[Django]-Clearing specific cache in Django

I make a function to delete key starting with some text. This help me to delete dynamic keys.

list posts cached

def get_posts(tag, page=1):
    cached_data = cache.get('list_posts_home_tag%s_page%s' % (tag, page))
    if not cached_data:
        cached_data = mycontroller.get_posts(tag, page)
        cache.set('list_posts_home_tag%s_page%s' % (tag, page), cached_data, 60)
    return cached_data

when update any post, call flush_cache

def update(data):
    response = mycontroller.update(data)
    flush_cache('list_posts_home')
    return response

flush_cache to delete any dynamic cache

def flush_cache(text):
    for key in list(cache._cache.keys()):
        if text in key:
            cache.delete(key.replace(':1:', ''))

Do not forget to import cache from django

from django.core.cache import cache

I had same problem and I found this solution.

if you are using this decorator:

django.views.decorators.cache.cache_page

you can use this function:

import hashlib
from typing import List
from django.core.cache import cache


def get_cache_keys_from_url(absolute_uri) -> List[str]:
    url = hashlib.md5(f"absolute_uri".encode('ascii'))
    return cache.keys(f'*{url.hexdigest()}*')

which you need to get cache keys for absolute_uri

and than use cache.delete_many(get_cache_keys_from_url(http://exemple.com/post/1234/)) for clear page with url – http://exemple.com/post/1234/

There’s a trick that might work for this. The cache_page decorator takes an optional argument for key_prefix. It’s supposed to be used when you have, say, multiple sites on a single cache. Here’s what the docs say:

CACHE_MIDDLEWARE_KEY_PREFIX – If the cache is shared across multiple sites using the same Django installation, set this to the name of the site, or some other string that is unique to this Django instance, to prevent key collisions. Use an empty string if you don’t care.

But if you don’t have multiple sites, you can abuse it thus:

cache_page(cache_length, key_prefx="20201215")

I used the date as the prefix so it’s easy to rotate through new invalidation keys if you want. This should work nicely.

However! Please note that if you are using this trick, some caches (e.g., the DB cache, filesystem cache, and probably others) do not clean up expired entries except when they’re accessed. If you use the trick above, you won’t ever access the cache entry again and it’ll linger until you clear it out. Probably doesn’t matter, but worth considering.

With delete() and delete_many(), you can delete the specific cache values in LocMemCache as shown below. *delele() and delete_many() can delete the specific version of single cache value and multiple cache values respectively and my answer explains how to set and get cache values with LocMemCache and the answer of my question explains the default version of a cache value with LocMemCache:

from django.http import HttpResponse
from django.core.cache import cache

def test(request):
    cache.set("first_name", "John")
    cache.set("first_name", "David", version=2)
    cache.set("last_name", "Smith")
    cache.set("last_name", "Miller", version=2)
    cache.set("age", 36)
    cache.set("age", 42, version=2)

    cache.delete("first_name") # Delete "John"
    cache.delete("first_name", 2) # Delete "David"
    cache.delete_many(["last_name", "age"]) # Detele "Smith" and 36
    cache.delete_many(["last_name", "age"], 2) # Detele "Miller" and 42
    return HttpResponse("Test")

In addition, clear() can delete all cache values as shown below:

from django.http import HttpResponse
from django.core.cache import cache

def test(request):
    cache.set("first_name", "John")
    cache.set("first_name", "David", version=2)
    cache.set("last_name", "Smith")
    cache.set("last_name", "Miller", version=2)
    cache.set("age", 36)
    cache.set("age", 42, version=2)

    cache.clear() # Delete all

    return HttpResponse("Test")