Google App Engine

Google Protocol RPC Library Overview

Note: If you wish to use authentication with Google Protocol RPC, you can use the authentication currently available for App Engine apps, as described in the article on authentication configuration setting in the Admin Console. You'll also need to specify the use login requirement in your app.yaml file. Other auth methodologies are not currently supported by the Google Protocol RPC library within App Engine.

The Google Protocol RPC library is a framework for implementing HTTP-based remote procedure call (RPC) services. An RPC service is a collection of message types and remote methods that provide a structured way for external applications to interact with web applications. Because you can define messages and services in the Python programming language, it's easy to develop Protocol RPC services, test those services, and scale them on App Engine.

While you can use the Google Protocol RPC library for any kind of HTTP-based RPC service, some common use cases include:

  • Publishing web APIs for use by third parties
  • Creating structured Ajax backends
  • Cloning to long-running server communication

You can define a Google Protocol RPC service in a single Python class that contains any number of declared remote methods. Each remote method accepts a specific set of parameters as a request and returns a specific response. These request and response parameters are user-defined classes known as messages.

  1. The Hello World of Google Protocol RPC
  2. Getting Started with Google Protocol RPC

The Hello World of Google Protocol RPC

This section presents an example of a very simple service definition that receives a message from a remote client. The message contains a user's name (HelloRequest.my_name) and sends back a greeting for that person (HelloResponse.hello):

from protorpc import messages
from protorpc import remote
from protorpc.wsgi import service

package = 'hello'

# Create the request string containing the user's name
class HelloRequest(messages.Message):
    my_name = messages.StringField(1, required=True)

# Create the response string
class HelloResponse(messages.Message):
    hello = messages.StringField(1, required=True)

# Create the RPC service to exchange messages
class HelloService(remote.Service):

    @remote.method(HelloRequest, HelloResponse)
    def hello(self, request):
        return HelloResponse(hello='Hello there, %s!' % request.my_name)

# Map the RPC service and path (/hello)
app = service.service_mappings([('/hello.*', HelloService)])

Getting Started with Google Protocol RPC

This section demonstrates how to get started with Google Protocol RPC using the guestbook application developed in the App Engine Getting Started Guide (Python). Users can visit the guestbook (also included as a demo in the Python SDK) online, write entries, and view entries from all users. Users interact with the interface directly, but there is no way for web applications to easily access that information.

That's where Protocol RPC comes in. In this tutorial, we'll apply Google Protocol RPC to this basic guestbook, enabling other web applications to access the guestbook's data. This tutorial only covers using Google Protocol RPC to extend the guestbook functionality; it's up to you what to do next. For example, you might want to write a tool that reads the messages posted by users and makes a time-series graph of posts per day. How you use Protocol RPC depends on your specific app; the important point is that Google Protocol RPC greatly expands what you can do with your application's data.

To begin, you'll create a file, postservice.py, which implements remote methods to access data in the guestbook application's datastore.

Creating the PostService Module

The first step to get started with Google Protocol RPC is to create a file called postservice.py in your application directory. You'll use this file to define the new service, which implements two methods—one that remotely posts data and another that remotely gets data.

You don't need to add anything to this file now—but this is the file where you'll put all the code defined in the subsequent sections. In the next section, you'll create a message that represents a note posted to the guestbook application's datastore.

Working with Messages

Messages are the fundamental data type used in Google Protocol RPC. Messages are defined by declaring a class that inherits from the Message base class. Then you specify class attributes that correspond to each of the message's fields.

For example, the guestbook service allows users to post a note. Let's define a message that represents such a note:

from protorpc import messages

class Note(messages.Message):

    text = messages.StringField(1, required=True)
    when = messages.IntegerField(2)

The note message is defined by two fields, text and when. Each field has a specific type. The text field is a unicode string representing the content of a user's post to the guestbook page. The when field is an integer representing the post's timestamp. In defining the string, we also:

  • Give each field a unique numerical value (1 for text and 2 for when) that the underlying network protocol uses to identify the field.
  • Make text a required field. Fields are optional by default, you can mark them as required by setting required=True. Messages must be initialized by setting required fields to a value. Google Protocol RPC service methods accept only properly initialized messages.

You can set values for the fields using the constructor of the Note class:

# Import the standard time Python library to handle the timestamp.
import time

note_instance = Note(text=u'Hello guestbook!', when=int(time.time()))

You can also read and set values on a message like normal Python attribute values. For example, to change the message:

print note_instance.text
note_instance.text = u'Good-bye guestbook!'
print note_instance.text
which outputs the following
Hello guestbook!
Good-bye guestbook!

Defining a Service

A service is a class definition that inherits from the Service base-class. Remote methods of a service are indicated by using the remote decorator. Every method of a service accepts a single message as its parameter and returns a single message as its response.

Let's define the first method of the PostService. If you haven't done so already, create a file called postservice.py in your application directory and review the guestbook tutorial if you need to. In that tutorial, guestbook greetings are put in the datastore using the guestbook.Greeting class. The PostService will also use the Greeting class to store a post in the datastore. Add the following to your newly created postservice.py file:

import datetime

from protorpc import message_types
from protorpc import remote

import guestbook

class PostService(remote.Service):

    # Add the remote decorator to indicate the service methods
    @remote.method(Note, message_types.VoidMessage)
    def post_note(self, request):

        # If the Note instance has a timestamp, use that timestamp
        if request.when is not None:
            when = datetime.datetime.utcfromtimestamp(request.when)

        # Else use the current time
        else:
            when = datetime.datetime.now()
        note = guestbook.Greeting(content=request.text, date=when)
        note.put()
        return message_types.VoidMessage()

The remote decorator takes two parameters:

  • The expected request type. The post_note() method accepts a Note instance as its request type.
  • The expected response type. The Google Protocol RPC library comes with a built-in type called a VoidMessage (in the protorpc.message_types module), which is defined as a message with no fields. This means that the post_note() message does not return anything useful to its caller. If it returns without error, the message is considered to have been posted.

Since Note.when is an optional field, it may not have been set by the caller. When this happens, the value of when is set to None. When Note.when is set to None, post_note() creates the timestamp using the time it received the message.

The response message is instantiated by the remote method and becomes the remote method's return value.

Registering the Service

You can publish your new service as a WSGI application using the protorpc.wsgi.service library. Create a new file called services.py in your application directory and add the following code to create your service:

from protorpc.wsgi import service

import postservice

# Map the RPC service and path (/PostService)
app = service.service_mappings([('/PostService', postservice.PostService)])

Now, add the following handler to your app.yaml file:

- url: /PostService.*
  script: services.app

Testing the Service from the Command Line

Now that you've created the service, you can test it using curl or a similar command-line tool.

# After starting the development web server:
# NOTE: ProtoRPC always expect a POST.
% curl -H \
   'content-type:application/json' \
   -d '{"text": "Hello guestbook!"}'\
   http://localhost:8080/PostService.post_note

An empty JSON response indicates that the note posted successfully. You can see the note by going to your guestbook application in your browser (http://localhost:8080/).

Adding Message Fields

Now that we can post messages to the PostService, let's add a new method to get messages from the PostService. First, we'll define a request message in postservice.py that defines some defaults and a new enum field that tells the server how to order notes in the response. Define it above the PostService class that you defined earlier:

class GetNotesRequest(messages.Message):
    limit = messages.IntegerField(1, default=10)
    on_or_before = messages.IntegerField(2)
  
    class Order(messages.Enum):
        WHEN = 1
        TEXT = 2
    order = messages.EnumField(Order, 3, default=Order.WHEN)

When sent to the PostService, this message requests a number of notes on or before a certain date and in a particular order. The limit field indicates the maximum number of notes to fetch. If not explicitly set, limit defaults to 10 notes (as indicated by the default=10 keyword argument).

The order field introduces the EnumField class, which enables the enum field type when the value of a field is restricted to a limited number of known symbolic values. In this case, the enum indicates to the server how to order notes in the response. To define the enum values, create a sub-class of the Enum class. Each name must be assigned a unique number for the type. Each number is converted to an instance of the enum type and can be accessed from the class.

print 'Enum value Order.%s has number %d' % (GetNotesRequest.Order.WHEN.name,
                                             GetNotesRequest.Order.WHEN.number)

Each enum value has a special characteristic that makes it easy to convert to their name or their number. Instead of accessing the name and number attribute, just convert each value to a string or an integer:

print 'Enum value Order.%s has number %d' % (GetNotesRequest.Order.WHEN,
                                             GetNotesRequest.Order.WHEN)

Enum fields are declared similarly to other fields except they must have the enum type as its first parameter before the field number. Enum fields can also have default values.

Defining the Response Message

Now let's define the get_notes() response message. The response needs to be a collection of Note messages. Messages can contain other messages. In the case of the Notes.notes field defined above, we indicate that it is a collection of messages by providing the Note class as the first parameter before the field number:

class Notes(messages.Message):
    notes = messages.MessageField(Note, 1, repeated=True)

The Notes.notes field is also a repeated field as indicated by the repeated=True keyword argument. Values of repeated fields must be lists of the field type of their declaration. In this case, Notes.notes must a list of Note instances. Lists are automatically created and cannot be assigned to None.

For example, here is how to create a Notes object:

response = Notes(notes=[Note(text='This is note 1'),
                        Note(text='This is note 2')])
print 'The first note is:', response.notes[0].text
print 'The second note is:', response.notes[1].text

Implement get_notes

Now we can add the get_notes() method to the PostService class:

import datetime
import time
from protorpc import remote

class PostService(remote.Service):
    @remote.method(GetNotesRequest, Notes)
    def get_notes(self, request):
        query = guestbook.Greeting.all().order('-date')
  
        if request.on_or_before:
            when = datetime.datetime.utcfromtimestamp(
                request.on_or_before)
            query.filter('date <=', when)
  
        notes = []
        for note_model in query.fetch(request.limit):
            if note_model.date:
                when = int(time.mktime(note_model.date.utctimetuple()))
            else:
                when = None
            note = Note(text=note_model.content, when=when)
            notes.append(note)
     
        if request.order == GetNotesRequest.Order.TEXT:
            notes.sort(key=lambda note: note.text)
     
        return Notes(notes=notes)

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