Pronoid Android

Over the summer, Google App Engine raised its limits for web requests and responses from 1MB to 10MB, but kept the maximum size of any single database element at 1MB. If you try to exceed this, you'll get a MemoryError. You can find a fair amount of grief and woe and gnashing of teeth and wearing of sackcloth and ashes about this online.

Which is kind of surprising, because it's not that hard to break files up into chunks and store those chunks in the database separately. Here's what I did today for my current project, which stores data - including photos - uploaded from smartphones:

First, we have to receive the uploaded image. Our uploads are two-phase - first data, then a photo - for various reasons. The data upload includes the image's file name; the photo upload is a basic form/multipart POST with exactly one argument (the filename) and its value (the file).

So, in "main.py":

class SaveImage(webapp.RequestHandler):
  def post(self):
    entryHandler=ec.EntryHandler()
    for arg in self.request.arguments():
      file = self.request.get(arg)
      response = entryHandler.saveImage(arg,file)
      self.response.out.write(response)

and in "ec.py":

class ImageChunk(db.Model):
  entryRef = db.ReferenceProperty(Entry)
  chunkIndex = db.IntegerProperty()
  chunk = db.BlobProperty()

class EntryHandler:
  def saveImage(self, fileName, file):
      results = Entry.all().filter("photoPath =", fileName).fetch(1)
      if len(results)==0:
        logging.warning("Error - could not find the entry associated with image name "+fileName)
        return "Failed"
      else:
        MaxBTSize=1000000
        entry = results[0]
        marker=0
        chunks=[]
        while marker*MaxBTSize<len(file):
          if MaxBTSize*(marker+1)>len(file):
            chunk = ImageChunk(entryRef=entry, chunkIndex=marker, chunk=db.Blob(file[MaxBTSize*marker:]))
          else:
            chunk = ImageChunk(entryRef=entry, chunkIndex=marker, chunk=db.Blob(file[MaxBTSize*marker:MaxBTSize*(marker+1)]))
          chunk.put()
          marker+=1
        logging.info("Successfully received image "+fileName)
        return "Successfully received image "+fileName

Pretty basic stuff: we chop the image up at each 1,000,000-byte mark, and put each chunk into its own ImageChunk DB object.

Then, when we need to retrieve the image, in 'main.py':

class ShowImageWithKey(webapp.RequestHandler):
  def get(self):
    key = self.request.get('entryKey')
    entryHandler = ec.EntryHandler()
    image = entryHandler.getImageByEntryKey(key)
    if image is not None:
      self.response.headers['Content-Type'] = 'image/jpeg'
      self.response.out.write(image)

and in 'ec.py':

  def getImageByEntryKey(self, key):
    chunks = db.GqlQuery("SELECT * FROM ImageChunk WHERE entryRef = :1 ORDER BY chunkIndex", key).fetch(100)
    if len(chunks)==0:
      return None

    image=""
    for chunkRow in chunks:
      image+=chunkRow.chunk
    return image

Since db.Blob is a subtype of str, that's all you have to do. I don't understand why some people are so upset about this: it's mildly annoying that I had to write the above, but hardly crippling. At least with JPEGs, which is what we use. (But I don't see why any other file type would be more difficult; they're ultimately all just a bunch of bytes). Could hardly be easier ... well, until App Engine rolls out their large file service.

(eta, Dec 14: which came out today! Meaning you can now disregard all the above and just use the new Blobstore instead.)

(eta, Dec 16: mmm, maybe not. Looked at the Blobstore in detail today, and it's really best suited for browser projects, not app or web-service stuff. The API for the blobs is very limited, and you can only access them via one-time-only URLs that App Engine puts in your HTML. You could scrape that, granted, but that's a pain in the ass, no less inelegant than the image-chunking solution above. It's experimental and subject to change, too. I think I'll hold out until its API improves.)

Labels: AppEngine, BigTable, chunking, chunks, Images, JPEG, JPG, limit, MemoryError, python, size