App Engine update blob.md

App Engine update blob

Writing to the blostore using the Files API is deprecated. But we can upload a blob to update the blob

This way we do not need GCS (Google Cloud Storage) To use GCS you have to pay for the application

UPDATE: GCS, using a default bucket is now FREE (1.9.0)

From the docs: An application can use the default GCS bucket, which provides an already configured bucket with free quota. https://developers.google.com/appengine/docs/python/googlecloudstorageclient/

updateblob.py

#!/usr/bin/python
# -*- coding: utf-8 -*-
from __future__ import unicode_literals, with_statement

import webapp2
import mimetypes
from google.appengine.ext.ndb import blobstore                                                      
from google.appengine.ext.webapp import blobstore_handlers
from google.appengine.api import urlfetch
from google.appengine.runtime import DeadlineExceededError
import logging
    
CRLF = b'\r\n'
BOUNDERY = b'--Th15-Is-ThE-BoUnDeRy--'
max_buffer_size = 1048576


def create_upload_url(for_url):

    return blobstore.create_upload_url(for_url, max_bytes_per_blob=max_buffer_size)


def _fetch_blob_update(payload):

    upload_url = create_upload_url(webapp2.uri_for('blob_update', _full=True))
    try:
        response = urlfetch.fetch(
            url=upload_url,
            payload=payload,
            method=urlfetch.POST,
            deadline=40,
            follow_redirects=False,
            headers={b'Content-Type': str('multipart/form-data; boundary=%s' % BOUNDERY)}
        )
        if response.status_code != 200:
            logging.error('URL : %s fetch ERROR : %d' % (upload_url, response.status_code))
            return None
        else:
            return response.content
    except (urlfetch.DownloadError, DeadlineExceededError), e:
        logging.error('fetch %s download or deadline error, exception : %s'
                      % (upload_url, str(e)))
        return None


def _encode_multipart(_fields=None, _files=None):
    """ Generator inserts fields and files to create a multipart payload """

    for (field_name, field_value) in _fields or []:
        yield b'--' + BOUNDERY + CRLF
        yield b'Content-Disposition: form-data; name="%s"' % str(field_name) + CRLF
        yield CRLF
        yield str(field_value) + CRLF

    for (file_name, file_data) in _files or []:
        yield b'--' + BOUNDERY + CRLF
        yield b'Content-Disposition: form-data; name="file"; filename="%s"' \
              % str(file_name) + CRLF
        yield b'Content-Type: %s' % mimetypes.guess_type(file_name)[0] + CRLF
        yield CRLF
        yield file_data + CRLF

    yield b'--%s--' % BOUNDERY + CRLF


def blob_update_by_upload(ndb_key, blob):
    """ update blob using blobstore upload """

    result = _fetch_blob_update(b''.join(_encode_multipart(_files=[(ndb_key.id(), blob)])))
    return blobstore.BlobKey(result)


class BlobUpdate(blobstore_handlers.BlobstoreUploadHandler):
    """ has named route : blob_update : 
        webapp2.Route(r'/blob_update', handler='....BlobUpdate', name='blob_update') """

    def post(self):
        """ blob upload handler returns the new blobkey"""

        blob_info = self.get_uploads('file')[0]
        logging.info('BlobUpdate : ' + blob_info.filename)
        self.response.headers[b'Content-Type'] = b'text/plain'
        self.response.write(str(blob_info.key()))