Python/Django support for distributed multi-tenant databases like Postgres+Citus

Enables easy scale-out by adding the tenant context to your queries, enabling the database (e.g. Citus) to efficiently route queries to the right database node.

There are architecures for building multi-tenant databases viz. Create one database per tenant, Create one schema per tenant and Have all tenants share the same table(s). This library is based on the 3rd design i.e Have all tenants share the same table(s), it assumes that all the tenant relates models/tables have a tenant_id column for representing a tenant.

The following link talks more about the trade-offs on when and how to choose the right architecture for your multi-tenat database:

https://www.citusdata.com/blog/2016/10/03/designing-your-saas-database-for-high-scalability/

Other useful links on multi-tenancy:

1. https://www.citusdata.com/blog/2017/03/09/multi-tenant-sharding-tutorial/
2. https://www.citusdata.com/blog/2017/06/02/scaling-complex-sql-transactions/

1. pip install django_multitenant

Tested with django 1.10 or higher.

1. In whichever files you want to use the library import it by just saying

   import django_multitenant
   from django_multitenant import *

2. All models should inherit the TenantModel class. Ex: class Product(TenantModel):
3. Define a static variable named tenant_id and specify the tenant column using this variable. Ex: tenant_id='store_id'
4. All foreign keys to TenantModel subclasses should use TenantForeignKey in place of models.ForeignKey
5. A sample model implementing the above 2 steps:

    class Product(TenantModel):
    	store = models.ForeignKey(Store)
    	tenant_id='store_id'
    	name = models.CharField(max_length=255)
    	description = models.TextField()
    	class Meta(object):
    		unique_together = ["id", "store"]
    class Purchase(TenantModel):
      store = models.ForeignKey(Store)
      tenant_id='store_id'
      product_purchased = TenantForeignKey(Product)

1. Write authentication logic using a middleware which also sets/unsets a tenant for each session/request. This way developers need not worry about setting a tenant on a per view basis. Just set it while authentication and the library would ensure the rest (adding tenant_id filters to the queries). A sample implementation of the above is as follows:

    class SetCurrentTenantFromUser(object):
    	def process_request(self, request):
     	        if not hasattr(self, 'authenticator'):
            		from rest_framework_jwt.authentication import JSONWebTokenAuthentication
            		self.authenticator = JSONWebTokenAuthentication()
                try:
            		user, _ = self.authenticator.authenticate(request)
                except:
            		# TODO: handle failure
            		return
                try:
            		#Assuming your app has a function to get the tenant associated for a user
            		current_tenant = get_tenant_for_user(user)
                except:
            		# TODO: handle failure
            		return
                set_current_tenant(current_tenant)
        def process_response(self, request, response):
                set_current_tenant(None)
                return response

      MIDDLEWARE_CLASSES = (
    	'our_app.utils.multitenancy.SetCurrentTenantFromUser',
      )

2. Set the tenant using set_current_tenant(t) api in all the views which you want to be scoped based on tenant. This would scope all the django API calls automatically(without specifying explicit filters) to a single tenant. If the current_tenant is not set, then the default/native API without tenant scoping is used.

    def application_function:
    	# current_tenant can be stored as a SESSION variable when a user logs in.
    	# This should be done by the app
    	t = current_tenant
    	#set the tenant
    	set_current_tenant(t);
    	#Django ORM API calls;
    	#Command 1;
    	#Command 2;
    	#Command 3;
    	#Command 4;
    	#Command 5;

1. Most of the APIs under Model.objects.* except select_related().
2. Model.save() injects tenant_id for tenant inherited models.

   s=Store.objects.all()[0]
   set_current_tenant(s)
   
   #All the below API calls would add suitable tenant filters.
   #Simple get_queryset()
   Product.objects.get_queryset()
   
   #Simple join
   Purchase.objects.filter(id=1).filter(store__name='The Awesome Store').filter(product__description='All products are awesome')
   
   #Update
   Purchase.objects.filter(id=1).update(id=1)
   
   #Save
   p=Product(8,1,'Awesome Shoe','These shoes are awesome')
   p.save()
   
   #Simple aggregates
   Product.objects.count()
   Product.objects.filter(store__name='The Awesome Store').count()
   
   #Subqueries
   Product.objects.filter(name='Awesome Shoe');
   Purchase.objects.filter(product__in=p);

This library uses similar logic of setting/getting tenant object as in django-simple-multitenant. We thank the authors for their efforts.