Caching Patterns

Configuration

Development (LocMemCache)

CACHES = {
    "default": {
        "BACKEND": "django.core.cache.backends.locmem.LocMemCache",
        "LOCATION": "unique-snowflake",
        "OPTIONS": {"MAX_ENTRIES": 1000},
    }
}

Production (Redis)

CACHES = {
    "default": {
        "BACKEND": "django_redis.cache.RedisCache",
        "LOCATION": os.environ.get("REDIS_URL"),
        "OPTIONS": {
            "CLIENT_CLASS": "django_redis.client.DefaultClient",
            "CONNECTION_POOL_KWARGS": {"max_connections": 50},
            "COMPRESSOR": "django_redis.compressors.zlib.ZlibCompressor",
        },
    }
}

Cache Timeout Constants

Use in core/cache.py:

CACHE_TIMEOUT_SHORT = 60        # 1 minute
CACHE_TIMEOUT_MEDIUM = 300      # 5 minutes
CACHE_TIMEOUT_LONG = 900        # 15 minutes
CACHE_TIMEOUT_VERY_LONG = 3600  # 1 hour
CACHE_TIMEOUT_DAY = 86400       # 24 hours

Caching Strategies

1. View-Level Caching

For static pages identical for all users:

from django.utils.decorators import method_decorator
from django.views.decorators.cache import cache_page

@method_decorator(cache_page(60 * 15), name='dispatch')
class MeritFlawListView(ListView):
    model = MeritFlaw
    template_name = "characters/core/meritflaw/list.html"

Use for: Reference data lists, public pages, rarely-changing content. Don't use for: User-specific pages, permission-gated content.

2. Queryset Caching

For expensive database queries:

from core.cache import cache_function, CACHE_TIMEOUT_MEDIUM

class CharacterDetailView(DetailView):
    @staticmethod
    @cache_function(timeout=CACHE_TIMEOUT_MEDIUM, key_prefix="char_scenes")
    def get_character_scenes(character_id):
        return list(
            Scene.objects.filter(characters__id=character_id)
            .select_related("chronicle", "location")
            .prefetch_related("characters")
            .order_by("-date_of_scene")
        )

    def get_context_data(self, **kwargs):
        context = super().get_context_data(**kwargs)
        context["scenes"] = self.get_character_scenes(self.object.id)
        return context

3. Manual Cache Access

from django.core.cache import cache

# Set with timeout
cache.set('my_key', data, timeout=300)

# Get with default
data = cache.get('my_key', default=None)

# Delete
cache.delete('my_key')

# Get or set pattern
def get_expensive_data():
    key = 'expensive_calculation'
    result = cache.get(key)
    if result is None:
        result = perform_expensive_calculation()
        cache.set(key, result, timeout=300)
    return result

4. Template Fragment Caching

{% load cache %}

{% cache 900 character_details character.id %}
    <div class="character-details">
        {{ character.name }}
        {{ character.description }}
    </div>
{% endcache %}

Cache Key Generator

class CacheKeyGenerator:
    PREFIX = "tg"

    @classmethod
    def make_model_key(cls, model_class, **filters):
        filter_str = ":".join(f"{k}={v}" for k, v in sorted(filters.items()))
        return f"{cls.PREFIX}:queryset:{model_class.__name__}:{filter_str}"

    @classmethod
    def make_view_key(cls, view_name, **params):
        param_str = ":".join(f"{k}={v}" for k, v in sorted(params.items()))
        return f"{cls.PREFIX}:view:{view_name}:{param_str}"

Cache Invalidation

On Model Save

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

@receiver([post_save, post_delete], sender=Character)
def invalidate_character_cache(sender, instance, **kwargs):
    cache.delete(f"tg:queryset:Character:id={instance.id}")
    cache.delete(f"tg:queryset:Character:chronicle={instance.chronicle_id}")

Manual Invalidation

# Single key
cache.delete('specific_key')

# Pattern-based (Redis only)
from django_redis import get_redis_connection
con = get_redis_connection("default")
keys = con.keys("tg:queryset:Character:*")
if keys:
    con.delete(*keys)

# Clear all (use sparingly!)
cache.clear()

Best Practices

Do Cache

• Reference data (merits/flaws, archetypes, clans)
• Expensive aggregations
• API responses
• Static content lists

Don't Cache

• User-specific permissions
• Frequently changing data
• Data displayed immediately after modification
• Session-dependent content

Cache Duration Guidelines

Combine with Query Optimization

# GOOD: Cache an optimized query
@cache_function(timeout=300)
def get_characters():
    return Character.objects.select_related('owner').prefetch_related('merits_and_flaws')

# BAD: Caching doesn't fix N+1
@cache_function(timeout=300)
def get_characters():
    return Character.objects.all()  # N+1 on access

Debugging

Check Cache Status

from django.core.cache import cache

# Test connection
cache.set('test', 'value')
assert cache.get('test') == 'value'
print("Cache working!")

# Check Redis keys (Redis only)
from django_redis import get_redis_connection
con = get_redis_connection("default")
print(f"Total keys: {len(con.keys('*'))}")

Clear Cache

# Django shell
python manage.py shell
>>> from django.core.cache import cache
>>> cache.clear()

# Redis CLI
redis-cli FLUSHDB