mozilla

Full tutorial

Let’s create a full working application with Cornice. We want to create a light messaging service.

You can find its whole source code at https://github.com/Cornices/examples/blob/master/messaging

Features:

  • users can register to the service
  • users can list all registered users
  • users can send messages
  • users can retrieve the latest messages
  • messages have three fields: sender, content, color (red or black)
  • adding a message is done through authentication

Limitations:

  • there’s a single channel for all messages.
  • if a user with the same name is already registered, he cannot register.
  • all messages and users are kept in memory.

Design

The application provides two services:

  • users, at /users: where you can list all users or register a new one
  • messages, at /: where you can read the messages or add new ones

On the server, the data is kept in memory.

We’ll provide a single CLI client in Python, using Curses.

Setting up the development environment

To begin, create a new directory and environment:

$ mkdir messaging
$ cd messaging
$ python3 -m venv ./

Once you have it, install Cornice in it with Pip:

$ bin/pip install cornice

You’ll also need waitress (see https://pypi.python.org/pypi/waitress):

$ bin/pip install waitress

We provide a Cookiecutter template you can use to create a new application:

$ bin/pip install cookiecutter
$ bin/cookiecutter gh:Cornices/cookiecutter-cornice
repo_name [myapp]: messaging
project_title [My Cornice application.]: Cornice tutorial

Once your application is generated, go there and call develop against it:

$ cd messaging
$ ../bin/python setup.py develop
...

The application can now be launched via embedded Pyramid pserve, it provides a default “Hello” service check:

$ ../bin/pserve messaging.ini
Starting server in PID 7618.
serving on 0.0.0.0:6543 view at http://127.0.0.1:6543

Once the application is running, visit http://127.0.0.1:6543 in your browser and make sure you get:

{'Hello': 'World'}

You should also get the same results calling the URL via Curl:

$ curl -i http://0.0.0.0:6543/

This will result:

HTTP/1.1 200 OK
Content-Length: 18
Content-Type: application/json; charset=UTF-8
Date: Tue, 12 May 2015 13:23:32 GMT
Server: waitress

{"Hello": "World"}

Defining the services

Let’s open the file in messaging/views.py, it contains all the Services:

from cornice import Service

hello = Service(name='hello', path='/', description="Simplest app")

@hello.get()
def get_info(request):
    """Returns Hello in JSON."""
    return {'Hello': 'World'}

Users management

We’re going to get rid of the Hello service, and change this file in order to add our first service - the users management

from cornice import Service

_USERS = {}

users = Service(name='users', path='/users', description="User registration")

@users.get(validators=valid_token)
def get_users(request):
    """Returns a list of all users."""
    return {'users': list(_USERS)}

@users.post(validators=unique)
def create_user(request):
    """Adds a new user."""
    user = request.validated['user']
    _USERS[user['name']] = user['token']
    return {'token': '%s-%s' % (user['name'], user['token'])}

@users.delete(validators=valid_token)
def delete_user(request):
    """Removes the user."""
    name = request.validated['user']
    del _USERS[name]
    return {'Goodbye': name}

What we have here is 3 methods on /users:

  • GET: returns the list of users names – the keys of _USERS
  • POST: adds a new user and returns a unique token
  • DELETE: removes the user.

Remarks:

  • POST uses the unique validator to make sure that the user name is not already taken. That validator is also in charge of generating a unique token associated with the user.
  • GET users the valid_token to verify that a X-Messaging-Token header is provided in the request, with a valid token. That also identifies the user.
  • DELETE also identifies the user then removes it.

These methods will use validators to fill the request.validated mapping. Add the following code to messaging/views.py:

import os
import binascii

from pyramid.httpexceptions import HTTPUnauthorized


def _create_token():
    return binascii.b2a_hex(os.urandom(20)).decode('utf-8')


def valid_token(request, **kargs):
    header = 'X-Messaging-Token'
    htoken = request.headers.get(header)
    if htoken is None:
        raise HTTPUnauthorized()
    try:
        user, token = htoken.split('-', 1)
    except ValueError:
        raise HTTPUnauthorized()

    valid = user in _USERS and _USERS[user] == token
    if not valid:
        raise HTTPUnauthorized()

    request.validated['user'] = user


def unique(request, **kargs):
    name = request.text
    if name in _USERS:
        request.errors.add('url', 'name', 'This user exists!')
    else:
        user = {'name': name, 'token': _create_token()}
        request.validated['user'] = user

The validators work by filling the request.validated dictionary. When the validator finds errors, it adds them to the request.errors dictionary, and that will return a 400 with the errors.

Let’s try our application so far with CURL:

$ curl http://localhost:6543/users
{"status": 401, "message": "Unauthorized"}

$ curl -X POST http://localhost:6543/users -d 'tarek'
{"token": "tarek-a15fa2ea620aac8aad3e1b97a64200ed77dc7524"}

$ curl http://localhost:6543/users -H "X-Messaging-Token:tarek-a15fa2ea620aac8aad3e1b97a64200ed77dc7524"
{"users": ["tarek"]}

$ curl -X DELETE http://localhost:6543/users -H "X-Messaging-Token:tarek-a15fa2ea620aac8aad3e1b97a64200ed77dc7524"
{"Goodbye": "tarek"}

Messages management

Now that we have users, let’s post and get messages. This is done via two very simple functions we’re adding in the views.py file:

_MESSAGES = []

messages = Service(name='messages', path='/', description="Messages")

@messages.get()
def get_messages(request):
    """Returns the 5 latest messages"""
    return _MESSAGES[:5]


@messages.post(validators=(valid_token, valid_message))
def post_message(request):
    """Adds a message"""
    _MESSAGES.insert(0, request.validated['message'])
    return {'status': 'added'}

The first one simply returns the five first messages in a list, and the second one inserts a new message in the beginning of the list.

The POST uses two validators:

  • valid_token(): the function we used previously that makes sure the user is registered
  • valid_message(): a function that looks at the message provided in the POST body, and puts it in the validated dict.

Here’s the valid_message() function:

import json

def valid_message(request):
    try:
        message = json.loads(request.body)
    except ValueError:
        request.errors.add('body', 'message', 'Not valid JSON')
        return

    # make sure we have the fields we want
    if 'text' not in message:
        request.errors.add('body', 'text', 'Missing text')
        return

    if 'color' in message and message['color'] not in ('red', 'black'):
        request.errors.add('body', 'color', 'only red and black supported')
    elif 'color' not in message:
        message['color'] = 'black'

    message['user'] = request.validated['user']
    request.validated['message'] = message

This function extracts the json body, then checks that it contains a text key at least. It adds a color or use the one that was provided, and reuse the user name provided by the previous validator with the token control.

The Client

A simple client to use against our service can do three things:

  1. let the user register a name
  2. poll for the latest messages
  3. let the user send a message !

Without going into great details, there’s a Python CLI against messaging that uses Curses.

See https://github.com/Cornices/examples/blob/master/messaging/messaging/client.py