This tutorial shows you how build a simple Python application with CockroachDB and the Django framework.
CockroachDB supports Django versions 2.2 and 3.0.
Before you begin
- Install CockroachDB.
- Start up a secure or insecure local cluster.
- Choose the instructions that correspond to whether your cluster is secure or insecure:
The example code and instructions on this page use Python 3 and Django 3.0.
Step 1. Install Django and the CockroachDB backend for Django
Install Django:
$ python -m pip install django==3.0.*
Before installing the CockroachDB backend for Django, you must install one of the following psycopg2 prerequisites:
psycopg2, which has some prerequisites of its own. This package is recommended for production environments.
psycopg2-binary. This package is recommended for development and testing.
After you install the psycopg2 prerequisite, install the CockroachDB Django backend:
$ python -m pip install django-cockroachdb==3.0.*
The major version of django-cockroachdb must correspond to the major version of django. The minor release numbers do not need to match.
Step 2. Create the django user and bank database and generate certificates
Open a SQL shell to the running CockroachDB cluster:
$ cockroach sql --certs-dir=certs --host=localhost:26257
In the SQL shell, issue the following statements to create the django user and bank database:
> CREATE USER IF NOT EXISTS django;
> CREATE DATABASE bank;
Give the django user the necessary permissions:
> GRANT ALL ON DATABASE bank TO django;
Exit the SQL shell:
> \q
Create a certificate and key for the django user by running the following command:
$ cockroach cert create-client django --certs-dir=certs --ca-key=my-safe-directory/ca.key
Step 2. Create the django user and bank database
Open a SQL shell to the running CockroachDB cluster:
$ cockroach sql --insecure --host=localhost:26257
In the SQL shell, issue the following statements to create the django user and bank database:
> CREATE USER IF NOT EXISTS django;
> CREATE DATABASE bank;
Give the django user the necessary permissions:
> GRANT ALL ON DATABASE bank TO django;
Exit the SQL shell:
> \q
Step 3. Create a Django project
In the directory where you'd like to store your code, use the django-admin command-line tool to create an application project:
$ django-admin startproject myproject
This creates a new project directory called myproject. myproject contains the manage.py script and a subdirectory, also named myproject, that contains some .py files.
Open myproject/myproject/settings.py, and add 0.0.0.0 to the ALLOWED_HOSTS in your settings.py file, so that it reads as follows:
ALLOWED_HOSTS = ['0.0.0.0']
In myproject/myproject/settings.py, add myproject to the list of INSTALLED_APPS, so that it reads as follows:
INSTALLED_APPS = [
'django.contrib.admin',
'django.contrib.auth',
'django.contrib.contenttypes',
'django.contrib.sessions',
'django.contrib.messages',
'django.contrib.staticfiles',
'myproject',
]
The other installed applications listed are added to all starter Django applications by default.
In myproject/myproject/settings.py, change DATABASES to the following:
DATABASES = {
'default': {
'ENGINE': 'django_cockroachdb',
'NAME': 'bank',
'USER': 'django',
'HOST': 'localhost',
'PORT': '26257',
'OPTIONS': {
'sslmode': 'require',
'sslrootcert': '<path>/certs/ca.crt',
'sslcert': '<path>/certs/client.django.crt',
'sslkey': '<path>/certs/client.django.key',
},
},
}
DATABASES = {
'default': {
'ENGINE': 'django_cockroachdb',
'NAME': 'bank',
'USER': 'django',
'HOST': 'localhost',
'PORT': '26257',
}
}
Step 4. Write the application logic
After you generate the initial Django project files, you need to build out the application with a few .py files in myproject/myproject.
Models
Start by building some models, defined in a file called models.py. You can copy the sample code below and paste it into a new file, or you can download the file directly.
from django.db import models
class Customers(models.Model):
id = models.AutoField(primary_key=True)
name = models.CharField(max_length=250)
class Products(models.Model):
id = models.AutoField(primary_key=True)
name = models.CharField(max_length=250)
price = models.DecimalField(max_digits=18, decimal_places=2)
class Orders(models.Model):
id = models.AutoField(primary_key=True)
subtotal = models.DecimalField(max_digits=18, decimal_places=2)
customer = models.ForeignKey(Customers, on_delete=models.CASCADE, null=True)
product = models.ManyToManyField(Products)
In this file, we define some simple classes that map to the tables in the example database bank.
Views
Next, build out some class-based views for the application in a file called views.py. You can copy the sample code below and paste it into a new file, or you can download the file directly.
from django.http import JsonResponse, HttpResponse
from django.utils.decorators import method_decorator
from django.views.generic import View
from django.views.decorators.csrf import csrf_exempt
from django.db import Error, IntegrityError
from django.db.transaction import atomic
from psycopg2 import errorcodes
import json
import sys
import time
from .models import *
# Warning: Do not use retry_on_exception in an inner nested transaction.
def retry_on_exception(num_retries=3, on_failure=HttpResponse(status=500), delay_=0.5, backoff_=1.5):
def retry(view):
def wrapper(*args, **kwargs):
delay = delay_
for i in range(num_retries):
try:
return view(*args, **kwargs)
except IntegrityError as ex:
if i == num_retries - 1:
return on_failure
elif getattr(ex.__cause__, 'pgcode', '') == errorcodes.SERIALIZATION_FAILURE:
time.sleep(delay)
delay *= backoff_
except Error as ex:
return on_failure
return wrapper
return retry
class PingView(View):
def get(self, request, *args, **kwargs):
return HttpResponse("python/django", status=200)
@method_decorator(csrf_exempt, name='dispatch')
class CustomersView(View):
def get(self, request, id=None, *args, **kwargs):
if id is None:
customers = list(Customers.objects.values())
else:
customers = list(Customers.objects.filter(id=id).values())
return JsonResponse(customers, safe=False)
@retry_on_exception(3)
@atomic
def post(self, request, *args, **kwargs):
form_data = json.loads(request.body.decode())
name = form_data['name']
c = Customers(name=name)
c.save()
return HttpResponse(status=200)
@retry_on_exception(3)
@atomic
def delete(self, request, id=None, *args, **kwargs):
if id is None:
return HttpResponse(status=404)
Customers.objects.filter(id=id).delete()
return HttpResponse(status=200)
# The PUT method is shadowed by the POST method, so there doesn't seem
# to be a reason to include it.
@method_decorator(csrf_exempt, name='dispatch')
class ProductView(View):
def get(self, request, id=None, *args, **kwargs):
if id is None:
products = list(Products.objects.values())
else:
products = list(Products.objects.filter(id=id).values())
return JsonResponse(products, safe=False)
@retry_on_exception(3)
@atomic
def post(self, request, *args, **kwargs):
form_data = json.loads(request.body.decode())
name, price = form_data['name'], form_data['price']
p = Products(name=name, price=price)
p.save()
return HttpResponse(status=200)
# The REST API outlined in the github does not say that /product/ needs
# a PUT and DELETE method
@method_decorator(csrf_exempt, name='dispatch')
class OrdersView(View):
def get(self, request, id=None, *args, **kwargs):
if id is None:
orders = list(Orders.objects.values())
else:
orders = list(Orders.objects.filter(id=id).values())
return JsonResponse(orders, safe=False)
@retry_on_exception(3)
@atomic
def post(self, request, *args, **kwargs):
form_data = json.loads(request.body.decode())
c = Customers.objects.get(id=form_data['customer']['id'])
o = Orders(subtotal=form_data['subtotal'], customer=c)
o.save()
for p in form_data['products']:
p = Products.objects.get(id=p['id'])
o.product.add(p)
o.save()
return HttpResponse(status=200)
This file defines the application's views as classes. Each view class corresponds to one of the table classes defined in models.py. The methods of these classes define read and write transactions on the tables in the database.
Importantly, the file defines a transaction retry loop in the decorator function retry_on_exception(). This function decorates each view method, ensuring that transaction ordering guarantees meet the ANSI SERIALIZABLE isolation level. For more information about how transactions (and retries) work, see Transactions.
URL routes
Lastly, define some URL routes in a file called urls.py. The django-admin command-line tool generated this file when you created the Django project, so it should already exist in myproject/myproject. You can copy the sample code below and paste it into the existing urls.py file, or you can download the file directly and replace the existing one.
from django.contrib import admin
from django.urls import path
from .views import CustomersView, OrdersView, PingView, ProductView
urlpatterns = [
path('admin/', admin.site.urls),
path('ping/', PingView.as_view()),
# Endpoints for customers URL.
path('customer/', CustomersView.as_view(), name='customers'),
path('customer/<int:id>/', CustomersView.as_view(), name='customers'),
# Endpoints for customers URL.
path('product/', ProductView.as_view(), name='product'),
path('product/<int:id>/', ProductView.as_view(), name='product'),
path('order/', OrdersView.as_view(), name='order'),
]
Models
Start by building some models, defined in a file called models.py. You can copy the sample code below and paste it into a new file, or you can download the file directly.
from django.db import models
class Customers(models.Model):
id = models.AutoField(primary_key=True)
name = models.CharField(max_length=250)
class Products(models.Model):
id = models.AutoField(primary_key=True)
name = models.CharField(max_length=250)
price = models.DecimalField(max_digits=18, decimal_places=2)
class Orders(models.Model):
id = models.AutoField(primary_key=True)
subtotal = models.DecimalField(max_digits=18, decimal_places=2)
customer = models.ForeignKey(Customers, on_delete=models.CASCADE, null=True)
product = models.ManyToManyField(Products)
In this file, we define some simple classes that map to the tables in the example database bank.
Views
Next, build out some class-based views for the application in a file called views.py. You can copy the sample code below and paste it into a new file, or you can download the file directly.
from django.http import JsonResponse, HttpResponse
from django.utils.decorators import method_decorator
from django.views.generic import View
from django.views.decorators.csrf import csrf_exempt
from django.db import Error, IntegrityError
from django.db.transaction import atomic
from psycopg2 import errorcodes
import json
import sys
import time
from .models import *
# Warning: Do not use retry_on_exception in an inner nested transaction.
def retry_on_exception(num_retries=3, on_failure=HttpResponse(status=500), delay_=0.5, backoff_=1.5):
def retry(view):
def wrapper(*args, **kwargs):
delay = delay_
for i in range(num_retries):
try:
return view(*args, **kwargs)
except IntegrityError as ex:
if i == num_retries - 1:
return on_failure
elif getattr(ex.__cause__, 'pgcode', '') == errorcodes.SERIALIZATION_FAILURE:
time.sleep(delay)
delay *= backoff_
except Error as ex:
return on_failure
return wrapper
return retry
class PingView(View):
def get(self, request, *args, **kwargs):
return HttpResponse("python/django", status=200)
@method_decorator(csrf_exempt, name='dispatch')
class CustomersView(View):
def get(self, request, id=None, *args, **kwargs):
if id is None:
customers = list(Customers.objects.values())
else:
customers = list(Customers.objects.filter(id=id).values())
return JsonResponse(customers, safe=False)
@retry_on_exception(3)
@atomic
def post(self, request, *args, **kwargs):
form_data = json.loads(request.body.decode())
name = form_data['name']
c = Customers(name=name)
c.save()
return HttpResponse(status=200)
@retry_on_exception(3)
@atomic
def delete(self, request, id=None, *args, **kwargs):
if id is None:
return HttpResponse(status=404)
Customers.objects.filter(id=id).delete()
return HttpResponse(status=200)
# The PUT method is shadowed by the POST method, so there doesn't seem
# to be a reason to include it.
@method_decorator(csrf_exempt, name='dispatch')
class ProductView(View):
def get(self, request, id=None, *args, **kwargs):
if id is None:
products = list(Products.objects.values())
else:
products = list(Products.objects.filter(id=id).values())
return JsonResponse(products, safe=False)
@retry_on_exception(3)
@atomic
def post(self, request, *args, **kwargs):
form_data = json.loads(request.body.decode())
name, price = form_data['name'], form_data['price']
p = Products(name=name, price=price)
p.save()
return HttpResponse(status=200)
# The REST API outlined in the github does not say that /product/ needs
# a PUT and DELETE method
@method_decorator(csrf_exempt, name='dispatch')
class OrdersView(View):
def get(self, request, id=None, *args, **kwargs):
if id is None:
orders = list(Orders.objects.values())
else:
orders = list(Orders.objects.filter(id=id).values())
return JsonResponse(orders, safe=False)
@retry_on_exception(3)
@atomic
def post(self, request, *args, **kwargs):
form_data = json.loads(request.body.decode())
c = Customers.objects.get(id=form_data['customer']['id'])
o = Orders(subtotal=form_data['subtotal'], customer=c)
o.save()
for p in form_data['products']:
p = Products.objects.get(id=p['id'])
o.product.add(p)
o.save()
return HttpResponse(status=200)
This file defines the application's views as classes. Each view class corresponds to one of the table classes defined in models.py. The methods of these classes define read and write transactions on the tables in the database.
Importantly, the file defines a transaction retry loop in the decorator function retry_on_exception(). This function decorates each view method, ensuring that transaction ordering guarantees meet the ANSI SERIALIZABLE isolation level. For more information about how transactions (and retries) work, see Transactions.
URL routes
Lastly, define some URL routes in a file called urls.py. The django-admin command-line tool generated this file when you created the Django project, so it should already exist in myproject/myproject. You can copy the sample code below and paste it into the existing urls.py file, or you can download the file directly and replace the existing one.
from django.contrib import admin
from django.urls import path
from .views import CustomersView, OrdersView, PingView, ProductView
urlpatterns = [
path('admin/', admin.site.urls),
path('ping/', PingView.as_view()),
# Endpoints for customers URL.
path('customer/', CustomersView.as_view(), name='customers'),
path('customer/<int:id>/', CustomersView.as_view(), name='customers'),
# Endpoints for customers URL.
path('product/', ProductView.as_view(), name='product'),
path('product/<int:id>/', ProductView.as_view(), name='product'),
path('order/', OrdersView.as_view(), name='order'),
]
Step 5. Set up and run the Django app
In the top myproject directory, use the manage.py script to create Django migrations that initialize the database for the application:
$ python manage.py makemigrations myproject
$ python manage.py migrate
This initializes the bank database with the tables defined in models.py, in addition to some other tables for the admin functionality included with Django's starter application.
To verify that the migration succeeded, open a SQL shell to the running CockroachDB cluster:
$ cockroach sql --certs-dir=certs --host=localhost:26257
To verify that the migration succeeded, open a SQL shell to the running CockroachDB cluster:
$ cockroach sql --insecure --host=localhost:26257
> USE bank;
> SHOW TABLES;
table_name
+----------------------------+
auth_group
auth_group_permissions
auth_permission
auth_user
auth_user_groups
auth_user_user_permissions
django_admin_log
django_content_type
django_migrations
django_session
myproject_customers
myproject_orders
myproject_orders_product
myproject_products
(14 rows)
In a new terminal, navigate to the top of the myproject directory, and start the app:
$ python manage.py runserver 0.0.0.0:8000
To perform simple reads and writes to the database, you can send HTTP requests to the application.
For example, in a new terminal, you can use curl to send a POST request to the application that inserts a new row into the customers table:
$ curl --header "Content-Type: application/json" \
--request POST \
--data '{"name":"Carl"}' http://0.0.0.0:8000/customer/
You can then send a GET request to read from that table:
$ curl http://0.0.0.0:8000/customer/
[{"id": 523377322022797313, "name": "Carl"}]
You can also query the tables directly in the SQL shell to see the changes:
> SELECT * FROM myproject_customers;
id | name
+--------------------+------+
523377322022797313 | Carl
(1 row)
What's next?
Read more about writing a Django app.
You might also be interested in the following pages:
- Client Connection Parameters
- Data Replication
- Fault Tolerance & Recovery
- Replication & Rebalancing
- Cross-Cloud Migration
- Follow-the-Workload
- Automated Operations
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