Sakai Conditional Release Developer's Guide

Motivation

Design

The first design attempt produced a plan for a system that would encapsulate into a Rule arbitrarily many queries to arbitrarily many Sakai services. In this way, if a particular resource were protected by a Rule, accessing the resource could cause a request to the GradebookService, another request to ContentHostingService, and so on. This design was deemed too chatty for reasonable performance in the Resources tool, which must conform to many of the performance characteristics of a file system.

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For example, we want to be able to restrict access to a particular folder to just those students who score above an 80 on a recent quiz. When the instructor grades her students, each assignment grade goes out into Sakai as a Excerpt

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gradebook.updateItemScore Excerpt

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event. The folder in question has an event-listener registered in its name. When all the events go out, the event listener examines each one for the specified condition (grade must be above 80). When a particular event (student's grade) meets the condition, that student's ID is added to the ACL for the folder under protection, and thereafter the student will be able to access it.

Sakai has a built-in mechanism for responding to events, called the NotificationService. Clients of the service ask it to create objects of type Notification. The Notification extends Sakai Entity and specifies three additional key pieces of information: Excerpthiddentrue

1. one or more strings containing event types to respond to. These are called functions.
2. a string called a resource filter which indicates which resource may be affected by the event.
3. an object of type NotificationAction which has the notify() method for taking action in response to the event.

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trueThe conditions module contains an implementation of NotificationAction called ResourceReleaseRule, which also implements Predicate from Apache commons-collections. The purpose of Predicate is to add a Excerpt

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boolean evaluate(Object arg) Excerpt

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method which answers the question "Has the condition been met?" The notify() method checks the condition and then takes the necessary action to update the state of the ContentHostingService.

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We now have all the pieces to describe exactly how conditional release works. Since the application of conditions is asynchronous, there can be an arbitrary space of time between steps 2 and 3: Excerpthiddentrue

1. Instructor edits details on an item in the Resources tool and is presented with "Only if the following conditions is met" option. The list of available assignments and their conditions is passed from the ConditionService with no direct dependency from the Resources tool and the GradebookService.
2. Instructor saves the details, and within ResourcesAction a Predicate is created from the condition parameters and is added to a fresh instance of ResourceReleaseRule which in turn is added to a fresh instance of Notification, along with the event type (function) to respond to and the resource filter (the resource's ID). The NotificationService takes care of persisting the Notification.
3. As events propagate through Sakai, BaseNotificationService examines each one for a matching function in its stored list of Notifications. If a stored Notification matches both the function and resource filter of the inbound event, its embedded NotificationAction is dereferenced and its notify() method is called.
4. In the case of a ResourceReleaseRule, an argument necessary for evaluating the embedded Predicate will be pulled from the event and the embedded Predicate will be evaluated. In response to the evaluation, the access rules of the underlying content resource will either be modified or left alone.

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BooleanExpression

The Predicate within the ResourceReleaseRule is delegated to another class called BooleanExpression. The BooleanExpression is how we programmatically encapsulate such English language questions as "Is the assignment score above 80?" We have a conventional boolean expression here, with a left term, an operator, and a right term. What makes this interesting is that at the time the instructor sets up the condition, a necessary piece of information to answer the question is missing. In our example, the missing piece is the left term, the assignment score in question. This data only materializes later, at the time the instructor submits one or more assignment grades.

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Sakai's BaseNotificationService has its limitations, notably that it requires any implementations of NotificationAction to be physically inside its component {{WEB-INF}} directory. Also, it uses manual manipulation of DOM objects for persistence, which is error-prone and yields object storage in the database as a lump of XML.

TO-DO

Here are a few less glamorous things that need doing. These are some suggestions that any volunteer developer could tackle.

• event pack should not have to declare a dependency on conditions-impl. This is in there now in order to avoid a ClassdefNotFound error when we ask the NotificationService to reconstitute a ResourceReleaseRule from the database. It should be able to get a Class object from the Sakai ComponentManager instead. Look in BaseNotificationService for where it does this:

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  m_action = (NotificationAction) Class.forName(className).newInstance();

• GradebookExternalAssessmentServiceImpl should not use a static cover of the EventTrackingService. This is a case of doing the quick thing rather than the right thing. There's a Spring bean configuration file in there. We just need to modify it to add an injected EventTrackingService.
• The behavior of the content tool UI should not depend on the index number of items in the <select> tags. There are hard-coded references to magic indexes in ResourcesAction, ListItem, and sakai_properties_scripts.vm. It's ugly.
• Speaking of ugly, the UI does not degrade gracefully if any parts of the form become very long. The box for adding a point value becomes orphaned on the next line. Someone with HTML chops could make it nice.