Build a Cassandra Python REST Service

Difficulty: Intermediate

Estimated Time: 20 minutes

The first step to creating a microservice is to create a service. Let's see how to add a REST front end on Cassandra with Python and Django!

In this scenario, we'll learn how to:

Install the necessary Django framework
Configure a REST service with Django
Connect the Django REST service to Cassandra

Python makes things easy and Django makes them even easier! We'll get you started!

ver 0.004

In this scenario, we learned how to:

Install the necessary Django framework
Configure a REST service with Django
Connect the Django REST service to Cassandra

This is a simple service, but it illustrates the important broad brush strokes.

Your environment is currently being packaged as a Docker container and the download will begin shortly. To run the image locally, once Docker has been installed, use the commands

cat scrapbook_datastax_cassandra-k8s/cassandra-django_container.tar | docker load

docker run -it /datastax_cassandra-k8s/cassandra-django:

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Step 1 of 7

Set up the Django server

In this step, we are going to set up the web server. Let's start by clicking the following to create a virtual environment.

python3 -m venv env
source env/bin/activate
apt -y install python3 python3-pip

With our virtual environment set up, we're ready to set up the Django framework and create our rest project. Click the following.

pip install djangorestframework
django-admin startproject rest .
python manage.py migrate

Let's start our service, by clicking the following, just to show that we can run the service. It won't do much... yet!

python manage.py runserver

Let's hit the service to verify it's up and running. If the service is running, the following command will print out the HTML from the server. We'll execute the following command from a separate terminal (just click the following).

curl 127.0.0.1:8000

Excellent! We have a running HTTP service!

Serve up some JSON

In the previous step we got the service running. But, it's only serving up an HTML page. In this step we'll figure out how to serve up some JSON.

For this, we need to create our REST app. Our app will be a simple service for keeping track of user credentials. We'll call it users.

Click on the following, which will stop the service in terminal 1 by sending a Ctrl-C and create the app.

python manage.py startapp users

We need to configure the users app, which includes:

Making Django aware of our new app by changing settings.py
Creating a User model to store user credentials (i.e., username and password) in users/models.py
Creating a serializer for our User model, which converts the credential info into JSON in users/serializers.py
Creating a view, or endpoint, for our users service in users/views.py
Adding a URL for the endpoint to rest/urls.py

Open the settings.py file by clicking the following.

Open rest/settings.py

You will want to make the INSTALLED_APPS section like the following, which adds two entries at the bottom of the list (find the list and replace it with the content shown). You can click the following to copy the contents to the clipboard.

INSTALLED_APPS = [
    'django.contrib.admin',
    'django.contrib.auth',
    'django.contrib.contenttypes',
    'django.contrib.sessions',
    'django.contrib.messages',
    'django.contrib.staticfiles',
    'rest_framework',
    'users.apps.UsersConfig',
]

What did we just do?

The INSTALLED_APPS list tells Django which apps to install. We need the REST framework, so we added that to the list. We also added our User app that we created above.

Next, add a User model to models.py. Click on the following to open the file.

Open users/models.py

Add the code to the file by clicking the following.

from django.db import models

class User(models.Model):
    username = models.CharField(max_length = 20)
    password = models.CharField(max_length = 20)

What did we just do?

We need a place to store Cassandra records before we write them and after we read them. The model contains classes to do that. In this example, we created a User model class that maps to the table we will use. The table will have two columns: username and password - just like our model.

We'll also create a serializers.py file to serialize the model. Click the following to open the file.

Open users/serializers.py

Here's the serializer code. Click on the code to copy it to the file.

from django.db import models
from rest_framework import serializers
from .models import User

class UserSerializer(serializers.ModelSerializer):
    class Meta:
        model = User
        fields = '__all__'

What did we just do?

The job of the serializer class is to convert model classes to JSON and JSON to models. Django does the heavy lifting for us, so we need to import the serializers package. We also need to import our User model. We name the class based on our model name (e.g., User) and add the word Serializer. Notice that the UserSerializer class derives from the Django ModelSerializer class.

In the inner Meta class, we specify which fields we want to include in the serialization. If we want to include all the fields, we can use '__all__'. Alternatively, we can specify the fields we want to include in a tuple (e.g., ('username', 'password')).

Finally, edit the views.py file to serve up the JSON. You see that we have hard-coded the returned values (i.e., MyName and MyPassword). We'll see how to extract these values from Cassandra in the next steps. Click the following to open the file.

Open users/views.py

Include this code by clicking the following.

from rest_framework.views import APIView
from rest_framework.response import Response
from rest_framework import status
from .models import User
from .serializers import UserSerializer

class User(APIView):

    def get(self, request):
        user = User()
        user.username = 'MyName'
        user.password = 'MyPassword'
        serializer = UserSerializer(user)
        return Response(serializer.data)

    def post(self, request):
        pass

What did we just do?

The users/view.py file contains the code that Django executes when the service receives a request. You notice the two methods get() and post() that correspond to the HTTP GET and POST requests. For now, we just create a User instance, load it up with some hard-coded values, pass it to the serializer and send the serialized results back to the client.

Finally, we'll add a url pattern to handle the request. Open the rest/urls.py file by clicking the following.

Open rest/urls.py

Replace the contents of the file with the following by clicking on the code.

from django.contrib import admin
from django.urls import path
from rest_framework.urlpatterns import format_suffix_patterns
from users import views

urlpatterns = [
    path('admin/', admin.site.urls),
    path('user/', views.User.as_view()),
]

urlpatterns = format_suffix_patterns(urlpatterns)

Let's restart the service in terminal 1 by clicking on the following.

python manage.py runserver

That's all there is to it. Let's hit the service in terminal 2 just to make sure it works. Click the following.

curl 127.0.0.1:8000/user/ | jq

Outstanding! The service is now serving up JSON!

Create the Python code

In this step, we'll demonstrate Python code to access the database.

Let's start by creating a user_credentials table. We've already installed and started Cassandra for you, so just click the following to stop the service and create the keyspace and table.

cqlsh -e "
CREATE KEYSPACE user_management
  WITH REPLICATION = {
    'class' : 'SimpleStrategy',
    'replication_factor' : 1
  };
CREATE TABLE user_management.user_credentials(
  username        text,
  password        text,
  PRIMARY KEY(username));"

Note: We would never store passwords in plain-text in a production system, but we will for this example to keep things simple.

We can add a handful of records to the table. Click the following.

cqlsh -e "
INSERT INTO user_management.user_credentials
  (username, password)
  VALUES ('User1', 'YouWontGuessThis');
INSERT INTO user_management.user_credentials
  (username, password)
  VALUES ('User2', 'SuperSecret');
INSERT INTO user_management.user_credentials
  (username, password)
  VALUES ('User3', 'UncrackablePassword');"

Next, let's create a Python program to access the table. We need to install the Python driver for Cassandra. Click the following.

pip install cassandra-driver

Let's use the Python interpreter to try out some code. Launch the interpreter by clicking the following.

python

We'll connect to the cluster and create a session. Click the following.

from cassandra.cluster import Cluster
cluster = Cluster(protocol_version = 3)
session = cluster.connect()

Now, let's query our table by clicking the following.

results = session.execute('SELECT * FROM user_management.user_credentials')
for result in results:
    print(result.username, result.password)

_ = '^^^ See the results!'

Exit the interpreter by clicking the following.

quit()

Perfect! The code you just completed gives you an example of how to access Cassandra from Python!

Plug the Python code into your service

In the previous step, we saw how to use Python to access the database. In this step, we'll access the database to retrieve a user's credentials based on a hard-coded username.

Let's modify the users/views.py file to access the database. As mentioned, for now we will use a hard-coded username for the request - User1. In the next step we'll see how to get the username from the URL.

Open users/views.py

In the following code, we add an import for the driver. We also replace the old code that just created a user with code that gets the user from the database. In both the old and new code, we serialize the User the same way.

from rest_framework.views import APIView
from rest_framework.response import Response
from rest_framework import status
from .models import User
from .serializers import UserSerializer

# Here is the import for the python driver
from cassandra.cluster import Cluster

class User(APIView):

    def get(self, request):
        # here's the old code that just creates a User:
        '''
        user = User()
        user.username = 'MyName'
        user.password = 'MyPassword'
        '''
        # here's the new code that gets the User from the database:
        cluster = Cluster(protocol_version = 3)
        session = cluster.connect()
        results = session.execute("SELECT * FROM user_management.user_credentials WHERE username = %s", ("User1", ))
        user = results.one()

        # serialize the user like before
        serializer = UserSerializer(user)
        return Response(serializer.data)

    def post(self, request):
        pass

Restart the service by clicking the following.

python manage.py runserver

Now, issue a request to the service to see our code work! Click the following.

curl 127.0.0.1:8000/user/ | jq

Great! we see that we can access the database from within the service and return the result!

Add the username to the URL

The last step in retrieving a user is to wire the username value from the URL to the query.

Modify the rest/urls.py file to add the username.

Open rest/urls.py

Add the username to the URL pattern.

from django.contrib import admin
from django.urls import path
from rest_framework.urlpatterns import format_suffix_patterns
from users import views

urlpatterns = [
    path('admin/', admin.site.urls),
    path('user/<str:username>/', views.User.as_view()),
]

urlpatterns = format_suffix_patterns(urlpatterns)

Then, add username as the third parameter to the get() method in users/views.py and use parameter in the query.

Open users/views.py

Here's the updated code.

from rest_framework.views import APIView
from rest_framework.response import Response
from rest_framework import status
from .models import User
from .serializers import UserSerializer

# Here is the import for the python driver
from cassandra.cluster import Cluster

class User(APIView):

    def get(self, request, username):
        cluster = Cluster(protocol_version = 3)
        session = cluster.connect()
        results = session.execute("SELECT * FROM user_management.user_credentials WHERE username = %s", (username, ))
        user = results.one()

        serializer = UserSerializer(user)
        return Response(serializer.data)

    def post(self, request):
        pass

Restart the service by clicking the following.

python manage.py runserver

Let's query for User2 by clicking the following.

curl 127.0.0.1:8000/user/User2/ | jq

Superb! Our REST service can handle GET requests!

POST user data to the service

Let's try writing data to our service. We won't put the data in the database until the next step. But, in this step we'll see how to capture the data in the post() method.

We need to add code to the post() method inside users/views.py.

Open users/views.py

Here's the code.

from django.http import JsonResponse
from rest_framework.views import APIView
from rest_framework.response import Response
from rest_framework import status
from .models import User
from .serializers import UserSerializer

# Here is the import for the python driver
from cassandra.cluster import Cluster

class User(APIView):

    def get(self, request, username):
        cluster = Cluster(protocol_version = 3)
        session = cluster.connect()
        results = self.session.execute("SELECT * FROM user_management.user_credentials WHERE username = %s", (username, ))
        user = results.one()

        serializer = UserSerializer(user)
        return Response(serializer.data)

    def post(self, request):
        serializer = UserSerializer(data=request.data)
        if serializer.is_valid():
            print("in post *************************************************")
            print("username = "+serializer.data.get("username"), "password = "+serializer.data.get("password"))
            return Response(serializer.data, status=status.HTTP_201_CREATED)
        return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)

Modify the rest/urls.py file to add the POST endpoint. Note that you must place this URL before the GET URL, otherwise the URL pattern is ambiguous.

Open rest/urls.py

Add the user/add/ endpoint URL pattern.

from django.contrib import admin
from django.urls import path
from rest_framework.urlpatterns import format_suffix_patterns
from users import views

urlpatterns = [
    path('admin/', admin.site.urls),
    path('user/add/', views.User.as_view()),
    path('user/<str:username>/', views.User.as_view()),
]

urlpatterns = format_suffix_patterns(urlpatterns)

Click the following to restart the service.

python manage.py runserver

Give it a try by clicking the following!

curl -X POST -d '{"username": "User4", "password": "PasswordFor4" }' -H 'Content-Type: application/json' 127.0.0.1:8000/user/add/ | jq

As you can see, the post() method prints out the correct username and password values. Now, we're all set to write the credentials to Cassandra - in the next step.

Woot! You see how to access POST data!

POST a user to the database

Let's complete the POST and write the data to the database.

Under the print statements in users/views.py, add the code to store the user credentials in the database.

Open users/views.py

We'll give you a chance to figure this out on your own, but if you need help, you can check out the solution.

Solution

from django.http import JsonResponse
from rest_framework.views import APIView
from rest_framework.response import Response
from rest_framework import status
from .models import User
from .serializers import UserSerializer

# Here is the import for the python driver
from cassandra.cluster import Cluster

class User(APIView):

    def get(self, request, username):
        cluster = Cluster(protocol_version = 3)
        session = cluster.connect()
        results = self.session.execute("SELECT  FROM user_management.user_credentials WHERE username = %s", (username, ))
        user = results.one()

        serializer = UserSerializer(user)
        return Response(serializer.data)

    def post(self, request):
        serializer = UserSerializer(data=request.data)
        if serializer.is_valid():
            print("in post **")
            print("username = "+serializer.data.get("username"), "password = "+serializer.data.get("password"))

            cluster = Cluster(protocol_version = 3)
            session = cluster.connect()
            results = session.execute("INSERT INTO user_management.user_credentials (username, password) VALUES(%s, %s)", (serializer.data.get("username"), serializer.data.get("password")))

            return Response(serializer.data, status=status.HTTP_201_CREATED)
        return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)

Before we post the new user, let's look at what's in the table.

cqlsh -e "SELECT * FROM user_management.user_credentials;"

We need to start the service again. Click the following.

python manage.py runserver

Now, post the new user.

curl -X POST -d '{"username": "User4", "password": "PasswordFor4" }' -H 'Content-Type: application/json' 127.0.0.1:8000/user/add/ | jq

Finally, verify that the post worked by looking at the contents of the table now. Click the following.

cqlsh -e "SELECT * FROM user_management.user_credentials;"

You now see the credentials for User4 in the table!

Amazing! That's all there is to creating a Cassandra REST service using Python and Django.

Terminal

cd <directory>	Change directory
ls	List directory
echo 'contents' > <file>	Write contents to a file
cat <file>	Output contents of file

i	In Command Mode	Change into Insert Mode, you can now insert and edit text in the file
esc	In Insert Mode	Change into Command Mode, you can now execute commands
:wq	In Command Mode	Save and Exit

Welcome!

Build a Cassandra Python REST Service

Congratulations!

You've completed the scenario!

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