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# ProtoRPC Python API Overview

#### Experimental!

ProtoRPC 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 ProtoRPC. We will inform the community when this feature is no longer experimental.

ProtoRPC is a simple way to send and receive 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 using only the Python programming language, it's easy to get started developing your own services, testing those services, and scaling them on App Engine.

While you can use ProtoRPC 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 ProtoRPC 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 ProtoRPC
- Getting Started with ProtoRPC

- Creating the PostService Module
- Working with Messages
- Defining a Service
- Registering the Service
- Testing the Service from the Command Line
- Message Fields
- Defining the Response Message

## The Hello World of ProtoRPC

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 google.appengine.ext import webapp
    from google.appengine.ext.webapp import util

    from protorpc import messages
    from protorpc.webapp import service_handlers
    from protorpc import remote

    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)
    service_mappings = service_handlers.service_mapping(
        [('/hello', HelloService),
        ])

    # Apply the service mappings to Webapp
    application = webapp.WSGIApplication(service_mappings)

    def main():
      util.run_wsgi_app(application)

    if __name__ == '__main__':
      main()

## Getting Started with ProtoRPC

This section demonstrates how to get started with ProtoRPC 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 to easily access that information.

That's where ProtoRPC comes in. In this tutorial, we'll apply ProtoRPC to this basic guestbook, enabling other web applications to access the guestbook's data. This tutorial only covers using ProtoRPC 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 ProtoRPC depends on your specific app; the important point is that ProtoRPC 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 ProtoRPC 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 ProtoRPC. You define messages 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. ProtoRPC 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. ProtoRPC 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 using the App Engine webapp framework. ProtoRPC has a small library (`protorpc.webapp.service_handlers`) making that easy. Create a new file called `services.py` in your application directory and add the following code to create your webapp-based service:

    from google.appengine.ext import webapp
    from google.appengine.ext.webapp import util

    from protorpc.webapp import service_handlers

    import PostService

    # Register mapping with application.
    application = webapp.WSGIApplication(
      service_handlers.service_mapping(
        [('/PostService', PostService.PostService)]),
      debug=True)

    def main():
      util.run_wsgi_app(application)

    if __name__ == '__main__':
      main()

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

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

### 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:
    % 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)