Persistent cached data #117

@Beforerr, the cached data is actually persistent in (stored on disk) but for each data request Speasy checks if there is any update on the remote server providing these data. The main issue here is that this mechanism can be tricky to get right. The current implementation can discard cache entries too eagerly because it is safer to refresh data that keeping an outdated product by default.
In my TODO list, I planed to add a no_refresh keyword to the get_data function to take data from cache without checking if remote servers have a newer version of a given product.

The pitfall here is that cache entries are fixed size (12h) data fragments that are stored when last server request was done. At any moment your cache is likely to have a mix of up to date and outdated entries for a given product. It means that using no_refresh (when implemented) could return a product that is a mix of different versions of the data for each fragment.

Manipulating the cache directly

This not documented in user documentation but the API is there.

You actually can play a bit with the cache manually in python, you can list entries with this method:

import speasy as spz
entries = spz.core.cache.entries()
# drop internal entries
entries = list(filter(lambda e: '__internal__' not in e, entries))
print(entries[:100])

it should print something like this:

image

It lists the keys that you can use to actually retrieve data from cache, the names as you can see are either cdf files URLs (entries for the archive module) or a string with the following format "{webservice name}/{product identifier}/{start time}".
Let say I want to directly get from cache the SpeasyVariable for amda/wnd_swe_n/2003-10-05T12:00:00+00:00 :

entry = spz.core.cache.get_item('amda/wnd_swe_n/2003-10-05T12:00:00+00:00')
v = spz.SpeasyVariable.from_dictionary(entry.data)

Let say you want to merge several cache entries:

entries = [ spz.core.cache.get_item('amda/wnd_swe_n/2003-10-05T00:00:00+00:00'), spz.core.cache.get_item('amda/wnd_swe_n/2003-10-05T12:00:00+00:00') ]
v = spz.products.variable.merge( [ spz.SpeasyVariable.from_dictionary(e.data) for e in entries ] )

Accessing the files directly

Since we use DiskCache. DiskCache uses both sqlite and pickle to store data, some data are stored in files using hashes for path and filenames and small values are directly stored in the database. This makes almost impossible to reason about files here.