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By Rafael H. Schloming, Mark Thomas
=======Almost all database-backed websites have users, and need to model the grouping of users. The OpenACS 4 Parties and Groups system is intended to provide the flexibility needed to model complex real-world organizational structures, particularly to support powerful subsite services; that is, where one OpenACS installation can support what appears to the user as distinct web services for different user communities.
A powerful web service that can meet the needs of large enterprises must be able to model the the real world's very rich organizational structures and many ways of decomposing the same organization. For example, a corporation can be broken into structures (the corporation, its divisions, and their departments) or regions (the Boston office, the LA office); a person who is employed by (is a member of) a specific department is also a member of the division and the corporation, and works at (is a member of, but in a different sense) a particular office. OpenACS's Parties and Groups system will support such complex relations faithfully.
Historical Motivations
The primary limitation of the OpenACS 3.x user group system is that it
restricts the application developer to representing a "flat group"
that contains only users: The user_groups table may contain the
group_id of a parent group, but parent-child relationship
support is limited because it only allows one kind of relationship between
groups to be represented. Moreover, the Oracle database's limited support
for tree-like structures makes the queries over these relationships
expensive.
In addition, the Module Scoping design in OpenACS 3.0 introduced a
party abstraction - a thing that is a person or a group of people -
though not in the form of an explicit table. Rather, the triple of
scope, user_id, and group_id columns
was used to identify the party. One disadvantage of this design convention is
that it increases a data model's complexity by requiring the programmer
to:
add these three columns to each "scoped" table
define a multi-column check constraint to protect against data corruption
(e.g., a row with a scope value of "group" but a null
group_id)
perform extra checks in Tcl and PL/SQL
functions and procedures to check both the user_id and
group_id values
In sum, the goal of the Parties and Groups system is to provide OpenACS programmers and site administrators with simple tools that fully describe the complex relationships that exist among groups in the real world.
Pat Developer has a client project and wants to model the company, its offices, its divisions, and its departments as groups and the employees as users.
We start with Groups, which contain members; the member can be either a person or another group (i.e. a member is a party).
In addition to membership, the party and groups system defines a composition relationship that may exist between groups: A group can be a component of another group. The child group is called a component group; the parent group is called a composite group.
A group Gc can be a member and/or a component of another group Gp; the difference is in the way the members of Gc are related to Gp:
If a party P is a member (or a component) of Gc and if Gc is a component of Gp, then P is also a member (or a component) of Gp
If a party P is a member (or a component) of Gc and if Gc is a member of Gp, then no relationship between P and Gp exists as a result of the relationship between Gp and Gp.
Consider an example to make this less abstract: Pretend that the Sierra Club is a member of Greenpeace. The Sierra Club has chapters; each chapter is a component of the Sierra Club. If Eddie Environmentalist is a member of the Massachusetts Chapter of the Sierra Club, Eddie is automatically a member of the Sierra Club, but being a Sierra Club member does not make Eddie a member of Greenpeace.
In the OpenACS, Greenpeace, Sierra Club, and the Sierra Club chapters would be modeled as groups, and Eddie would be a user. There would be a composition relationship between each Sierra Club chapter and the Sierra Club. Membership relationships would exist between Eddie and the Massachusetts Chapter, between Eddie and the Sierra Club (due to Eddie's membership in the Massachusetts chapter), and between the Sierra Club and Greenpeace.
Membership requirements can vary from group to group. The parties and groups system must provide a base type that specifies the bare minimum necessary to join a group.
The parties and groups system must support constraints between a composite group GP and any of its component groups, GC. For example, the system should be able to enforce a rule like: Do not allow a party P to become a member of GC unless P is already a member of GP.
The data model for the parties and groups system must provide support for the following types of entities:
A party is an entity used to represent either a group or a person.
The data model should enforce these constraints:
10.10 A party has an email address, which can be empty.
10.20 A party may have multiple email addresses associated with it.
10.30 The email address of a party must be unique within an OpenACS system.
A group is a collection of zero or more parties.
20.10 The data model should support the subclassing of groups via OpenACS Objects.
A person represents an actual human being, past or present.
A user is a person who has registered with an OpenACS site. A user may have additional attributes, such as a screen name.
The data model should enforce these constraints:
40.10 A user must have a non-empty email address.
40.20 Two different users may not have the same email address on a single OpenACS installation; i.e., an email address identifies a single user on the system.
40.30 A user may have multiple email addresses; for example, two or more email addresses may identify a single user.
40.40 A user must have password field which can be empty.
The data model for the parties and groups system must provide support for the following types of relationships between entities:
A party P is considered a member of a group G
when a direct membership relationship exists between P and G
or when there exists a direct membership relationship between P and some group GC and GC has a composition relationship (c.f., 60.0) with G.
50.10 A party may be a member of multiple groups.
50.20 A party may be a member of the same group multiple times only when all the memberships have different types; for example, Jane may be a member of The Company by being both an Employee and an Executive.
50.30 A party as a member of itself is not supported.
50.40 The data model must support membership constraints.
50.50The data model should support the subclassing of membership via OpenACS Relationships.
A group GC is considered a component of a second group GP
when a direct composition relationship exists between GC and GP
or when there exists a direct composition relationship between GC and some group Gi and Gi has a composition relationship with GP.
60.10A group may be a component of multiple groups.
60.20A group as a component of itself is not supported.
60.30The data model must support component constraints.
60.40The data model should support the subclassing of composition via OpenACS Relationships.
The API should let programmers accomplish the following tasks:
The parties and groups system provides a well defined API call that creates a new group by running the appropriate transactions on the parties and groups system data model. This API is subject to the constraints laid out in the data model.
The parties and groups system provides a well defined API call that creates a new person by running the appropriate transactions on the parties and groups system data model. This API is subject to the constraints laid out in the data model.
The parties and groups system provides a well defined API call that creates a new user by running the appropriate transactions on the parties and groups system data model. This API is subject to the constraints laid out in the data model.
The parties and groups system provides a well defined API call that creates a new user by running the appropriate transactions on an existing person entity. This API is subject to the constraints laid out in the data model.
The parties and groups system provides a well defined API call that demotes an existing user entity to a person entity by running the appropriate transactions on the existing user. This API is subject to the constraints laid out in the data model.
The programmer should be able to modify, add, and delete attributes on any party. This API is subject to the constraints laid out in the data model.
The programmer should be able to view the attributes on any party. This API is subject to the constraints laid out in the data model.
The system provides an API for deleting a party. This API is subject to the constraints laid out in the data model.
100.30 The system may provide a single API call to remove the party from all groups and then delete the party.
100.40 In the case of a group, the system may provide a single API call to remove all parties from a group and then delete the group.
The parties and groups system provides an API for adding a party as a member of a group. This API is subject to the constraints laid out in the data model.
The parties and groups system provides an API for adding a group as a component of a second group. This API is subject to the constraints laid out in the data model.
The parties and groups system provides an API for deleting a party's membership in a group. This API is subject to the constraints laid out in the data model.
The parties and groups system provides an API for deleting a group's composition in a second group. This API is subject to the constraints laid out in the data model.
The parties and groups system provides an API for answering the question: "Is party P a member of group G?"
The parties and groups system provides an API for answering the question: "Is group GC a component of group GP?"
The parties and groups system provides an API for answering the question: "Which parties are members of group G?"
The parties and groups system provides an API for answering the question: "Which groups are components of group G?"
The parties and groups system provides an API for answering the question: "Of which groups is party P a member?"
The parties and groups system provides an API for answering the question: "Of which groups is group G a component?"
The parties and groups system provides an API for answering the question: "Is party P allowed to become a member of group G?"
The parties and groups system provides an API for answering the question: "Is group GC allowed to become a component of group GP?"
Since many pages at a site may check membership in a group before serving a page (e.g., as part of a general permissions check), the data model must support the efficient storage and retrieval of party attributes and membership.
Since many SQL queries will check membership in a group as part of the
where clause, whatever mechanism is used to check membership in SQL
should be fairly small and simple.
The user interface is a set of HTML pages that are used to drive the underlying API. The user interface may provide the following functions:
200.0 Create a party
210.0 View the attributes of a party
220.0 Update the attributes of a party
240.0 Delete a party
250.0 Add a party to a group
260.0 Remove a party from a group
270.0 Perform the membership and composition checks outlined in 130.x to 165.x
| Document Revision # | Action Taken, Notes | When? | By Whom? |
| 0.1 | Creation | 08/16/2000 | Rafael Schloming |
| 0.2 | Initial revision | 08/19/2000 | Mark Thomas |
| 0.3 | Edited and reviewed, conforms to requirements template | 08/23/2000 | Kai Wu |
| 0.4 | Further revised, added UI requirements | 08/24/2000 | Mark Thomas |
| 0.5 | Final edits, pending freeze | 08/24/2000 | Kai Wu |
| 0.6 | More revisions, added composition requirements | 08/30/2000 | Mark Thomas |
| 0.7 | More revisions, added composition requirements | 09/08/2000 | Mark Thomas |