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EclipseLink/DesignDocs/MultiTenantFeatures

UNDER CONSTRUCTION

Purpose

EclipseLink provides a number of features that help with the challenges of developing multi tenant applications. This document will provide an overview of some of the ways EclipseLink can be used to support multi tenant applications and suggest some enhancements that could be made to further enable thos applications.

Some of the challenges involved in developing multitenant applications include:

  1. You must choose if many tenants reside on the same DB, or the same schema
  2. Will the schema be flexible? If so, hom much?
  3. How do I differentiate data for tenants that share the same DB?

EclipseLink provides features that support various decision patchs about the above issues.

Here are some examples of the types of multi-tenant applications and how you could enable them in Eclipselink.

Static Application

In this application, neither the object model nor the database schema is variable. Tenants, for the most part, use the application as it has been provided. If they want to make use of data that is not part of the initial application they are provided a limited number of predefined database fields and cooresponding fields in the object model.

For Example:

Table Customer contains fields: id, name, attribute1, attribute2 Class Customer contains fields: id, name, attribute1, attribute2

In the above example, the id and name fields have predefined functionality in the application. Fields attribute1, and attribute2 can be configured to sore any data the customer wants. Typically these fields store data that is mapped direct from Object to database. (In JPA, a Basic mapping)

This application resembles a basic JPA application and has at its disposal, all the functionality available in JPA and the extended functionality available in EclipseLink. Basically, the application acquires an EntityManagerFactory for each tenant and that EntityManagerFactory serves that tenant only.

Some features of interest that exist in EclipseLink:

  1. Converters - If the attribute1 and attribute2 fields need to be stored as different types in the database and in the application, converters could be used to address this
  2. VPD Support - When using an Oracle Database, VPD provides an excellent way of securly segregating data between clients. (i.e. with VPD, it is easy to make the same table contain data for different tenants have automatically have only data for a particular tenant used in queries). VPD users can be chosen through EclipseLink's proxy authentication feature and specified through properties in creation of EntityManager factories.
  3. Additional Criteria Support - If VPD is not available, it is possible to segregate data by setting an Additional Criteria for a descriptor. This criteria could be used to ensure only data appropriate to a specific client would be returned.
  4. SessionCustomizer/DescriptorCustomizer - these could be used in the configuration of persistence units for each tenant. The Additional Criteria listed above is an example of what could be specified
  5. Dynamic Specification of persistence.xml - EclipseLink will allow the persistence.xml for a persistence unit to be specified at runtime. This is an additional way to support different configurations per tenant. In this case, each tenant would have a persistence.xml that configured the persistence unit for their part of the application.

Some features that could be implemented to help:

  1. VPD Equivalents for databases other that Oracle - Other vendors provide similar functionality and this could be support if there was demand.
  2. Dynamic specification of mapping xml. If mapping xml file could be specified at EntityManagerFactory creation time that would reduce the need to specify different persistence.xml files and any mapping/descriptor overrides could be specified in that file.

Mapped Attributes for Extension

To provide slightly more flexibility than the Static Application, a Map could be used for extended attributes.

In this case a model could be designed like this:

Table Customer contains fields: id, name Table Customer_extensions contains fields: customer_id, attributeName, attributeValue Class Customer contains fields: id, name, extensions

extensions would be defined using a Map mapping and hold any additional fields the customer wanted to use. For instance, an Employee Number would be stored with key = "EmployeeNumber" and the value = <employee number>. Business logic is used to expose the data in extensions as attributes.

Some features of interest that exist in EclipseLink:

  1. Map Mapping Strategies: EclipseLink Supports map mapping strategies with many different types of keys and values.




As part of the data-access layer of an application, EclipseLink features that enable multi-tenant application fall into one of two categories.

  1. Features that enable data-models for multi-tenant applications
  2. Features that enable object models for multi-tenant applications


EclipseLink provides a number of features that help with the challenges of developing multi tenant applications. This document will provide an overview of some of the ways EclipseLink can be used to support multi tenant applications.

In particular, this document will focus on applications where the application provider provides a core application and the tenant needs to extend the data provided in the application. (like Oracle Applications flex columns)

e.g. The application provider provides an Employee class that includes name and address. The tenant also wants to store the Employee Number. How can the application be architected to make storing that kind of extra data easy.

From a persistence-layer point of view, there are two areas where you must design to allow extra data.

  1. The object model and metadata
  2. The database

We will outline the options for both areas.

Object Model and Metadata

There are a number of different ways EclipseLink can support added fields in the object model.

Predefined fields

In this case, a class is designed with several generically named and typed fields. Those fields may be used by the tenant to store any data they choose. Converters could be used to control the data types coming out of the fields.

e.g. Employee contains fields called name, address, attribute1, attribute2 etc... The tenant chooses to store employee number in attribute1.

Dynamic JPA

Dynamic JPA allows the construction of a completely metadata driven application. The provider writes a dynamic JPA application that defines the set of classes and attributes that exist. The tenant calls API or provides XML that describes the additional fields on those classes.

e.g. Employee is a dynamically created entity containing name and address attributes. The tenant is allowed to construct a mapping for Employee Number by constructing an EclipseLink-orm.xml file that defines Employee with the new attribute.

Static Classes + Dynamic JPA

A static object model is provided by the provider that defines all the shared attributes. The tenant is provided with mechanism to provide extensions through Dynamic JPA. They are allowed to construct new mappings on the static classes by providing XML that describes fields that will be added to the existing object model through dynamic JPA.

e.g. Employee is a class that contains name and address attributes. The tenant is allowed to construct an eclipselink-orm.xml file that adds a virtual Employee Number field to Employee. EclipseLink's dynamic JPA functionality handles adding the field.

See https://bugs.eclipse.org/bugs/show_bug.cgi?id=331915 for the remaining work required for this configuration.

Map

Additional attributes are specified in a Map Mapping. Business logic is used to expose the fields in the Map as standard data.

e.g. Employee has attributes name and address and also a map for additional properties. To add Employee number, the tenant adds a Map Entry with key=EmployeeNumber and Value=<employees Employee Number>

Database

There are many different database architectures that can be used to support storing extra data.


Different Schema

If the tenants are all deployed on different databases or on different table spaces, the tables can quite simply be different. This can be enabled by EclipseLink's table creation feature. Additionally, it would be possible to introduce an EclipseLink feature that could alter tables in an initial schema based on different metadata.

e.g. A base tenant would have an Employee table with just a name and address field. Another tenant could have an Employee table with a name, address, and employee number field.

Static Fields

Simply define extra fields in each database table that can used to store extra data.

e.g. A table for Employee would contain name, address, attribute1, attribute2 etc...

This table architecture can be used with most of the object model strategies above.

Secondary Tables

EclipseLink supports secondary tables and these could be used to extend object models.

e.g. A base Employee table could contain name and address, a secondary table could contain a foreign key to the base table, and a set of additional field. (e.g. employee number, or attribute1 etc)

Secondary tables could be provided with preset fields in much the same way as the Static Field model above, or secondary tables could be created for each tenant that needed extra data. Dynamic metadata could be provided to change each tenants mapping to access their tables.

Map Table

Attributes are stored in a table that contains a key, an attribute name, and an attribute value. The table could optionally have a foreign key that mapped to a source table.

e.g. Employee table contains an id (and possibly some of the data). Attributes are contained in a table that contains a foreign key to the Employee table, an attribute name and an attribute value.

With this strategy, the map table could contain all the attributes, or the main table could contain some of the attributes and the map table could contain the rest.

Differentiating between tenants

EclipseLink provide 2 features that could be used to differentiate between tenants on the database

  1. Support for Oracle VPD - Oracle users can use Oracle's VPD feature through EclipseLink and that feature can be used to differentiate between tenants.
  2. Additional Join Criteria - EclipseLink allows additional join criteria to be specified on each descriptor that could be used for tenant differentiation

Other databases provide VPD-like features and support could be added for those as they are required.

EclipseLink Config

Configuration Strategies

The following configuration options can be used to configure the different stratgies

Single Persistence Unit

A single persistence unit could be provided that maps all the attributes. This strategy is useful in the case where there is no dynamic component to the application, all the db fields and all the attributes remain the same.

Deployment per tenant

A full persistence unit could be configured per tenant. That persistence unit could contain different persistence.xml, mapping files, classes and resources. This could be used with any of the above strategies.

Persistence xml per tenant

In this case, each tenant has a persistence.xml file which creates them a separate persistence unit. The classes used by each tenant remain the same. EclipseLink supports dynamic selection of persistence.xml so as long as the persistence xml for each tenant could be provided, this could all be configured in one deployment.

eclipselink-orm.xml per tenant

It is possible to add a persistence unit property that allows orm.xml files to be provided by on a per-tenant basis. In this case, everything about the persistence unit would be the same except a single eclipselink-orm.xml file which defines the mappings that are particular to a single tenant.

Customizer per tenant

A customizer could be used to configure tenant data. Each tenant would have a customizer that cusomized their persistence unit.

Metadata in the db

Ideally if there are multiple config files they should be stored in the DB along with other tenant information. We will have to add extensions to allow config files to be retrieved from the DB.

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