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

Concept Flow - Cache invalidation strategies

Data changes in DB

↓

Identify cached data affected

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Choose invalidation strategy

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Delete cache

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Serve fresh data from cache

When data changes, Django identifies which cached data is affected, then either deletes or updates the cache to keep it fresh.

Execution Sample

Django

from django.core.cache import cache

# Invalidate cache key after data update
cache.delete('user_profile_42')

This code deletes a specific cache entry after user data changes to ensure fresh data is served next time.

Execution Table

Step	Action	Cache Key	Cache State Before	Cache State After	Result
1	Initial cache set	user_profile_42	None	{'user_profile_42': 'old_data'}	Cache stores old user data
2	User data updated in DB	-	{'user_profile_42': 'old_data'}	{'user_profile_42': 'old_data'}	Cache still has old data
3	Cache invalidation triggered	user_profile_42	{'user_profile_42': 'old_data'}	{}	Cache entry deleted
4	Next request fetches fresh data	user_profile_42	{}	{'user_profile_42': 'fresh_data'}	Cache updated with fresh data
5	Serve fresh data from cache	user_profile_42	{'user_profile_42': 'fresh_data'}	{'user_profile_42': 'fresh_data'}	User sees updated profile

💡 Cache invalidation ensures stale data is removed so fresh data is served after DB update

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
cache	{}	{'user_profile_42': 'old_data'}	{'user_profile_42': 'old_data'}	{}	{'user_profile_42': 'fresh_data'}	{'user_profile_42': 'fresh_data'}

Key Moments - 3 Insights

Why does the cache still have old data after the database update in step 2?

Because updating the database does not automatically update or delete the cache. The cache must be explicitly invalidated as shown in step 3.

What happens if we don't delete or update the cache after data changes?

The cache will serve stale data, causing users to see outdated information. Step 3 shows the importance of invalidation.

Can we update the cache directly instead of deleting it?

Yes, updating the cache with fresh data after DB changes is another strategy, as shown in step 4.

Visual Quiz - 3 Questions

Look at the execution table, what is the cache state after step 3?

A {}

B {'user_profile_42': 'old_data'}

C {'user_profile_42': 'fresh_data'}

D None

💡 Hint

Check the 'Cache State After' column for step 3 in the execution table.

At which step does the cache get updated with fresh data?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look for when 'Cache State After' shows fresh data in the execution table.

If we skip cache deletion in step 3, what will the cache state be at step 4?

A {}

B {'user_profile_42': 'old_data'}

C {'user_profile_42': 'fresh_data'}

D None

💡 Hint

Refer to the cache state before and after deletion in the execution table.

Concept Snapshot

Cache Invalidation Strategies in Django:
- When data changes, identify affected cache keys.
- Invalidate cache by deleting keys or updating with fresh data.
- Use cache.delete('key') to remove stale entries.
- Always invalidate cache after DB updates to avoid stale data.
- Fresh data is cached on next request after invalidation.

Full Transcript

Cache invalidation in Django means removing or updating cached data when the original data changes. This prevents users from seeing old information. The process starts when data in the database changes. Django then finds which cached data is affected. It either deletes that cache entry or updates it with new data. For example, after updating a user's profile, the cache key 'user_profile_42' is deleted. The next time the profile is requested, fresh data is fetched from the database and stored in the cache. This way, the cache always serves up-to-date information. Without invalidation, the cache would keep serving old data, confusing users. So, always remember to invalidate cache after data changes.