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Suggestions on mechanism or existing code - maintain persistence of file download history


On 30/01/20 10:38 AM, jkn wrote:
> On Wednesday, January 29, 2020 at 8:27:03 PM UTC, Chris Angelico wrote:
>> On Thu, Jan 30, 2020 at 7:06 AM jkn <jkn_gg at nicorp.f9.co.uk> wrote:
>>> I want to be a able to use a simple 'download manager' which I was going to write
>>> (in Python), but then wondered if there was something suitable already out there.
>>> I haven't found it, but thought people here might have some ideas for existing work, or approaches.
>>>
>>> The situation is this - I have a long list of file URLs and want to download these
>>> as a 'background task'. I want this to process to be 'crudely persistent' - you
>>> can CTRL-C out, and next time you run things it will pick up where it left off.
>>
>> A decent project. I've done this before but in restricted ways.
>>
>>> The download part is not difficult. Is is the persistence bit I am thinking about.
>>> It is not easy to tell the name of the downloaded file from the URL.
>>>
>>> I could have a file with all the URLs listed and work through each line in turn.
>>> But then I would have to rewrite the file (say, with the previously-successful
>>> lines commented out) as I go.
...

>      Thanks for the idea. I should perhaps have said more clearly that it is not
> easy (though perhaps not impossible) to infer the name of the downloaded data
> from the URL - it is not a 'simple' file URL, more of a tag.
> 
> However I guess your scheme would work if I just hashed the URL and created
> a marker file - "a39321604c.downloaded" once downloaded. The downloaded content
> would be separately (and somewhat opaquely) named, but that doesn't matter.
> 
> MRAB's scheme does have the disadvantages to me that Chris has pointed out.

Accordingly, +1 to @Dan's suggestion of a database*:

- it can be structured to act as a queue, for URLs yet to be downloaded
- when downloading starts, the pertinent row can be updated to include 
the fileNM in use (a separate field from the URL)
- when the download is complete, further update the row with a suitable 
'flag'
- as long as each write/update is commit-ed, the system will be 
interrupt-able (^c).

Upon resumption, query the DB looking for entries without 
completion-flags, and re-start/resume the download process.

If a downloaded file is (later) found to be corrupt, either add the 
details to the queue again, or remove the 'flag' from the original entry.

This method could also be extended/complicated to work if you (are smart 
about) implement multiple retrieval threads...


* NB I don't use SQLite (in favor of going 'full-fat') and thus cannot 
vouch for its behavior under load/queuing mechanism/concurrent 
accesses... but I'm biased and probably think/write SQL more readily 
than Python - oops!
-- 
Regards =dn