Question

I am trying to save large files from Google App Engine’s Blobstore to Google Cloud Storage to facilitate backup.

It works fine for small files (<10 mb) but for larger files it get gets unstable and GAE throws and FileNotOpenedError.

My code:

PATH = '/gs/backupbucket/'
for df in DocumentFile.all():           
  fn = df.blob.filename
  br = blobstore.BlobReader(df.blob)
  write_path = files.gs.create(self.PATH+fn.encode('utf-8'), mime_type='application/zip',acl='project-private') 
  with files.open(write_path, 'a') as fp:
    while True:
      buf = br.read(100000)
      if buf=="": break
      fp.write(buf)
  files.finalize(write_path)

(Runs in a taskeque to avoid exceeding execution time).

Throws a FileNotOpenedError:

Traceback (most recent call last):
  File "/base/python27_runtime/python27_lib/versions/third_party/webapp2-2.3/webapp2.py", line 1511, in __call__
    rv = self.handle_exception(request, response, e)
  File "/base/python27_runtime/python27_lib/versions/third_party/webapp2-2.3/webapp2.py", line 1505, in __call__
    rv = self.router.dispatch(request, response)
  File "/base/python27_runtime/python27_lib/versions/third_party/webapp2-2.3/webapp2.py", line 1253, in default_dispatcher
    return route.handler_adapter(request, response)
  File "/base/python27_runtime/python27_lib/versions/third_party/webapp2-2.3/webapp2.py", line 1077, in __call__
    return handler.dispatch()
  File "/base/python27_runtime/python27_lib/versions/third_party/webapp2-2.3/webapp2.py", line 547, in dispatch
    return self.handle_exception(e, self.app.debug)
  File "/base/python27_runtime/python27_lib/versions/third_party/webapp2-2.3/webapp2.py", line 545, in dispatch
    return method(*args, **kwargs)
  File "/base/data/home/apps/s~simplerepository/1.354754771592783168/processFiles.py", line 249, in post
    fp.write(buf)
  File "/base/python27_runtime/python27_lib/versions/1/google/appengine/api/files/file.py", line 281, in __exit__
    self.close()
  File "/base/python27_runtime/python27_lib/versions/1/google/appengine/api/files/file.py", line 275, in close
    self._make_rpc_call_with_retry('Close', request, response)
  File "/base/python27_runtime/python27_lib/versions/1/google/appengine/api/files/file.py", line 388, in _make_rpc_call_with_retry
    _make_call(method, request, response)
  File "/base/python27_runtime/python27_lib/versions/1/google/appengine/api/files/file.py", line 236, in _make_call
    _raise_app_error(e)
  File "/base/python27_runtime/python27_lib/versions/1/google/appengine/api/files/file.py", line 179, in _raise_app_error
    raise FileNotOpenedError()

I have investigated further and according to a comment to GAE Issue 5371 the Files API closes the file every 30 seconds. I have not seen this documented anywhere else.

I have tried to work around this by closing and opening the file at intervals but now I get an WrongOpenModeError. The code below is edited from the first version of this post I have added a 0.5 second pause between the close and the open of the file. It now throws a WrongOpenModeError.

My code (updated):

PATH = '/gs/backupbucket/'
for df in DocumentFile.all():           
  fn = df.blob.filename
  br = blobstore.BlobReader(df.blob)
  write_path = files.gs.create(self.PATH+fn.encode('utf-8'), mime_type='application/zip',acl='project-private') 
  fp = files.open(write_path, 'a')
  c = 0
  while True:       
    if (c == 5):
      c = 0
      fp.close()
      files.finalize(write_path)
      time.sleep(0.5)
      fp = files.open(write_path, 'a')
    c = c + 1
    buf = br.read(100000)
    if buf=="": break
    fp.write(buf)
  files.finalize(write_path)

Stacktrace:

Traceback (most recent call last):
  File "/base/python27_runtime/python27_lib/versions/third_party/webapp2-2.3/webapp2.py", line 1511, in __call__
    rv = self.handle_exception(request, response, e)
  File "/base/python27_runtime/python27_lib/versions/third_party/webapp2-2.3/webapp2.py", line 1505, in __call__
    rv = self.router.dispatch(request, response)
  File "/base/python27_runtime/python27_lib/versions/third_party/webapp2-2.3/webapp2.py", line 1253, in default_dispatcher
    return route.handler_adapter(request, response)
  File "/base/python27_runtime/python27_lib/versions/third_party/webapp2-2.3/webapp2.py", line 1077, in __call__
    return handler.dispatch()
  File "/base/python27_runtime/python27_lib/versions/third_party/webapp2-2.3/webapp2.py", line 547, in dispatch
    return self.handle_exception(e, self.app.debug)
  File "/base/python27_runtime/python27_lib/versions/third_party/webapp2-2.3/webapp2.py", line 545, in dispatch
    return method(*args, **kwargs)
  File "/base/data/home/apps/s~simplerepository/1.354894420907462278/processFiles.py", line 267, in get
    fp.write(buf)
  File "/base/python27_runtime/python27_lib/versions/1/google/appengine/api/files/file.py", line 310, in write
    self._make_rpc_call_with_retry('Append', request, response)
  File "/base/python27_runtime/python27_lib/versions/1/google/appengine/api/files/file.py", line 388, in _make_rpc_call_with_retry
    _make_call(method, request, response)
  File "/base/python27_runtime/python27_lib/versions/1/google/appengine/api/files/file.py", line 236, in _make_call
    _raise_app_error(e)
  File "/base/python27_runtime/python27_lib/versions/1/google/appengine/api/files/file.py", line 188, in _raise_app_error
    raise WrongOpenModeError()

I have tried to find information about the WrongOpenModeError but the only place it is mentioned is in the appengine.api.files.file.py itself.

Suggestions on how to get around this and be able to save also large files to Google Cloud storage would be greatly appreciated. Thanks!

Answer 1

I was having the same issue, endup writing an iterator around fetch data and catch the exception, works but is a work-around.

Re-writing your code would be something like:

from google.appengine.ext import blobstore
from google.appengine.api import files

def iter_blobstore(blob, fetch_size=524288):
  start_index = 0
  end_index = fetch_size

  while True:
    read = blobstore.fetch_data(blob, start_index, end_index)

    if read == "":
      break

    start_index += fetch_size
    end_index += fetch_size

    yield read


PATH = '/gs/backupbucket/'
for df in DocumentFile.all():           
  fn = df.blob.filename
  br = blobstore.BlobReader(df.blob)
  write_path = files.gs.create(self.PATH+fn.encode('utf-8'), mime_type='application/zip',acl='project-private') 
  with files.open(write_path, 'a') as fp:
    for buf in iter_blobstore(df.blob):
      try:
        fp.write(buf)
      except files.FileNotOpenedError:
        pass
  files.finalize(write_path)