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Add persistence to cache.FileCache #90
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Is it possible to split the
preload
and thefreeze
of cache?In the current implementation, user has to do following to
freeze
the cache and get multi-process safe:load => preserve => preload
If we can split these two functionalities, then we can simply do "load" => "freeze". Moreover, the cache can be modified before "freeze".
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What do you mean by load?
If "load" means writing data into cache files, that means creating file cache with specific name if I understand it correctly. But it is unacceptable because it leads to partially written named files left after job failure and they'll definitely mess up the cache and following re-run of the job.
Also the principle of cache in this module is data being immutable and deterministic, there should be no modification. What do you mean by "modified"?
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Sorry to cause confusion, by "load" I mean reading data from remote storage. If I understand correctly, in order to use this module, user has to do following:
My suggestion is to have a "freeze" function, so that we can:
to support the multiprocess. I do not think it would increase the risk of having a broken cache.
By modifying, I do not mean changing the data itself, but the ability of adding new cache entities.
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I believe definitely it happens. For example,
put(1, b'bar')
Even in this case partial write may happen like
b'ba'
was written to disk butb'r'
wasn't. Then, restart the job,put(1, b'bar')
as the data 1 is immutableAlso in this case it is not possible to put data nor to run
get(1)
if partial write happened. This is typical scenario of broken data and broken cache files must be thrown away.There was a problem hiding this comment.
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I think we both are misunderstanding each other. I think in your code, you integrated the "freeze" into preserve, which I misunderstood, so the 3 is not necessary. But I think that design forces the user to preserve the cache, which is not always necessary. For example, they just want to enable multiprocessing, which only needs "freezing" the cache. Preserving the data disables the automatic deletion of the cache.
In my example, the cache should not be frozen until all the data is loaded, and after "freeze", no data can be added to the cache. Then when the job fails at a "put", which is definitely before the "freeze", the cache will be neither frozen nor preserved. That is why I think such design will not increase the risk of having broken case.
Sorry for making confusion.
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Automatic deletion of the cache among multiple processes does not work correctly because there are no synchronization on closing the file objects, where child processes can't read contents after parent process exit. This is because Python's
tempfile
module explicitly unlinks named temporary files. Please try following script:You'll see this:
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I see, thank you for the explanation. Can you fix the other issues?