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IV.17.2 Object Model Requirements

By Pete Su

OpenACS docs are written by the named authors, and may be edited by OpenACS documentation staff.

A major goal in OpenACS 4 is to unify and normalize many of the core services of the system into a coherent common data model and API. In the past, these services were provided to applications in an ad-hoc and irregular fashion. Examples of such services include:

  • General Comments

  • User/groups

  • Attribute storage in user/groups

  • General Permissions

  • Site wide search

  • General Auditing

All of these services involve relating extra information and services to application data objects, examples of which include:

  • Bboard messages

  • A user home page

  • A ticket in the Ticket Tracker

  • A photograph in the PhotoDB

In the past, developers had to use ad-hoc and inconsistent schemes to interface to the various "general" services mentioned above. Since each service used its own scheme for storing its metadata and mapping this data to application objects, we could not implement any kind of centralized management system or consistent administrative pages for all the services. Consequently, a large amount of duplicate code appeared throughout the system for dealing with these services.

Unifying and "normalizing" these interfaces, to minimize the amount of code repetition in applications, is a primary goal of OpenACS 4. Thus the Object Model (OM, also referred to later as the object system) is concerned primarily with the storage and management of metadata, on any object within a given instance of OpenACS 4. The term "metadata" refers to any extra data the OM stores on behalf of the application - outside of the application's data model - in order to enable certain generic services. The term "object" refers to any entity being represented within the OpenACS, and typically corresponds to a single row within the relational database.

The OpenACS 4 Object Model must address five high-level requirements that repeatedly exhibit themselves in the context of existing services in OpenACS 3.x, as described below.

Object Identifiers for General Services

Generic services require a single unambiguous way of identifying application objects that they manage or manipulate. In OpenACS 3.x, there are several different idioms that construct object identifiers from other data. Many modules use a (user_id, group_id, scope) triple combination for the purpose of recording ownership information on objects for access control. User/groups also uses (user_id, group_id) pairs in its user_group_map table as a way to identify data associated with a single membership relation.

Also in OpenACS 3.x, many utility modules exist that do nothing more than attach some extra attributes to existing application data. For example, general comments maintains a mapping table that maps application "page" data (static or dynamic) to one or more user comments on the page, by constructing a unique identifier for each page. This identifier is usually a combination of the table in which the data is stored, and the value of the primary key value for the particular page. This idiom is referred to as the "(on_which_table + on_what_id)" method for identifying application data. General comments stores its map from pages to comments using a "(on_which_table + on_what_id)" key, plus the id of the comment itself.

All of these composite key constructions are implicit object identifiers: they build a unique ID out of other pieces of the data model. The problem is that their definition and use is ad-hoc and inconsistent. This makes the construction of generic application-independent services difficult. Therefore, the OpenACS 4 Object Model should provide a centralized and uniform mechanism for tagging application objects with unique identifiers.

Support for Unified Access Control

Access control should be as transparent as possible to the application developer. Until the implementation of the general permissions system, every OpenACS application had to manage access control to its data separately. Later on, a notion of "scoping" was introduced into the core data model.

"Scope" is a term best explained by example. Consider some hypothetical rows in the address_book table:

... scope user_id group_id ...
... user 123 ...
... group 456 ...
... public ...

The first row represents an entry in User 123's personal address book, the second row represents an entry in User Group 456's shared address book, and the third row represents an entry in the site's public address book.

In this way, the scoping columns identify the security context in which a given object belongs, where each context is either a person or a group of people or the general public (itself a group of people).

The problem with this scheme is that we are limited to using only users and groups as scopes for access control, limiting applications to a single level of hierarchy. Worse, the scoping system demanded that every page needing access to a given application had to do an explicit scope check to make sure access was allowed - if a developer was careless on just one site page, a security problem could result.

Thus the OpenACS 4 Object Model must support a more general access control system that allows access control domains to be hierarchical, and specifiable with a single piece of data, instead of the old composite keys described above.

Extensible Data Models

Another problem with previous OpenACS data models is that many of the central tables in the system became bloated as they were extended to support an increasing number of modules. The users table is the best case in point: it became full of columns that exist for various special applications (e.g. user portraits), but that aren't really related to each other in any way except that they store information on users, i.e. the table became grossly denormalized. Normalizing (breaking-down) this table into several pieces, each of which is specific to a particular application, would improve maintainability greatly. Furthermore, the ability to allow applications or users to define new extensions to existing tables, and have some central metadata facility for keeping track of what data belong to which tables, would be very useful.

Thus the motivation for providing object types and subtyping in the OpenACS 4 Object Model. The OM should allow developers to define a hierarchy of metadata object types with subtyping and inheritance. Developers can then use the framework to allow users to define custom extensions to the existing data models, and the OM does the bookkeeping necessary to make this easier, providing a generic API for object creation that automatically keeps track of the location and relationships between data.

Design Note: While this doesn't really belong in a requirements document, the fact that we are constrained to using relational databases means that certain constraints on the overall design of the object data model exist, which you can read about in Summary and Design Considerations.

Modifiable Data Models

Another recurring applications problem is how to store a modifiable data model, or how to store information that may change extensively between releases or in different client installations. Furthermore, we want to avoid changes to an application's database queries in the face of any custom extensions, since such changes are difficult or dangerous to make at runtime, and can make updating the system difficult. Some example applications in OpenACS 3.x with modifiable data models include:

  • User/groups: developers and users can attach custom data to group types, groups, and members of groups.

  • In the Ecommerce data model, the ec_custom_product_fields table defines attributes for catalog products, and the ec_custom_product_field_values table stores values for those attributes.

  • In the PhotoDB data model, the ph_custom_photo_fields table defines attributes for the photographs owned by a specific user, and tables named according to the convention "ph_user_<user_id>_custom_info" are used to store values for those attributes.

Thus the Object Model must provide a general mechanism for applications and developers to modify or extend data models, without requiring changes to the SQL schema of the system. This ensures that all applications use the same base schema, resulting in a uniform and more maintainable system.

Generic Relations

Many OpenACS applications define simple relationships between application objects, and tag those relationships with extra data. In OpenACS 3.x, this was done using mapping tables. The user/groups module has the most highly developed data model for this purpose, using a single table called user_group_map that mapped users to groups. In addition, it uses the the user_group_member_fields and user_group_member_fields_map tables to allow developers to attach custom attributes to group members. In fact, these custom attributes were not really attached to the users, but to the fact that a user was a member of a particular group - a subtle but important distinction. As a historical note, in OpenACS 3.x, user/groups was the only part of the system that provided this kind of data model in a reusable way. Therefore, applications that needed this capability often hooked into user/groups for no other reason than to use this part of its data model.

The OpenACS 4 data model must support generic relations by allowing developers to define a special kind of object type called a relation type. Relation types are themselves object types that do nothing but represent relations. They can be used by applications that previously used user/groups for the same purpose, but without the extraneous, artificial dependencies.

The Object Model package is a combination of data model and a procedural API for manipulating application objects within an OpenACS instance. The OM allows developers to describe a hierarchical system of object types that store metadata on application objects. The object type system supports subtyping with inheritance, so new object types can be defined in terms of existing object types.

The OM data model forms the main part of the OpenACS 4 Kernel data model. The other parts of the Kernel data model include:

  • Parties and Groups

  • Permissions

Each of these is documented elsewhere at length.

(Pending as of 8/27/00)

The data model for the object system provides support for the following kinds of schema patterns that are used by many existing OpenACS modules:

10.0 Object Identification and Storage

Object identification is a central mechanism in the new metadata system. The fact that every object has a known unique identifier means that the core can deal with all objects in a generic way. Thus the only action required of an application to obtain any general service is to "hook into" the object system.

In OpenACS 3.x, modules use ad-hoc means to construct unique identifiers for objects that they manage. Generally, these unique IDs are built from other IDs that happen to be in the data model. Because there is no consistency in these implementations, every application must hook into every service separately.

Examples of utilities that do this in OpenACS 3.x system are:

  • User/groups: Information is attached to group membership relations.

  • General Comments: Comments are attached to objects representing some kind of document.

  • General Permissions: Stores access control information on application data.

  • User Profiling: Maps users to pieces of content that they have looked at; content identifiers must be managed in a uniform way.

  • Site Wide Search: Stores all content in a single flat table, with object identifiers pointing to the object containing the content in the first place. This way, we can search the contents of many different types of objects in a uniform way.

The OM will support and unify this programming idiom by providing objects with unique identifiers (unique within a given OpenACS instance) and with information about where the application data associated with the object is stored. The identifier can be used to refer to collections of heterogeneous application data. More importantly, object identifiers will enable developers to readily build and use generic services that work globally across a system.

The object identifiers should be subject to the following requirements:

10.10 Uniqueness

The object ID should be unique among all the IDs in the entire OpenACS system in which the object lives.

10.20 Useful as a Reference

Applications should be able to use the unique object ID as a reference, with which they can fetch any or all of the object's attributes.

10.30 Storable

Object IDs should be storable in tables. e.g. you should be able to use them to implement mapping tables between objects, to represent relationships.

10.40 Moveable

Objects should be mobile between databases. That is, information will often need to be moved between multiple servers (development, staging, and production), so a mechanism for moving this data is necessary. In addition, a mechanism for tagging these objects in a way similar to CVS would be useful in determining which objects need to be synchronized.

20.0 Object Types

An object type refers to a specification of one or more attributes to be managed along with a piece of application data.

The object system should provide a data model for describing and representing object types. This data model is somewhat analogous to the Oracle data dictionary, which stores information about all user defined tables in the system.

The canonical example of this kind of data model occurs in the current OpenACS 3.x user/groups module, which allows the developer to create new group types that can contain not only generic system level attributes but also extended, developer-defined attributes. In addition, these attributes can either be attached to the group type itself, and shared by all instances, or they can be different for each instance. At its core, the OpenACS 4 object system is meant to be a generalization of this mechanism. The data model should allow developers to at least do everything they used to with user/groups, but without its administrative hassles.

Therefore, the data model must be able to represent object types that have the following characteristics:

20.10 Type Name

A human readable name for the object type.

20.20 Type Attributes

Attributes whose values are shared by all instances of the object type.

20.30 Object Attributes

Attributes that are specific to each particular object belonging to a given type.

The data model must also enforce certain constraints on object types:

20.40 Type Uniqueness

Object type names must be unique.

20.50 Attribute Name Uniqueness

Attribute names must be unique in the scope of a single object type and any of its parent types.

30.0 Type Extension

The Object Model must support the definition of object types that are subtypes of existing types. A subtype inherits all the attributes of its parent type, and defines some attributes of its own. A critical aspect of the OM is parent types may be altered, and any such change must propagate to child subtypes.

The OM data model must enforce constraints on subtypes that are similar to the ones on general object types.

30.10 Subtype Uniqueness

Subtype names must be unique (this parallels requirement 10.40).

30.20 Subtype Attribute Name Uniqueness

Attribute names must be unique in the scope of a single object subtype.

30.30 Parent Type Prerequisite

Subtypes must be defined in terms of parent types that, in fact, already exist.


The extended attribute names in a subtype must not be the same as those in its parent type.

35.0 Methods

35.10 Method and Type Association

The OM data model should define a mechanism for associating procedural code, called methods, with objects of a given type. Methods are associated with the each object type - not each object instance.

35.20 Method Sharing

All instances of a given object type should share the same set of defined methods for that type.

40.0 Object Attribute Value Storage

In addition to information on types, the OM data model provides for the centralized storage of object attribute values. This facility unifies the many ad-hoc attribute/value tables that exist in various OpenACS 3.x data models, such as:

  • User groups: Each instance of a group type can have custom data.

  • Photo DB: Users can define their own custom metadata to attach to photograph objects.

  • Ecommerce: Vendors can attach custom fields to the data model describing their products.

40.10 Generic Retrieval

Attributes should be stored so that they are retrievable in a way that is independent of the type of the object that they belong to. That is, the only data needed to retrieve an attribute should be the system-wide ID of an object (see requirement 10.20 above) and the attribute name.

40.20 Inherited Attributes

The system should allow for the automatic retrieval of inherited attribute values, for an object belonging to a subtype.

40.30. Constraints on Attributes

The system should allow the developer to put down constraints on the values that an attribute may hold, for the purposes of maintaining application specific integrity rules.

50.0 Object Contexts

In OpenACS 3.x, there was a notion of "scope" for application objects. An object could be belong to one of three scopes: public, group or user. This provided a crude way to associate objects with particular scopes in the system, but it was awkward to use and limited in flexibility.

The OpenACS 4 Object Model provides a generalized notion of scope that allows developers to represent a hierarchy of object contexts. These contexts are used as the basis for the permissions system. In general, if an object has no explicit permissions attached to it, then it inherits permissions from its context.

The context data model also forms the basis of the subsites system, and is a basic part of the permissions system, described in separate documents.

The context data model should provide the following facilities:

50.10 Unique ID

Every context should have a unique ID in the system.

50.20 Tree Structure

The data model should support a tree structured organization of contexts. That is, contexts can be logically "contained" within other contexts (i.e. contexts have parents) and contexts can contain other contexts (i.e. contexts can have children).

50.30 Data Model Constraints

All objects must have a context ID. This ID must refer to an existing context or be NULL. The meaning of a NULL context is determined by the implementation.


The current system interprets the NULL context as meaning the default "site-wide" context in some sense. I wanted to note this fact for others, but there is no need to make this a requirement of the system. I think it would be reasonable to have a NULL context be an error (psu 8/24/2000).

55.0 Object Relations

The data model should include a notion of pair-wise relations between objects. Relations should be able to record simple facts of the form "object X is related to object Y by relationship R," and also be able to attach attributes to these facts.

The API should let programmers accomplish the following actions:

60.0 Object Type Creation

60.10 Create a New Object Type

The object system API should provide a procedure call that creates a new object type by running the appropriate transactions on the object system data model. This API call is subject to the constraints laid out in the data model. We call this operation "instantiating" an object.

60.20 Create a New Object Subtype

The object system API should provide a procedure call for creating subtypes of a given type. Operationally, this API is the same as requirement 60.10. Instances of subtypes automatically contain all attributes of the parent type in addition to all attributes of the subtype. This API is subject to the constraints laid out in the data model.

60.30 Create a New Relation Type

There should be an API call to create a new type of object relation. Relation types can be modeled as object types. The API below for manipulating attributes can then be used to add attributes to relation types.

70.0 Update an Object Type

The object system API must allow the programmer to modify, add, and delete attributes from any object type. Updates should be propagated to any child subtypes. This API is subject to the constraints laid out in the data model.

80.0 Delete an Object Type

The system provides an API call for deleting an object type.


Deleting an object type destroys all instances of the type. It should be an error to delete types that have dependent subtypes. This API is subject to the constraints laid out in the data model.


However, the programmer should also be able to specify that all the subtypes and instances of those subtypes be destroyed before destroying the object type. This is similar to a "delete cascade" constraint in SQL.

90.0 Object Instance Creation and Destruction

The system must provide API calls to manage the creation and destruction of object instances.

90.10 Create an Instance of an Object Type

The system should provide an API call for creating a new instance of a given object type. The new instance should be populated with values for each of the attributes specified in the definition of the type. In addition, it should be possible to create the new instance with an optional context ID that refers to the default context that the object will live in.

90.20 Delete an Object Instance

The OM should provide an API call for object deletion. Objects can be deleted only when no other objects in the system refer to them. Since it might not be practical to provide a mechanism like "delete cascade" here in a reliable way, providing such a facility in the system is optional.

94.0 Object Relation Creation and Destruction

The system must provide API calls to manage the creation and destruction of object relations.

94.10 Create an Object Relation

The OM must provide an API call to declare that two objects are related to each other by a given relation type. This API call should also allow programmers to attach attributes to this object relation.

94.20 Destroy an Object Relation

There should be an API call for destroying object relations and their attributes.

95.10 Create and Destroy Contexts

The system should provide an API to create and destroy object contexts.

100.10 Set Attribute Values for an Object

The system should provide an API for updating the attribute values of a particular instance of an object type.

110.10 Get Attribute Values for an Object

The system should provide an API for retrieving attribute values from a particular instance of an object type.

120.10 Efficiency

The Object Model must support the efficient storage and retrieval of object attributes. Since the OM is intended to form the core of many general services in the OpenACS, and these services will likely make extensive use of the OM tables, queries on these tables must be fast. The major problem here seems to be supporting subtyping and inheritance in a way that does not severely impact query performance.

130.10 Ease of Use

Most OpenACS packages will be expected to use the Object Model in one way or another. Since it is important that the largest audience of developers possible adopts and uses the OM, it must be easy to incorporate into applications, and it must not impose undue requirements on an application's data model. In other words, it should be easy to "hook into" the object model, and that ability should not have a major impact on the application data model.

Note: Is the API the only way to obtain values? How does this integrate with application level SQL queries?

Document Revision # Action Taken, Notes When? By Whom?
0.1 Creation 08/10/2000 Bryan Quinn
0.2 Major re-write 08/11/2000 Pete Su
0.3 Draft completed after initial reviews 08/22/2000 Pete Su
0.4 Edited, updated to conform to requirements template, pending freeze 08/23/2000 Kai Wu
Final edits before freeze 08/24/2000 Pete Su
0.5 Edited for consistency 08/27/2000 Kai Wu
0.6 Put Object ID stuff first, because it makes more sense 08/28/2000 Pete Su
0.7 Added requirement that knowledge-level objects must be moveable between databases. 08/29/2000 Richard Li
0.8 Rewrote intro to match language and concepts in the design document. Also cleaned up usage a bit in the requirements section. Added short vague requirements on relation types. 09/06/2000 Pete Su
0.9 Edited for ACS 4 Beta release. 09/30/2000 Kai Wu