for finer control).

This is very common for caching temporary session tokens or frequently changing data.

SETEX user:123:session 300 "active"

This key will expire in 5 minutes (300 seconds).

You can also set or change the TTL for an existing key, or check how much time is left.

• EXPIRE key seconds: Sets a TTL for an existing key.
• PERSIST key: Removes the expiration from a key, making it permanent.
• TTL key: Returns the remaining TTL in seconds (or -1 if no TTL, -2 if key doesn't exist).

EXPIRE user:123:session 60
TTL user:123:session

The first command updates the TTL to 60 seconds. The second checks it.

To "update" a cache entry, you simply use the SET command again with the same key. Redis will overwrite the old value with the new one.

If the key had a TTL, it will be removed unless you explicitly set a new one with SETEX or EXPIRE.

SET user:123:name "Bob"
SET user:123:name "Robert"

After the second command, GET user:123:name would return "Robert".

When data is no longer needed or becomes completely invalid, you can remove it from the cache using the DEL command.

• DEL key: Deletes a single key.
• DEL key1 key2 key3: Deletes multiple keys at once.

This frees up memory and ensures your cache only holds relevant data.

DEL myapp:homepage
DEL user:123:session user:456:session

These commands remove the specified keys from Redis.

Let's simulate a simple caching flow for a web page using these commands:

SET page:about "HTML content for about page" EX 3600
GET page:about
# ... time passes ...
DEL page:about  # If content changes, invalidate it!

Here, we set the page content with a 1-hour TTL, then retrieve it. If the page is updated, we explicitly delete the old cache entry.

Consider the following sequence of Redis commands:

SET mykey "valueA"
EXPIRE mykey 5
SET mykey "valueB"
TTL mykey

What will be the output of the final TTL mykey command?

Great job! You've learned the essential Redis commands for managing your cache:

• SET: To store key-value pairs.
• GET: To retrieve values.
• SETEX/PSETEX: To set a value with an expiration.
• EXPIRE/PEXPIRE/PERSIST: To manage existing key expirations.
• DEL: To remove keys.

These commands are the building blocks for any Redis caching strategy. Next, we'll explore more advanced data structures Redis offers for caching!