Support page-specific cache purging

[tomdyson]

Could clear_cache() take an optional page argument, supporting page-specific cache purging? It would be great to match the standard behaviour for Wagtail's front-end cache invalidation:

http://docs.wagtail.io/en/latest/reference/contrib/frontendcache.html

[vsalvino]

Assuming that CacheMiddleware.process_response() stores the keys based on URL, this should be possible. Will have to research CacheMiddleware more.

[vsalvino]

Finally figured out how to do this. The Django cache utilities generate cache keys based on a variety of factors, most of which makes it impossible to guess the cache entries for a given page, as the keys contain a combination of request URI (hashed), locale, method, etc.

Now that wagtail-cache is no longer using the django cache middleware, we have a bit more control over the caching process. To be able to purge based on URI alone, and not based on specific locales and request headers, we would need to maintain a separate cache index of keys based on URI. This can be accomplished by storing the cache keys in a list, in a dictionary keyed by URI or something easy to look up.

# Create caches key dict.
if not cache.get('keyring'):
    cache.set('keyring', {})

# Process response here, which yields a
cache_key = ...

# Track cache keys.
uri = request.build_absolute_uri()
keyring = cache.get('keyring')
keyring[uri] = keyring.get(uri, []) + [cache_key]
cache.set('keyring', keyring)

From here, to purge and individual page, one would get a list of cache keys from the keyring based on the URI of that page, and then delete those individually.

[marcus-steinbach]

I have taken the inspiration and expanded it.

Now you can clean up the cache in e.g. wagtail_hooks.py .

EXAMPLE:

@hooks.register('after_create_page')
@hooks.register('after_edit_page')
def clear_wagtailcache(request, page):
    purge_cache(
        [
            page.full_url, # page
            page.get_parent().full_url, # category page
            page.get_url_parts()[1], # root page
        ]
    )

[vsalvino]

Implemented in #22 and #39