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

Concept Flow - Cache invalidation strategies

Data Update Happens

↓

Choose Invalidation Strategy

↓

Time-based

↓

Expire keys

↓

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.

Practice

1. What is the main purpose of cache invalidation in Redis?

easy

A. To keep cached data fresh and accurate

B. To increase the size of the cache

C. To slow down data retrieval

D. To permanently store data without changes

Solution

1. Step 1: Understand cache invalidation

   Cache invalidation means removing or updating cached data to avoid stale information.

2. Step 2: Identify the main goal

   The goal is to keep the cached data fresh and accurate so users get the latest information.

3. Final Answer:

   To keep cached data fresh and accurate -> Option A

4. Quick Check:

   Cache invalidation = fresh data [OK]

Hint: Cache invalidation means keeping data fresh [OK]

Common Mistakes:

• Thinking cache invalidation increases cache size
• Believing it slows down data retrieval
• Assuming cache data never changes

2. Which Redis command is used to manually delete a cached key for invalidation?

easy

A. EXPIRE

B. GET

C. SETEX

D. DEL

Solution

1. Step 1: Identify commands for cache invalidation

   Manual invalidation requires deleting keys, so the command must remove keys.

2. Step 2: Match command to action

   DEL command deletes keys immediately, while EXPIRE sets a timeout, SETEX sets value with expiration, GET retrieves value.

3. Final Answer:

   DEL -> Option D

4. Quick Check:

   Delete key = DEL command [OK]

Hint: DEL deletes keys instantly for invalidation [OK]

Common Mistakes:

• Confusing EXPIRE with immediate deletion
• Using GET which only reads data
• Thinking SETEX deletes keys

3. Consider this Redis usage:
SET user:1 "Alice" EX 10
What happens after 10 seconds?

medium

A. The key user:1 remains forever

B. The key user:1 is deleted automatically

C. The key user:1 value changes to null

D. The key user:1 expires but stays in memory

Solution

1. Step 1: Understand the EX option in SET

   EX 10 means the key expires after 10 seconds automatically.

2. Step 2: Identify what happens on expiration

   When a key expires, Redis deletes it from memory automatically.

3. Final Answer:

   The key user:1 is deleted automatically -> Option B

4. Quick Check:

   EX sets expiration = auto delete [OK]

Hint: EX sets auto-delete time for keys [OK]

Common Mistakes:

• Thinking key stays forever after expiration
• Believing value changes to null instead of deletion
• Assuming expired keys remain in memory

4. You wrote this Redis command to expire a key:
EXPIRE user:2 0
What is the problem with this command?

medium

A. It sets expiration to zero seconds, effectively deleting the key

B. It deletes the key immediately

C. It sets the key to never expire

D. It causes a syntax error

Solution

1. Step 1: Understand EXPIRE with zero seconds

   EXPIRE key 0 removes any existing expiration timeout (equivalent to PERSIST command).

2. Step 2: Identify the problem

   The intent was to expire the key, but this sets it to never expire instead.

3. Final Answer:

   It sets the key to never expire -> Option C

4. Quick Check:

   EXPIRE 0 = remove expiration [OK]

Hint: EXPIRE 0 removes expiration (never expire) [OK]

Common Mistakes:

• Thinking it deletes the key immediately
• Believing it causes a syntax error
• Assuming zero seconds expires immediately

5. You want to keep cached product data fresh but avoid deleting keys too often. Which strategy best balances speed and data correctness in Redis?

hard

A. Use a long expiration time and rely on manual invalidation when data changes

B. Never expire keys and update them only manually

C. Delete keys immediately after every read

D. Set a short expiration time and manually delete keys on updates

Solution

1. Step 1: Understand trade-offs of expiration and manual deletion

   Short expiration causes frequent reloads; manual deletion alone risks stale data.

2. Step 2: Choose balanced approach

   Long expiration reduces reloads, manual invalidation on updates keeps data correct without overhead.

3. Final Answer:

   Use a long expiration time and rely on manual invalidation when data changes -> Option A

4. Quick Check:

   Balance speed and correctness = long expiry + manual invalidation [OK]

Hint: Long expiry plus manual delete balances freshness and speed [OK]

Common Mistakes:

• Choosing very short expiration causing overhead
• Never expiring keys causing stale data
• Deleting keys after every read causing slow performance