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.
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 (
1fortextand2forwhen) that the underlying network protocol uses to identify the field. -
Make
texta required field. Fields are optional by default, you can mark them as required by settingrequired=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.textwhich 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_typesmodule), 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)