Cache invalidation strategies in Redis - Step-by-Step Execution

Concept Flow - Cache invalidation strategies

Data Update Happens

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Choose Invalidation Strategy

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Time-based

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Expire keys

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Cache Miss -> Fetch fresh data

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Cache Updated

When data changes, choose a strategy to remove or update cached data so users get fresh info.

Execution Sample

Redis

SET user:1 "Alice"
EXPIRE user:1 10
GET user:1
# After 10 seconds
GET user:1

Cache a user name with a 10-second expiry, then try to get it before and after expiry.

Execution Table

Step	Command	Action	Cache State	Output
1	SET user:1 "Alice"	Store key 'user:1' with value 'Alice'	{user:1: "Alice"}	OK
2	EXPIRE user:1 10	Set key 'user:1' to expire in 10 seconds	{user:1: "Alice" (expires in 10s)}	1
3	GET user:1 (within 10s)	Retrieve 'user:1' from cache	{user:1: "Alice" (expires in 7s)}	"Alice"
4	Wait 10 seconds	Key 'user:1' expires and is removed	{}	Key not found
5	GET user:1 (after expiry)	Cache miss, no key found	{}	nil

💡 Cache key expires after 10 seconds, so GET returns nil after expiry.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5
user:1	nil	"Alice"	"Alice" (expires in 10s)	"Alice" (expires in 7s)	expired (removed)	nil

Key Moments - 3 Insights

Why does GET return nil after 10 seconds even though we set the key before?

Because EXPIRE sets a time limit on the key. After 10 seconds, Redis automatically deletes it, so GET finds no key (see execution_table step 4 and 5).

What happens if we don't set EXPIRE after SET?

The key stays in cache forever until manually deleted. GET will always return the value (not shown in table but implied).

How does Redis know when to delete the key after expiry?

Redis tracks expiry times internally and removes keys once expired, shown in step 4 where the cache state becomes empty.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the output of GET user:1?

A "Alice"

B nil

C Key not found

D Error

💡 Hint

Check the Output column at step 3 in execution_table.

At which step does the cache key expire and get removed?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the Cache State column showing the key removal.

If we remove the EXPIRE command, what would happen at step 5?

A GET returns nil

B GET returns "Alice"

C GET returns error

D Cache clears automatically

💡 Hint

Refer to key_moments about what happens without EXPIRE.

Concept Snapshot

Cache Invalidation Strategies in Redis:
- Use EXPIRE to auto-remove keys after time (time-based).
- Use DEL to remove keys on data change (event-based).
- Manual cache clear for full reset.
- Cache miss triggers fresh data fetch and cache update.
- Proper invalidation keeps cache fresh and consistent.

Full Transcript

This visual trace shows how Redis cache invalidation works using the time-based strategy. First, we store a key 'user:1' with value 'Alice'. Then, we set it to expire in 10 seconds. When we GET the key before expiry, Redis returns 'Alice'. After waiting 10 seconds, the key expires and is removed automatically. A GET after expiry returns nil, indicating a cache miss. This process ensures cached data does not become stale. Key moments clarify why the key disappears after expiry and what happens if expiry is not set. The quiz tests understanding of cache state and expiry timing.