Thanks for the responses, guys.
Regarding my deployment, I've been doing my development on my local machine
using manage.py. I'll give this a go in WSGI and see what happens.
Graham, I totally agree that there are ways to get around my problem outside
of trying to send HTTP headers before the message body. A simple way would
be to give the user a "please wait" page while the tarball is generated.
I'm mostly just lazy and don't want to do that, haha. That said, thanks for
the suggestion.
Rick, the issue with your suggestion is that I need to actually create the
tarball, which takes time. I'm trying to avoid having the user wait while
the tarball is created by giving them the download dialogue before
generation happens. Using your BigTarFileWrapper assumes that the tarball
has been created already, at least from what I can tell.
So again, I'll give this a go in WSGI and see what happens. In the
meantime, I suppose the empty gzipped file gets the job done.
Thanks again for all the responses. Go Django!
Alex
On Wed, Apr 15, 2009 at 8:36 AM, Rick Wagner <rwag...@physics.ucsd.edu>wrote:
>
>
>
> On Apr 14, 6:55 pm, Graham Dumpleton <graham.dumple...@gmail.com>
> wrote:
> > On Apr 15, 7:49 am, Alex Loddengaard <a...@cloudera.com> wrote:
> >
> >
> >
> > > I've found several messages on this list discussing ways to send large
> files
> > > in a HttpResponse. One can use FileWrapper, or one can use a generator
> and
> > > yield chunks of the large file. What about the case when the large
> file is
> > > generated at HTTP request time? In this case, it would be annoying to
> have
> > > the user wait for the page to generate the large file and then stream
> the
> > > file. Instead we would want a way to start the HTTP response (so that
> the
> > > user gets the download dialogue), generate the large file, and then
> stream
> > > the file. Let's take the following example:
> >
> > > def create_tarball():
> >
> > > > path = create_some_big_tarball()
> >
> > > > chunk = None
> > > > fh = open(path, 'r')
> > > > while True:
> > > > chunk = fh.read(1024 * 128)
> > > > if chunk == '':
> > > > break
> > > > yield chunk
> >
> > > > def sample_view(request):
> > > > response = HttpResponse(create_tarball(),
> > > > mimetype='application/x-compressed')
> > > > response['Content-Disposition'] =
> "attachment;filename=mytarball.tar.gz"
> >
> > > The above example nearly accomplishes what we want, but it doesn't
> start the
> > > HTTP response before the tarball is created, hence making the user wait
> a
> > > long time before the download dialogue box shows up. Let's try
> something
> > > like this (notice the addition of a noop yield):
> >
> > > def create_tarball():
> >
> > > yield '' # noop to send the HTTP headers
> >
> > > > path = create_some_big_tarball()
> >
> > > > chunk = None
> > > > fh = open(path, 'r')
> > > > while True:
> > > > chunk = fh.read(1024 * 128)
> > > > if chunk == '':
> > > > break
> > > > yield chunk
> >
> > > > def sample_view(request):
> > > > response = HttpResponse(create_tarball(),
> > > > mimetype='application/x-compressed')
> > > > response['Content-Disposition'] =
> "attachment;filename=mytarball.tar.gz"
> >
> > > The issue with the above example is that the "yield ''" seems to be
> > > ignored. HTTP headers are not sent before the tarball is created.
> > > Similarly, "yield ' '" and "yield None" don't work, because they
> corrupt the
> > > tarball (HttpResponse calls str() on the iterable items given to the
> > > HttpResponse constructor). As a temporary solution, we're writing an
> empty
> > > gzip file in the first yield. Our large tarball is gzipped, and since
> gzip
> > > files can be concatenated to one and other, our hack seems to be
> working.
> > > In the above example, replace the first "yield ''" with:
> >
> > > noop = StringIO.StringIO()
> >
> > > > empty = gzip.GzipFile(mode='w', fileobj=noop)
> > > > empty.write("")
> > > > empty.close()
> > > > yield noop.getvalue()
> >
> > > I'm wondering if there is a better way to accomplish this? I don't
> quite
> > > understand why HTTP responses are written to stdout. Possibly
> orthogonal to
> > > that, it seems like, in theory, yielding an empty value in the
> generator
> > > should work, because a flush is called after the HTTP headers are
> written.
> > > Any ideas, either on how to solve this problem with the Django API, or
> on
> > > why Django doesn't send HTTP headers on a "yield ''"?
> >
> > From memory, file wrappers at django level, in order to work across
> > different hosting mechanisms supported, only allow a file name to be
> > supplied. At the WSGI level the file wrapper actually takes a file
> > like object. If you were doing this in raw WSGI, you could run your
> > tar ball creation as a separately exec'd pipeline and rather than
> > create a file in the file system, have tar output to the pipeline,
> > ie., use '-' instead of filename. The file object resulting from the
> > pipeline could then be used as input to the WSGI file wrapper object.
> >
> > So, if this operation isn't somehow bound into needing Django itself,
> > and this is important to you, maybe you should create a separate
> > little WSGI application just for this purpose.
> >
> > Actually, even if bound into needing Django you may still be able to
> > do it. Using mod_wsgi, you could even delegate the special WSGI
> > application to run in same process as Django and mount it at a URL
> > which appears within Django application. Because though you are side
> > stepping Django dispatch, you couldn't though have it be protected by
> > Django based form authentication.
> >
> > Graham
>
> Hi,
>
> First, the FileWrapper class in django.core.servers.basehttp.py
> accepts file-like objects, i.e., ones that have a read method. Which
> is what leads me to suggest that your solution may be to write your
> own FileWrapper class, that get the file on the first iteration.
> Here's a modified, untested, version of FileWrapper:
>
> class BigTarFileWrapper(object):
> """Wrapper to convert file-like objects to iterables"""
>
> def __init__(self, tar_args, blksize=8192):
> self.filelike = None
> self.tar_args = tar_args
> self.blksize = blksize
>
> def __getitem__(self,key):
> if not self.filelike:
> self.filelike = get_some_big_tarball(self.tar_args)
> data = self.filelike.read(self.blksize)
> if data:
> return data
> raise IndexError
>
> def __iter__(self):
> return self
>
> def next(self):
> if not self.filelike:
> self.filelike = get_some_big_tarball(self.tar_args)
> data = self.filelike.read(self.blksize)
> if data:
> return data
> raise StopIteration
>
> Then your response becomes something this:
>
> def sample_view(request, args):
> tar_iterator = BigTarFileWrapper(args)
> response = HttpResponse(tar_iterator,
> mimetype='application/x-compressed')
> response['Content-Disposition'] =
> "attachment;filename=mytarball.tar.gz"
> return response
>
> This was inspired by the snippet that you may have seen [1], and my
> experience in needing to return files from an external storage system
> using my own iterator class.
>
> --Rick
>
> [1] http://www.djangosnippets.org/snippets/365/
>
> >
>
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