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# Google Cloud Storage Python API Overview

#### Experimental!

Google Cloud Storage API for Google App Engine is an experimental, innovative, and rapidly changing new feature for App Engine. Unfortunately, being on the bleeding edge means that we may make backwards-incompatible changes to Google Cloud Storage API for Google App Engine. We will inform the community when this feature is no longer experimental.

The Google Cloud Storage API allows your application to store and serve your data as opaque blobs known as "objects". While Cloud Storage offers a RESTful API, this documentation describes how to use Cloud Storage on App Engine.

- Introducing Google Cloud Storage
- Prerequisites
- Using Google Cloud Storage
- Complete Sample App
- Quotas and Limits

## Introducing Google Cloud Storage

Google App Engine provides an easier way to read from and write to Google Cloud Storage objects , which allows applications to create and serve data objects. These objects are stored within buckets in Cloud Storage but can be additionally accessed by Google App Engine applications through the Google Cloud Storage API. You can interact with the Google Cloud Storage API using the RESTful interface or through the Google Cloud Storage Python API for Google App Engine applications, which is discussed in this document.

Cloud Storage is useful for storing and serving large files because it does not have a size limit for your objects. Additionally, Cloud Storage offers the use of access control lists (ACLs), the ability to resume upload operations if they're interrupted, use of the `POST` policy document, and many other features.

Once you have created an object, it cannot be modified. To modify an existing object, you need to overwrite the object with a new object that contains your desired changes.

For more information, see the Google Cloud Storage pricing.

## Prerequisites

To use the Cloud Storage API, you must complete the following prerequisites:

1. Download and install the App Engine 1.5.5 SDK.

   Google Cloud Storage API for Python is available for App Engine SDKs 1.5.5 and later. For instructions to update or download the App Engine SDK, see the downloads page.

2. Activate Cloud Storage.

   Activate the Cloud Storage service in the Google APIs Console.

3. Set up billing.

   Cloud Storage offers a free trial quota term until December 31, 2011, but requires that you provide billing information. You are not charged unless you exceed the free trial quota.

4. Create a bucket.

   Cloud Storage is made up of buckets, basic containers that hold your data. Buckets hold all your data in the form of objects, individual pieces of data that you upload to Google Cloud Storage. Before you can use the Google Cloud Storage API, you need to create a bucket(s) where you would like to store your data. The easiest way to do so is using the GSUtil tool or the online Google Storage browser that is accessible through the Google APIs Console.

5. Give permissions to your bucket or objects.

   The samples in this document describe how to create new objects in Cloud Storage. To be able to create new objects, you need to give `WRITE` permission for your App Engine application to your Cloud Storage bucket using the following steps:

   1. Log into the App Engine Admin Console.
   2. Click on the application you want to authorize for your Cloud Storage bucket.
   3. Click on Application Settings under the Administration section on the left-hand side.
6. Copy the value under Service Account Name. This is the service account name of your application, in the format application-id@appspot.gserviceaccount.com. If you are using an App Engine Premier Account, the service account name for your application is in the format application-id.example.com@appspot.gserviceaccount.com.
7. Add the service account as a project editor to the Google APIs Console project that the bucket belongs to. For information about permissions in Cloud Storage, see Scopes and Permissions on the Cloud Storage documentation.

`READ-ONLY` Access for Objects

Alternatively, if you want your Google App Engine application to only be able to read certain objects, you can:

- Set the predefined project-private ACL on your objects manually
- Store your objects in a bucket with the default project-private object ACL

Then, add the service account as a project viewer in the Google APIs Console, giving your App Engine application `READ-ONLY` access to your objects. You can also set the predefined public-read ACL, which allows all users, whether or not they are authenticated, access to read your object. You won't need to add your application's service account name as a project viewer, but if you have data you do not want to be publicly accessible, do not set the `public-read` ACL on your object.

Note that setting `READ` permission only on your buckets does not provide `READ` permission to any objects inside those buckets. You must set `READ` permission on objects individually.

Tip: There are various ways to set the permissions for your App Engine application. The steps described here are the most straightforward, but you can also use Cloud Storage's ACLs to exercise more explicit or fine-grained control.

## Using the Cloud Storage API

Applications can use Cloud Storage to read from and write to large files of any type, or to upload, store, and serve large files from users. Files are called "objects" in Google Cloud Storage once they're uploaded.

### Before You Begin

You can use the Cloud Storage API from your App Engine application to read and write data to Cloud Storage. In order to use the Cloud Storage Python API, you must include the following `import` statements at the beginning of your code:

    from __future__ import with_statement
    from google.appengine.api import files

### Creating an Object

To create an object, call the files.gs.create() function. You need to pass a file name, which must be of the form `/gs/bucket_name/desired_object_name`, and a `MIME` type:

    # Create a file
    filename = '/gs/my_bucket/my_file'
    writable_file_name = files.gs.create(filename, mime_type='application/octet-stream', acl='public-read')

In this example, we also pass in the `acl='public-read'` parameter, as described below. If you do not set this parameter, Cloud Storage sets this parameter as null and uses the default object ACL for that bucket (by default, this is `project-private`).

You can use the following optional parameters to set ACLs and HTTP headers on your object:

Parameter | Description | Usage | Default
--- | --- | --- | ---
`acl` | Set a predefined ACL on your object. | `acl='public_read'` If you do not set this parameter, Cloud Storage sets this parameter as null and uses the default object ACL for that bucket (by default, this is `project-private`). | None
`cache_control` | Set a Cache-Control header on your object. | `cache_control='no-cache'` | `'max-age=3600'`
`content_encoding` | If your object is compressed, specify the compression method using the Content-Encoding header. | `content_encoding='gzip'` | None
`content_disposition` | Set the Content-Disposition header for your object. | `content_disposition='attachment;filename=filename.ext'` | None

`user_metadata`

Set a Dictionary data set of custom headers and values.

`params = {'header1':'value1', 'header2':'value2'}`
`gs.create(... user_metadata=params)`

These custom headers are added to the object in the format:

    x-goog-meta-custom_header: custom_value

None

The following code snippet provides an example using some of these optional parameters:

    params = {'date-created':'092011', 'owner':'Jon'}
    files.gs.create(filename='/gs/my_bucket/my_object',
              acl='public-read',
              mime_type='text/html',
              cache_control='no-cache',
              user_metadata=params)

### Opening and Writing to the Object

Before you can write to the object, you must open the object for writing by calling files.open(), passing in the file and `'a'` as parameters. Then, write to the object using files.write():

    # Open and write the file.
    with files.open(writable_file_name, 'a') as f:
      f.write('Hello World!')
      f.write('This is my first Google Cloud Storage object!')
      f.write('How exciting!')

The parameter `'a'` opens the file for writing. If you want to open a file for read, pass in the parameter `'r'`, as described below.

### Finalizing the Object

Once you are done writing to the object, close it by calling files.finalize():

    # Finalize the file.
    files.finalize(writable_file_name)

Once you finalize an object, you can no longer write to it. If you want to modify your file, you will need to create a new object with the same name to overwrite it.

### Reading the Object

Before you can read an object, you must finalize it.

To read an object, call `files.open()`, passing in the full file name in the format '/gs/bucket/object', and `'r'` as parameters. Passing `'r'` opens the file for reads, as opposed to passing `'a'`, which opens the file for write. f.read() takes the number of bytes to read as a parameter:

    # Open and read the file.
    print 'Opening file', filename
    with files.open(filename, 'r') as f:
      data = f.read(1000)
      while data != "":
        print data
        data = f.read(1000)

The maximum size of bytes that can be read by an application with one API call is 32 megabytes.

## Complete Sample App

The following is a sample application that demonstrates one use of the Cloud Storage API within an App Engine application. In order to successfully run the application, you must be an owner or have `WRITE` access to a bucket.

The sample application demonstrates two different use cases of the Cloud Storage API:

- Making one-time requests

  The EchoPage function allows you to make a one-time request to create, write, and read to an object using the request format http://myapp.appspot.com/echo?message=Your+Message.

- Making multiple requests over time

  The MainPage, CreatePage, AppendPage, and ReadPage functions create and maintain an event log that stores individual events as objects. This demonstrates how to use the Cloud Storage API to make multiple requests to an object.

  The main page of this application loads a form field with the name Title for the title of the event (and the object). After you provide a name and click the New Event Log button, the next page provides another form field to enter a message to write to the object. Enter your message and click Append Field. If you want to finalize the object, which means you cannot append to it later, check the Finalize log box. Your new object will appear on the main page.

To set up and run this application:

1. Follow the instructions to create a simple HelloWorld app on the first Python HelloWorld page.
2. Edit your `app.yaml` file and replace the value of the `application:` setting with the registered ID of your App Engine application.
3. Replace the contents of `helloworld.py` with the following sample code, replacing the parts where it asks for a bucket name.

       # Copyright 2011 Google Inc. All Rights Reserved.
       """Create, Write, Read and Finalize Google Cloud Storage objects.

       EchoPage will create, write and read the Cloud Storage object in one request:
         http://your_app_id.appspot.com/echo?message=Leeroy+Jenkins

       MainPage, CreatePage, AppendPage and ReadPage will do the same in multiple
       requests:
         http://your_app_id.appspot.com/
       """

       from __future__ import with_statement

       import cgi
       import urllib

       from google.appengine.api import files
       from google.appengine.ext import db
       from google.appengine.ext import webapp
       from google.appengine.ext.webapp.util import run_wsgi_app

       try:
         files.gs
       except AttributeError:
         import gs
         files.gs = gs

       class EchoPage(webapp.RequestHandler):
         """A simple echo page that writes and reads the message parameter."""

         # TODO: Change to a bucket your app can write to.
         READ_PATH = '/gs/bucket/obj'

         def get(self):
           # Create a file that writes to Cloud Storage and is readable by everyone in the
           # project.
           write_path = files.gs.create(self.READ_PATH, mime_type='text/plain',
                                                                   acl='public-read')
           # Write to the file.
           with files.open(write_path, 'a') as fp:
             fp.write(self.request.get('message'))

           # Finalize the file so it is readable in Google Cloud Storage.
           files.finalize(write_path)

           # Read the file from Cloud Storage.
           with files.open(self.READ_PATH, 'r') as fp:
             buf = fp.read(1000000)
             while buf:
               self.response.out.write(buf)
               buf = fp.read(1000000)

       class EventLog(db.Model):
         """Stores information used between requests."""
         title = db.StringProperty(required=True)
         read_path = db.StringProperty(required=True)
         write_path = db.TextProperty(required=True)  # Too long for StringProperty
         finalized = db.BooleanProperty(default=False)

       class MainPage(webapp.RequestHandler):
         """Prints a list of event logs and a link to create a new one."""

         def get(self):
           """Page to list event logs or create a new one.

           Web page looks like the following:
             Event Logs
               * Dog barking
               * Annoying Squirrels (ongoing)
               * Buried Bones
             [New Event Log]
           """
           self.response.out.write(
               """
               <html> <body>
               <h1>Event Logs</h1>
               <ul>
               """)
           # List all the event logs in the datastore.
           for event_log in db.Query(EventLog):
             # Each EventLog has a unique key.
             key_id = event_log.key().id()
             if event_log.finalized:
               # Finalized events must be read
               url = '/read/%d' % key_id
               title = '%s' % cgi.escape(event_log.title)
             else:
               # Can only append to writable events.
               url = '/append/%d' % key_id
               title = '%s (ongoing)' % cgi.escape(event_log.title)
             self.response.out.write(
                 """
                 <li><a href="%s">%s</a></li>
                 """ % (url, title))
           # A form to allow the user to create a new Cloud Storage object.
           self.response.out.write(
               """
               </ul>
               <form action="create" method="post">
                 Title: <input type="text" name="title" />
                 <input type="submit" value="New Event Log" />
               </form>
               </body> </html>
               """)

       class CreatePage(webapp.RequestHandler):
         """Creates a Cloud Storage object that multiple requests can write to."""

         BUCKET = 'my-bucket'  # TODO: Change to a bucket your app can write to.

         def post(self):
           """Create a event log that multiple requests can build.

           This creates an appendable Cloud Storage object and redirects the user to
           the append page.
           """
           # Choose an interesting title for the event log.
           title = self.request.get('title') or 'Unnamed'

           # We will store the event log in a Google Cloud Storage object.
           # The Google Cloud Storage object must be in a bucket the app has access to, and
           # use the title for the key.
           read_path = '/gs/%s/%s' % (self.BUCKET, title)
           # Create a writable file that eventually become our Google Cloud Storage object
           # after we finalize it.
           write_path = files.gs.create(read_path, mime_type='text/plain')
           # Save these paths as well as the title in the datastore so we can find this
           # during later requests.
           event_log = EventLog(
               read_path=read_path, write_path=write_path, title=title)
           event_log.put()
           # Redirect the user to the append page, where they can start creating the
           # file.
           self.redirect('/append/%d?info=%s' % (
               event_log.key().id(), urllib.quote('Created %s' % title)))

       class AppendPage(webapp.RequestHandler):
         """Appends data to a Cloud Storage object between multiple requests."""

         @property
         def key_id(self):
           """Extract 123 from /append/123."""
           return int(self.request.path[len('/append/'):])

         def get(self):
           """Display a form the user can use to build the event log.

           Web page looks like:
             Append to Event Title

             /--------------\
             |Log detail... |
             |              |
             |              |
             \--------------/

             [ ] Finalize log
             [ Append message ]
           """

           # Grab the title, which we saved to an EventLog object in the datastore.
           event_log = db.get(db.Key.from_path('EventLog', self.key_id))
           title = event_log.title
           # Display a form that allows the user to append a message to the log.
           self.response.out.write(
               """
               <html> <body>
               <h1>Append to %s</h1>
               <div>%s</div>
               <form method="post">
               <div><textarea name="message" rows="5" cols="80"></textarea></div>
               <input type="checkbox" name="finalize" value="1">Finalize log</input>
               <input type="submit" value="Append message" />
               </form>
               </body> </html>
               """ % (
                   cgi.escape(title),
                   cgi.escape(self.request.get('info')),
                   ))

         def post(self):
           """Append the message to the event log.

           Find the writable Cloud Storage path from the specified EventLog.
           Append the form's message to this path.
           Optionally finalize the object if the user selected the checkbox.
           Redirect the user to a page to append more or read the finalized object.
           """
           # Use the id in the post path to find the EventLog object we saved in the
           # datastore.
           event_log = db.get(db.Key.from_path('EventLog', self.key_id))
           # Get writable Google Cloud Storage path, which we saved to an EventLog object in
           # the datastore.
           write_path = event_log.write_path
           # Get the posted message from the form.
           message = self.request.get('message')
           # Append the message to the Google Cloud Storage object.
           with files.open(event_log.write_path, 'a') as fp:
             fp.write(message)
           # Check to see if the user is finished writing.
           if self.request.get('finalize'):
             # Finished writing.  Finalize the object so it becomes readable.
             files.finalize(write_path)
             # Tell the datastore that we finalized this object. This makes the main
             # page display a link that reads the object.
             event_log.finalized = True
             event_log.put()
             self.redirect('/read/%d' % self.key_id)
           else:
             # User is not finished writing.  Redirect to the append form.
             self.redirect('/append/%d?info=%s' % (
                 self.key_id, urllib.quote('Appended %d bytes' % len(message))))

       class ReadPage(webapp.RequestHandler):
         """Reads a Cloud Storage object and prints it to the page."""

         @property
         def key_id(self):
           """Extract 123 from /read/123."""
           return int(self.request.path[len('/read/'):])

         def get(self):
           """Display the EventLog to the user.

           Web page looks like the following:
             Event Log Title

             Event log description.
             [ Download from Cloud Storage ]
           """
           self.response.out.write(
               """
               <html> <body>
               """)
           # Use the get request path to find the event log in the datastore.
           event_log = db.get(db.Key.from_path('EventLog', self.key_id))
           read_path = event_log.read_path
           title = event_log.title
           # Print the title
           self.response.out.write(
               """
               <h1>%s</h1>
               """ % cgi.escape(title))
           # Read the object from Cloud Storage and write it out to the web page.
           self.response.out.write(
               """
               <pre>
               """)
           with files.open(read_path, 'r') as fp:
             buf = fp.read(1000000)
             while buf:
               self.response.out.write(cgi.escape(buf))
               buf = fp.read(1000000)
           self.response.out.write(
               """
               </pre>
               """)

           self.response.out.write(
               """
               <div><a href="/">Event Logs</a></div>
               </body> </html>
               """)

       application = webapp.WSGIApplication(
           [
               ('/create', CreatePage),
               ('/append/.*', AppendPage),
               ('/read/.*', ReadPage),
               ('/echo', EchoPage),
               ('/.*', MainPage),
           ], debug=True)

       def main():
         run_wsgi_app(application)

       if __name__ == '__main__':
         main()

4. Upload your application using the following command:

       appcfg.py update helloworld/

5. View your application at `http://application-id.appspot.com`

## Quotas and Limits

Cloud Storage is a pay-to-use service that offers a generous free trial quota. If you exceed the free trial quota, you will be charged according to the Cloud Storage price sheet.