Understanding the sync method from the python shelve library

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The python documentation says this about the sync method:

Write back all entries in the cache if the shelf was opened with writeback set to True. Also empty the cache and synchronize the persistent dictionary on disk, if feasible. This is called automatically when the shelf is closed with close().

I am really having a hard time understanding this. How does accessing data from cache differ from accessing data from disk? And does emptying the cache affect how we can access the data stored in a shelve?

shelve python
1 Answers

For whoever is using the data in the Shelve object, it is transparent whether the data is cached or is on disk. If it is not on the cache, the file is read, the cache filled, and the value returned. Otherwise, the value as it is on the cache is used.

If the cache is emptied on calling sync, that means only that on the next value fetched from the same Shelve instance, the file will be read again. Since it is all automatic, there is no difference. The documentation is mostly describing how it is implemented.

If you are trying to open the same "shelve" file with two concurrent apps, or even two instances of shelve on the same program, chances are you are bound to big problems. Other than that, it just behaves as a "persistent dictionary" and that is it.

This pattern of writing to disk and re-reading from a single file makes no difference for a workload of a single user in an interactive program. For a Python program running as a server with tens to thousands of clients, or even a single big-data processing script, where this could impact actual performance, Shelve is hardly a usable thing anyway.

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