# Overview

This app uses PostgreSQL schemas to support data multi-tenancy in a single Django project. Schemas are a layer of separation between databases and tables, so that one database can have multiple schemas, which in turn can have multiple (and possibly identical) tables. For an accurate description on schemas, see the official documentation on PostgreSQL schemas.

PostgreSQL uses a “search path” to denote in which schemas it should look for the appropriate tables. If there are three schemas: `client1`, `common` and `public` and the search path is set to `["client1", "public"]`, PostgreSQL will look for tables first on schema `client1`, and then, if not found, will look on schema `public`. The tables on schema `common` would never be searched. Also, if there is a table with the same name on both `client1` and `public` schemas (i.e. `django_migrations`), only the table in `client1` will be found by that search path. Table creation always takes place on the first schema in the search path.

`django-pgschemas`, as well as it’s predecessors `django-tenants` and `django-tenant-schemas`, takes advantage of PostgreSQL schemas to emulate multi-tenancy, by mapping certain URL patterns to schemas, and setting the search path accordingly. It also provides an API to smartly change the search path outside the request/response cycle, in order to perform schema-specific tasks.

## Multi-tenancy

There are typically three solutions for solving the multi-tenancy problem.

1. Isolated approach: Separate databases. Each tenant has it’s own database.

2. Semi-isolated approach: Shared database, separate schemas. One database for all tenants, but one schema per tenant.

3. Shared approach: Shared database, shared schema. All tenants share the same database and schema. There is a main tenant-table, where all other tables have a foreign key pointing to.

Each solution has its up and down sides, for a more in-depth discussion, see Microsoft’s excellent article on Multi-Tenant Data Architecture.

This application implements the second approach, which in our opinion, represents a good compromise between simplicity and performance.

Tip

If you are looking for an implementation of the third approach, you might be interested in django-multitenant. For other solutions of the multi-tenancy problem, you could also look here.

The semi-isolated approach through PostgreSQL schemas has some advantages and disadvantages:

- Simplicity: barely make any changes to your current code to support multi-tenancy. Plus, you only manage one database.

- Performance: make use of shared connections, buffers and memory.

vs.

- Scalability: for a large number of tenants (thousands) the schema approach might not be feasible, and as of now, there is no clear way for implementing tenant sharding.

## Schemas vs. Tenants

The terms schema and tenant are used indistinctly all over the documentation. However, it is important to note some subtle differences between the two. We consider a tenant to be a subset of data that can be accessed with a URL (routed), and we use database schemas for that purpose. Still, there can be schemas that shouldn’t be considered tenants according to our definition. One good example is the `public` schema, which most typically contains data shared across all tenants. Then, every tenant is a schema, but not every schema is a tenant.

## Static vs. Dynamic

In a typical software-as-a-service (SaaS), there is a number of static sites that are related to enterprise level operations. Using `mydomain.com` as example, one could think of these enterprise level sites:

    mydomain.com
    www.mydomain.com
    blog.mydomain.com
    help-center.mydomain.com

Likewise, there are going to be multiple sites for tenant specific operations. Those sites are dynamic in nature, as cannot be determined at the time of implementation – and hopefully, will be thousands ;) The dynamic sites could follow a subdomain routing approach like:

    customer1.mydomain.com
    customer2.mydomain.com

A subfolder routing approach like:

    customers.mydomain.com/customer1
    customers.mydomain.com/customer2

Or a mixed approach, where even a-la-carte domains are used (say, for VIP clients) like:

    customer1.mydomain.com
    customers.mydomain.com/customer1
    customers.mydomain.com/customer2
    www.thevipcustomer.com

This app allows you to manage both static and dynamic tenants, and the three kinds of routing.

Attention

For static tenants, only the subdomain routing is available.

In order to manage dynamic tenants, we provide two model mixins you must inherit in your models: `TenantMixin` and `DomainMixin`. The former controls the tenants and the latter controls the domain/folder combinations that will route each tenant.