Cache Invalidation Strategies

Introduction

Cache invalidation is notoriously difficult. Poor invalidation leads to stale data or cache misses.

Key Concepts

TTL-Based: Cache expires after time period.

Event-Based: Invalidate on data changes.

Version-Based: Change cache key on updates.

Deep Dive

Signal-Based Invalidation

python
from django.db.models.signals import post_save, post_delete
from django.dispatch import receiver

@receiver([post_save, post_delete], sender=Article)
def invalidate_article_cache(sender, instance, **kwargs):
    cache.delete(f'article:{instance.slug}')
    cache.delete('article_list')
    cache.delete(f'author:{instance.author_id}:articles')

Version-Based Keys

python
class Article(models.Model):
    version = models.IntegerField(default=0)
    
    def save(self, *args, **kwargs):
        self.version += 1
        super().save(*args, **kwargs)
    
    @property
    def cache_key(self):
        return f'article:{self.pk}:v{self.version}'

Cache Tags (with django-redis)

python
from django_redis import get_redis_connection

def cache_with_tags(key, value, tags, timeout=3600):
    cache.set(key, value, timeout)
    r = get_redis_connection('default')
    for tag in tags:
        r.sadd(f'tag:{tag}', key)

def invalidate_tag(tag):
    r = get_redis_connection('default')
    keys = r.smembers(f'tag:{tag}')
    if keys:
        cache.delete_many([k.decode() for k in keys])
    r.delete(f'tag:{tag}')

# Usage
cache_with_tags('article:123', article, ['articles', f'author:{author_id}'])
invalidate_tag('articles')  # Clears all article caches

Best Practices

1. Prefer TTL + event invalidation: Belt and suspenders.
2. Use cache prefixes: Easier to invalidate by category.
3. Log cache invalidations: Helps debug stale data issues.

Summary

Combine TTL with signal-based invalidation for reliability. Version-based keys avoid race conditions. Cache tags simplify bulk invalidation.