[Django]-How to invalidate several django cached values efficiently?

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https://martinfowler.com/bliki/TwoHardThings.html

There are only two hard things in Computer Science: cache invalidation and naming things.

— Phil Karlton

Presuming you are using Django’s Cache Middleware, you’ll need to target the cache keys that are relevant. You can see how they generate the cache key from these two files in the Django Project:

–
https://github.com/django/django/blob/master/django/middleware/cache.py#L99
– https://github.com/django/django/blob/master/django/utils/cache.py#L367
– https://github.com/django/django/blob/master/django/utils/cache.py#L324

_generate_cache_key

def _generate_cache_key(request, method, headerlist, key_prefix):
    """Return a cache key from the headers given in the header list."""
        ctx = hashlib.md5()
        for header in headerlist:
            value = request.META.get(header)
            if value is not None:
                ctx.update(force_bytes(value))
        url = hashlib.md5(force_bytes(iri_to_uri(request.build_absolute_uri())))
        cache_key = 'views.decorators.cache.cache_page.%s.%s.%s.%s' % (key_prefix, method, url.hexdigest(), ctx.hexdigest())
        return _i18n_cache_key_suffix(request, cache_key)

The cache key is generated based on attributes and headers from the request and hashed values (i.e. the url is hashed and used as part of the key). The Vary header in your response specifies other headers to use as part of the cache it.

If you understand how Django is caching your views and calculating your cache keys, then you can use this to target appropriate cache entries, but this is still very difficult because the url is hashed you can’t target url patterns (you could use https://stackoverflow.com/a/35629796/784648 cache.delete_patterns(...) otherwise).

Django primarily relies on timeout to invalidate the cache.

I would recommend looking into Django Cacheops, this package is designed to work with Django’s ORM to cache and invalidate QuerySets. This seems a lot more practical for your needs because you want fine-grained invalidation on your Item QuerySets, you simply will not get that from Django’s Cache Middleware. Take a look at the github repo, I’ve used it and it works well if you take the time to read the docs and understand it.

👤A. J. Parr

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