# Multi-tenancy

## What is multi-tenancy?

The term multi-tenancy in general is applied to software development to indicate an architecture in which a single running instance of an application simultaneously serves multiple clients (tenants). This is highly common in SaaS solutions. Isolating information (data, customizations, etc) pertaining to the various tenants is a particular challenge in these systems. This includes the data owned by each tenant stored in the database. It is this last piece, sometimes called multi-tenant data, on which we will focus.

## Multi-tenant data approaches

There are 3 main approaches to isolating information in these multi-tenant systems which goes hand-in-hand with different database schema definitions and JDBC setups.

Each approach has pros and cons as well as specific techniques and considerations. Such topics are beyond the scope of this documentation. Many resources exist which delve into these other topics. One example is which does a great job of covering these topics.

### Separate database

Each tenant's data is kept in a physically separate database instance. JDBC Connections would point specifically to each database, so any pooling would be per-tenant. A general application approach here would be to define a JDBC Connection pool per-tenant and to select the pool to use based on the tenant identifier associated with the currently logged in user.

### Separate schema

Each tenant's data is kept in a distinct database schema on a single database instance. There are 2 different ways to define JDBC Connections here:

- Connections could point specifically to each schema, as we saw with the `Separate database` approach. This is an option provided that the driver supports naming the default schema in the connection URL or if the pooling mechanism supports naming a schema to use for its Connections. Using this approach, we would have a distinct JDBC Connection pool per-tenant where the pool to use would be selected based on the tenant identifier associated with the currently logged in user.

- Connections could point to the database itself (using some default schema) but the Connections would be altered using the SQL `SET SCHEMA` (or similar) command. Using this approach, we would have a single JDBC Connection pool for use to service all tenants, but before using the Connection it would be altered to reference the schema named by the tenant identifier associated with the currently logged in user.

### Partitioned (discriminator) data

All data is kept in a single database schema. The data for each tenant is partitioned by the use of partition value or discriminator. The complexity of this discriminator might range from a simple column value to a complex SQL formula. Again, this approach would use a single Connection pool to service all tenants. However, in this approach the application needs to alter each and every SQL statement sent to the database to reference the tenant identifier discriminator.

## Multi-tenancy in Hibernate

Using Hibernate with multi-tenant data comes down to both an API and then integration piece(s). As usual Hibernate strives to keep the API simple and isolated from any underlying integration complexities. The API is really just defined by passing the tenant identifier as part of opening any session.

Specifying tenant identifier from SessionFactory

Additionally, when specifying configuration, a org.hibernate.MultiTenancyStrategy should be named using the hibernate.multiTenancy setting. Hibernate will perform validations based on the type of strategy you specify. The strategy here correlates to the isolation approach discussed above.

NONE

  (the default) No multi-tenancy is expected. In fact, it is considered an error if a tenant identifier is specified when opening a session using this strategy.

SCHEMA

  Correlates to the separate schema approach. It is an error to attempt to open a session without a tenant identifier using this strategy. Additionally, a org.hibernate.service.jdbc.connections.spi.MultiTenantConnectionProvider must be specified.

DATABASE

  Correlates to the separate database approach. It is an error to attempt to open a session without a tenant identifier using this strategy. Additionally, a org.hibernate.service.jdbc.connections.spi.MultiTenantConnectionProvider must be specified.

DISCRIMINATOR

  Correlates to the partitioned (discriminator) approach. It is an error to attempt to open a session without a tenant identifier using this strategy. This strategy is not yet implemented in Hibernate as of 4.0 and 4.1. Its support is planned for 5.0.

### MultiTenantConnectionProvider

When using either the DATABASE or SCHEMA approach, Hibernate needs to be able to obtain Connections in a tenant specific manner. That is the role of the org.hibernate.service.jdbc.connections.spi.MultiTenantConnectionProvider contract. Application developers will need to provide an implementation of this contract. Most of its methods are extremely self-explanatory. The only ones which might not be are getAnyConnection and releaseAnyConnection. It is important to note also that these methods do not accept the tenant identifier. Hibernate uses these methods during startup to perform various configuration, mainly via the java.sql.DatabaseMetaData object.

The MultiTenantConnectionProvider to use can be specified in a number of ways:

- Use the hibernate.multi_tenant_connection_provider setting. It could name a MultiTenantConnectionProvider instance, a MultiTenantConnectionProvider implementation class reference or a MultiTenantConnectionProvider implementation class name.

- Passed directly to the org.hibernate.service.ServiceRegistryBuilder.

- If none of the above options match, but the settings do specify a hibernate.connection.datasource value, Hibernate will assume it should use the specific org.hibernate.service.jdbc.connections.spi.DataSourceBasedMultiTenantConnectionProviderImpl implementation which works on a number of pretty reasonable assumptions when running inside of an app server and using one javax.sql.DataSource per tenant. See its javadocs for more details.

### CurrentTenantIdentifierResolver

When applications use either the standard jta or thread based implementations of the org.hibernate.context.spi.CurrentSessionContext feature, Hibernate will need to open a session if it cannot find an existing one in scope. However, when a session is opened in a multi-tenant environment the tenant identifier has to be specified. This is the role of the org.hibernate.context.spi.CurrentTenantIdentifierResolver contract. It will resolve the tenant identifier to use. The implementation to use is either passed directly to Configuration via its setCurrentTenantIdentifierResolver method. It can also be specified via the hibernate.tenant_identifier_resolver setting.

Additionally, if the CurrentTenantIdentifierResolver implementation returns `true` for its validateExistingCurrentSessions method, Hibernate will make sure any existing sessions that are found in scope have a matching tenant identifier.

### Caching

Multi-tenancy support in Hibernate works seamlessly with the Hibernate second level cache. The key used to cache data encodes the tenant identifier.

### Odds and ends

Currently schema export will not really work with multi-tenancy. That may not change.

## Strategies for MultiTenantConnectionProvider implementors

Implementing MultiTenantConnectionProvider using different connection pools

The approach above is valid for the DATABASE approach. It is also valid for the SCHEMA approach provided the underlying database allows naming the schema to which to connect in the connection URL.

Implementing MultiTenantConnectionProvider using single connection pool

This approach is only relevant to the SCHEMA approach.