[web-devel] Generating, serving, and deleting temporary files with Wai/Yesod

Michael Snoyman michael at snoyman.com
Sun Oct 24 11:20:05 CEST 2010

I think it's great; I've just released wai 0.2.2 with those code changes.


On Sat, Oct 23, 2010 at 11:56 PM, Matt Brown <matt at softmechanics.net> wrote:
> Thanks for the feedback.  I reverted my last commit and pushed a
> rewrite, trying to follow your advice.  Seems much cleaner now; what
> do you think?
> http://github.com/softmechanics/wai/commit/b37747ab9e2fd031512661ea57dc52c960edb902
> -matt
> On Sat, Oct 23, 2010 at 10:54 AM, Michael Snoyman <michael at snoyman.com> wrote:
>> On Fri, Oct 22, 2010 at 8:21 PM, Matt Brown <matt at softmechanics.net> wrote:
>>> Hello all,
>>> I've been experimenting with generating graphs from log files using
>>> timeplot and serving them with yesod.  Each generated graph is only
>>> served once, so I need a way to clean them up.  A cron job would be
>>> ok, but it's one more thing I'd have to remember to do, and I'm lazy.
>>> Instead, I'd like to pass a "clean up action" to the enumerator
>>> sending the file, to be performed after EOF is reached.  For most
>>> files, this would just close the handle.  For temp files, it would
>>> also delete the file.  Here's the gist of it:
>>> http://gist.github.com/640902
>>> The first revision defines a monolithic fromTempFile :: FilePath ->
>>> Enumerator.  The second imports a factored version from
>>> Network.Wai.Handler (see my wai commit:
>>> http://github.com/softmechanics/wai/commit/2cb5ea47ca458f4b91a73e01a1dd0a29e84e001e).
>>> Not sure wai is the place for it.  Perhaps wai-extra would be a better?
>>> Does this seem like a good approach?
>> This does seem like the right approach. Just as fair warning: using an
>> enumerator for serving files loses out on the possible optimization of
>> a sendfile system call. Another possibility may be to keep a pool of
>> file names to use as temporary files and keep rotating their usage.
>> But I *do* think your approach is the correct one.
>> Regarding the API changes to wai: they seem reasonable, and I'd
>> probably accept them. Some concerns about the implementation:
>> * Instead of fromHandle' and fromFile', perhaps fromHandleFinally and
>> fromFileFinally?
>> * In fromFile', why are you calling hClose? withBinaryFile
>> automatically closes the file handle. Are you worried about the
>> temporary file getting deleted while the file handle is still open?
>> * In fromHandle', onEOF won't get called if the iteratee returns Left.
>> Is this intended behavior?
>> * Your code won't work in the presence of exceptions. Ideally the body
>> of fromHandle' would be wrapped with a call to finally. This point is
>> related to the previous one.
>> Michael

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