The Coad Letter: Modeling and Design Edition, Issue 76, Modeling User Roles, by Stephen Palmer

By: Coad Letter Modeling Editor

Abstract: In this issue, we look at another archetype of color modeling, roles.

Welcome to the first issue of the CoadLetter for 2001.

Some more object modeling in color to start the year off:

A question that keeps being asked by the teams I work with is, "How do we model the roles of employees and users of a system?"

So this month lets consider some options...

Have fun

Steve



Modeling User Roles

OM:SPA Actor-Participant Pattern

A long time ago (1997 in other words) Pete, with David North and Mark Mayfield, published the second edition of the Object Models: Strategies, Patterns, & Applications book. Page 435 describes a small object model pattern called 'Actor-Participant'. Today this is reflected in the Party and Role class archetypes with added detail including the typical types of attributes and operations you would find in classes of those archetypes.

By the way, it is also very interesting to compare the 'Overview of transactional patterns' on the same page of the OM:SPA book with today's Domain Neutral Component.

Anyway enough of history. Here are five strategies to consider when modeling employee and user roles.

Strategy 2001-01-01: Model Each User Role as a Separate Class

Separate attributes and operations that are specific to each role into separate classes. Associate each role class with the relevant  role-player class (either a Party, Place or Thing). Make the role-player class responsible for ensuring only valid combinations of roles are played by a role-player.

Example

In the example below, the person class is the role-player class and controls the roles that maybe played. Each role is represented by a separate class. Possible attributes and operations are shown for some of the roles. Others are left as an exercise for the reader :-) and to keep the diagram small. Note: It is common in larger systems for Person to be a subclass of a more generic Party class that holds the attributes and operations that are common to both people and organizations.

Advantages:

This strategy leads to a conceptually simple model.
There is good separation of responsibilities.

Disadvantages:

In larger systems this strategy could result in a large number of almost trivial classes.
The constraints on which combinations of roles may be played when are hard-coded into the role-player class.
The role player class has to specifically know about each different role class.

Strategy 2001-01-02: Use a Role superclass and a Description class to simplify the Role-Player class

Introduce an abstract superclass for the individual role classes. Link the role-player class to zero or more instances of the superclass. Add a Description class for the role-player class with an optional Plug-in point for the policy on valid role combinations. Have the role-player class delegate to the Description class (which in turn delegates to its plugin if it has one) when asked questions about valid combinations of roles.

Example

We introduce the PersonRole superclass. This super class contains anything that is common across all the role classes. Often this is just the knowledge of the role-player of the role (if even this were not needed then the superclass could be replaced by an interface in Java). Rather than have a plugin for each instance of the Person class use a description class to hold the policy. (Note: static or class attributes and operations are a slightly less flexible alternative to using a separate Description class).

When it comes to adding or removing a role, the Person class consults the PersonDescription class to see whether, with the combination of roles the Person already holds, the addition or removal is allowed.

Advantages:

The role-player class is simpler, more loosely coupled to the role classes and more reusable as a result.
It is easier to change the policy on which combination of roles can be played and when.

Disadvantages:

Extra classes are required for the superclass, the PersonDescription and the plugin policy classes.

Strategy 2001-01-03: Subsequent Role Strategy

Coding constraints on which combinations of roles can be played can become complex. Model a rigid constraint of 'You can only play role B if you already play role A' as a subsequent role. Instead of linking role B directly to the role-player, link it instead to role A (see Java Design 2nd ed. pg. 133 for more subsequent roles and Java interfaces).

Example

In the example below, the constraint 'A person must be an employee before they can use the system' is modeled as a subsequent role.  Instead of being linked directly to the Person role-player class, the SystemUser role class is linked to the Employee role class. Now for a Person object to be able to play a SystemUser role they must first play an Employee role. Once a Person object  is indirectly associated with a SystemUser role object it can be given further roles (Cashier, Admin, Manager, InventoryClerk).


 

Advantages:

The constraint is modeled explicitly through associations rather than being left to code or an external policy.

Disadvantages:

This only works if the constraint will never change. Imagine a system that was used only by employees but now needs to be opened up to allow internet users limited access. The example above would need some 'refactoring'.
 

Strategy 2001-01-04: Challenge a role. Is it anything more than a set of authorizations or permissions?

Often a role can be little more than a name for a particular combination of application permissions, assignments or authorizations. In these cases, use a more generic role class and derive the specific roles from the associated permissions, assignments or authorizations.

Example

A person playing an employee role could be assigned to manage a department for a time. That assignment makes them a manager.
A Point Of Sale system user may be authorized to work on a particular cash-register. That authorization makes them a Cashier.
In general many user 'roles' can be modeled as a single User role class associated with a particular combination of permissions (operations that the person playing that role may perform).
 

Advantages:

The reduced number of  role classes make it easier to manage inter-role constraints.

Disadvantages:

Bigger more complex role classes requiring deductive logic to identify business roles.

Strategy 2001-01-05: Use a Rules or Policy Engine

For large enterprise systems with many complex combinations of permissions and access rules, have the User role class delegate to a rules engine.

Example

The SystemUser class requests a list of permissions from the rules engine when the user logs on. The SystemUser class hides the use of the rules engine from the rest of the system.

Advantages:

More flexible permissions policy administration.
Third party software available to buy rather than building your own.

Disadvantages:

Using a separate rules engine may impose a performance overhead (in complex cases it may actually improve performance however).
The permissions policy is constrained by limits of the rules engine.

Conclusion

As usual, trade-offs! Design is all about them. There is almost never a single right design. Show me three or four possible solutions, tell me the advantages and disadvantages of each so I can make an informed selection of the solution I want for my context. And, of course, the best answer this time may be a combination of two or three of the possible approaches. This core principle of design can be found at the heart of the analysis and design steps in Feature Driven Development.

Published on: 2/25/2002 12:00:00 AM

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