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django-db-multitenant

Provides a simple multi-tenancy solution for Django based on the concept of having a single tenant per database.

This application is still experimental, but is being used in production by the authors. Contributions and discussion are welcome.

Read the Changelog

Background

Multi-tenancy is the ability to support multiple distinct datasets from the same application server. Each dataset usually maps to a customer (the tenant) and is partially or fully partitioned from all other tenant data.

Among the possible approaches are:

  • Isolated approach: Separate database per tenant.
  • Semi-isolated approach: Shared database, separate namespaces (PostgreSQL schemas) or table names/prefix per tenant.
  • Shared approach: Single database for all tenants. Each table has a column identifying the tenant for that row of data.

This application supports two backends, MySQL and PostgreSQL:

  • With MySQL, this application implements a variation of the isolated approach, each tenant has its own database, however their connection details are shared (such as password, database user).
  • For PostgreSQL, this application implements a semi-isolated approach, each tenant has its own schema and the connection details are shared via the public schema.

django-db-multitenant makes it possible (even easy) to take a Django application designed for a single tenant and use it with multiple tenants.

Operation

The main technique is as follows:

  1. When a request first arrives, determine desired the tenant from the request object, and save it in thread-local storage.
  2. Later in the request, when a database cursor is accquired, issue an SQL USE <tenant database name> for the desired tenant with MySQL or SET search_path TO <tenant name> with PostgreSQL.

Step 1 is accomplished by implementing a mapper class. Your mapper takes a request object and returns a database name or tenant name, using whatever logic you like (translate hostname, inspect a HTTP header, etc). The mapper result is saved in thread-local storage for later use.

Step 2 determines whether the desired database or schema has already been selected, and is skipped if so. This is implemented using a thin database backend wrapper for MySQL and for PostgreSQL which must be set in settings.DATABASES as the backend.

Usage

1. Install

Install django-db-multitenant (or add it to your setup.py).

$ pip install django-db-multitenant

2. Implement a mapper

You must implement a subclass of db_multitenant.mapper which determines the database name and cache prefix from the request.

To help you to write your mapper, the repository contains examples of mappers which extracts the hostname of URL to determine the tenant name (eg. in https://foo.example.com/bar/, foo will be the tenant name):

  • mapper for MySQL, which uses a portion of the hostname directly as the database name.
  • mapper for PostgreSQL, which uses a portion of the hostname as search path (schema). PostgreSQL allows complex setups with sharing of common tables (public accounts for example), see the comment in the mapper for more details.
  • mapper for Redis, which looks up the tenant using the hostname, throwing a 404 if unrecognized.

Feel free to copy an example mapper in your project then adjust it to your needs.

3. Update settings.py

Set the multitenant mapper by specifying the full dotted path to your implementation (in this example, mapper is the name of file mapper.py):

MULTITENANT_MAPPER_CLASS = 'myapp.mapper.TenantMapper'

Install the multitenant middleware as the first middleware of the list (prior to Django 1.10, you must use the MIDDLEWARE_CLASSES setting):

MIDDLEWARE = [
    'db_multitenant.middleware.MultiTenantMiddleware',
    ....
]

Change your database backend to the multitenant wrapper:

DATABASES = {
    'default': {
        'ENGINE': 'db_multitenant.db.backends.mysql',
        'NAME': 'devnull',
    }
}

Note: the NAME is useless for MySQL but due to a current limitation, the named database must exist. It may be empty and read-only.

Or for PostgreSQL:

DATABASES = {
    'default': {
        'ENGINE': 'db_multitenant.db.backends.postgresql',
        'NAME': 'mydb',
    }
}

Optionally, add the multitenant helper KEY_FUNCTION to your cache definition, which will cause cache keys to be prefixed with the value of mapper.get_cache_prefix:

CACHES = {
  'default' : {
        'LOCATION': '127.0.0.1:11211',
        'BACKEND': 'django.core.cache.backends.memcached.MemcachedCache',
        'KEY_FUNCTION': 'db_multitenant.cache.helper.multitenant_key_func'
    }
}

4. Tests

If the tenant name of your application is extracted from the URL (as in the provided examples of mappers), you can add a host to your /etc/hosts such as foo.example.com to redirect to your localhost server.

You should add foo.example.com to ALLOWED_HOSTS list in your Django settings and just try to reach your application from your browser with http://foo.example.com:8000.

The examples of mappers provide information about the way to create a tenant zone.

Management Commands

In order to use management commands (like migrate) with the correct tenant, inject this little hack at the end of your settings.py:

from db_multitenant.utils import update_from_env
update_from_env(database_settings=DATABASES['default'],
    cache_settings=CACHES['default'])

If you didn't set CACHES in your settings and you don't intend to use a cache system, you don't have to pass the cache_settings argument to the function.

You can then export TENANT_DATABASE_NAME for MySQL or TENANT_NAME for PostgreSQL and TENANT_CACHE_PREFIX on the command line, for example:

$ TENANT_DATABASE_NAME=example.com ./manage.py migrate

Don't forget to create the database (MySQL) or the required schema first (PostgreSQL).

That’s it. Because django-db-multitenant does not define any models, there’s no need to add it to INSTALLED_APPS.

Advantages and Limitations

There is no one-size-fits-all solution for a data modeling problem such as multi-tenancy (see ‘Alternatives’).

Advantages

  • Compatibility: Your Django application doesn’t need any awareness of multi-tenancy. Database-level tools (such as mysqldump or pgdump) just work.
  • Isolation: One tenant, one database means there’s no intermingling of tenant data (excepted if you share tables with PostgreSQL).
  • Simplicity: Your application schemas don’t need to be cluttered with ‘Tenant’ foreign key relationships.
  • Should work well with Django 1.6 connection persistence and connection pooling.

Limitations

  • Unorthodox. Django does not expect this kind of dynamic database connection tinkering, and there could be unexpected bugs.
  • Limited isolation. Since the same DB credentials are used for all tenants, bugs in the mapper (or anywhere else in the app) could cause data corruption.
  • A valid database still needs to be specified in settings.DATABASE for use when the connection is first established with MySQL (this should be fixed eventually).
  • Overhead: requests may add up to one extra query (the USE <db_name> statement for MySQL or the SET search_path TO <tenant_name> for PostgreSQL).

License

The project is distributed under the 3-clause BSD license.

Alternatives and Further Reading

  • django-tenant-schemas implements a semi-isolated approach using PostgreSQL schemas (and inspired this project, as well as the ‘Overview’ section above).