What does `key_prefix` do for flask-cache?

Question

For example like this, is it necessary to use key_prefix?

@cache.cached(timeout=50, key_prefix='all_comments')
def get_all_comments():
    comments = do_serious_dbio()
    return [x.author for x in comments]

cached_comments = get_all_comments()

In the document, it says that the key_prefix's default value is request.path cache_key.:, what does cache_key mean, how can I use it? What does key_prefix do?

Answer 1

First the request.path is everything (except params) after your script_root. For example:

1. For a url like, http://127.0.0.1:5000/users/login/, request data is:

   request.path is: /users/login/
   

2. For a url like in the example from the link above, http://www.example.com/myapplication/page.html?x=y, request data is:

   request.path is: /page.html
   

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Q. What does cache_key mean, how can I use it?

The cache_key is the key that is used to access the particular cached value. As you know that the cache is a key-value store.

In Flask-Cache the cache_key is generated by the extension and we are not supposed to use it ourselves.

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Q. What does key_prefix do?

key_prefix is used to generate the cache_key for a cached value. See make_cache_key source to see how exactly it is done.

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Q. Is it necessary to use key_prefix?

Let's say you are calling get_all_comments from 2 different view functions, say manage(), and view(). And you don't specify a key_prefix, while caching get_all_comments with @cached.

The first time you view a post through view the get_all_comments output is cached with a default key like: view/view or view/module/view, or whatever the value of view/%s is, where %s is request.path.

Next when you manage a post through manage, the get_all_comments output is not read from the cache, since the cache_key applied to get data from the cache has changed to view/manage and is not the old view/view, because the request.path has now changed.

The whole point of caching get_all_comments here was to get the data from the cache whenever possible and not the db, but since the keys have changed between view functions the data is actually retrieved both the times from the db itself.

However in case you had specified a key_prefix like all_comments, then the first time data is retrieved from the db, and the next time the cache_key is still all_comments and value is found, and data is accessed from the cache instead of db.

So when you have cases like the above it is obviously better to use a key_prefix, in other cases when the function is always called from a single path/ view function then it is ok to let the default one be used.

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Note: The cache_key is generated/calculated for every request, see the source:

cache_key = decorated_function.make_cache_key(*args, **kwargs)