Django 大文件下载实现过程解析

django提供文件下载时，若果文件较小，解决办法是先将要传送的内容全生成在内存中，然后再一次性传入Response对象中：

def simple_file_download(request):
  # do something...
  content = open("simplefile", "rb").read()

如果文件非常大时，最简单的办法就是使用静态文件服务器，比如Apache或者Nginx服务器来处理下载。不过有时候，我们需要对用户的权限做一下限定，或者不想向用户暴露文件的真实地址，或者这个大内容是临时生成的(比如临时将多个文件合并而成的)，这时就不能使用静态文件服务器了。

django文档中提到，可以向HttpResponse传递一个迭代器，流式的向客户端传递数据。

要自己写迭代器的话，可以用yield：

def read_file(filename, buf_size=8192):
  with open(filename, "rb") as f:
    while True:
      content = f.read(buf_size)
      if content:
        yield content
      else:
        break
def big_file_download(request):
  filename = "filename"
  response = HttpResponse(read_file(filename))
  return response

或者使用生成器表达式，下面是django文档中提供csv大文件下载的例子：

import csv

from django.utils.six.moves import range
from django.http import StreamingHttpResponse

class Echo(object):
  """An object that implements just the write method of the file-like
  interface.
  """
  def write(self, value):
    """Write the value by returning it, instead of storing in a buffer."""
    return value

def some_streaming_csv_view(request):
  """A view that streams a large CSV file."""
  # Generate a sequence of rows. The range is based on the maximum number of
  # rows that can be handled by a single sheet in most spreadsheet
  # applications.
  rows = (["Row {0}".format(idx), str(idx)] for idx in range(65536))
  pseudo_buffer = Echo()
  writer = csv.writer(pseudo_buffer)
  response = StreamingHttpResponse((writer.writerow(row) for row in rows),
                   content_type="text/csv")
  response['Content-Disposition'] = 'attachment; filename="somefilename.csv"'
  return response

python也提供一个文件包装器，将类文件对象包装成一个迭代器：

class FileWrapper:
  """Wrapper to convert file-like objects to iterables"""
  def __init__(self, filelike, blksize=8192):
    self.filelike = filelike
    self.blksize = blksize
    if hasattr(filelike,'close'):
      self.close = filelike.close
  def __getitem__(self,key):
    data = self.filelike.read(self.blksize)
    if data:
      return data
    raise IndexError
  def __iter__(self):
    return self
  def next(self):
    data = self.filelike.read(self.blksize)
    if data:
      return data
    raise StopIteration

使用时：

from django.core.servers.basehttp import FileWrapper
from django.http import HttpResponse
import os
def file_download(request,filename):

  wrapper = FileWrapper(open(filename, 'rb'))
  response = HttpResponse(wrapper, content_type='application/octet-stream')
  response['Content-Length'] = os.path.getsize(path)
  response['Content-Disposition'] = 'attachment; filename=%s' % filename
  return response

django也提供了StreamingHttpResponse类来代替HttpResponse对流数据进行处理。

压缩为zip文件下载：

import os, tempfile, zipfile
from django.http import HttpResponse
from django.core.servers.basehttp import FileWrapper
def send_zipfile(request):
  """
  Create a ZIP file on disk and transmit it in chunks of 8KB,
  without loading the whole file into memory. A similar approach can
  be used for large dynamic PDF files.
  """
  temp = tempfile.TemporaryFile()
  archive = zipfile.ZipFile(temp, 'w', zipfile.ZIP_DEFLATED)
  for index in range(10):
    filename = __file__ # Select your files here.
    archive.write(filename, 'file%d.txt' % index)
  archive.close()
  wrapper = FileWrapper(temp)
  response = HttpResponse(wrapper, content_type='application/zip')
  response['Content-Disposition'] = 'attachment; filename=test.zip'
  response['Content-Length'] = temp.tell()
  temp.seek(0)
  return response

不过不管怎么样，使用django来处理大文件下载都不是一个很好的注意，最好的办法是django做权限判断，然后让静态服务器处理下载。

这需要使用sendfile的机制："传统的Web服务器在处理文件下载的时候，总是先读入文件内容到应用程序内存，然后再把内存当中的内容发送给客户端浏览器。这种方式在应付当今大负载网站会消耗更多的服务器资源。sendfile是现代操作系统支持的一种高性能网络IO方式，操作系统内核的sendfile调用可以将文件内容直接推送到网卡的buffer当中，从而避免了Web服务器读写文件的开销，实现了“零拷贝”模式。 "

Apache服务器里需要mod_xsendfile模块来实现，而Nginx是通过称为X-Accel-Redirect的特性来实现。

nginx配置文件：

# Will serve /var/www/files/myfile.tar.gz
# When passed URI /protected_files/myfile.tar.gz
location /protected_files {
  internal;
  alias /var/www/files;
}

或者

# Will serve /var/www/protected_files/myfile.tar.gz
# When passed URI /protected_files/myfile.tar.gz
location /protected_files {
  internal;
  root /var/www;
}

注意alias和root的区别。

django中：

response['X-Accel-Redirect']='/protected_files/%s'%filename

这样当向django view函数发起request时，django负责对用户权限进行判断或者做些其它事情，然后向nginx转发url为/protected_files/filename的请求，nginx服务器负责文件/var/www/protected_files/filename的下载：

@login_required
def document_view(request, document_id):
  book = Book.objects.get(id=document_id)
  response = HttpResponse()
  name=book.myBook.name.split('/')[-1]
  response['Content_Type']='application/octet-stream'
  response["Content-Disposition"] = "attachment; filename={0}".format(
      name.encode('utf-8'))
  response['Content-Length'] = os.path.getsize(book.myBook.path)
  response['X-Accel-Redirect'] = "/protected/{0}".format(book.myBook.name)
  return response

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