Summary

Rollover bundles are used to enable transfer some of the remaining free units from one period to an another later period. The purpose of this document is to describe, how this functionality is implemented as a part of the core project.

Rollover Concept

Rollover means, that some part of the free units from one SubscriptionBundle which have not been used in one period, can be used later on by an another later SubscriptionBundle. The amount to be 'rolled over' to another bundle can also be called "surplus". The Rollover can be described with four parameters:

1. Maximum rollover amount per SubscriptionBundle - how much can be transferred from one period to another.
2. Rollover validity\Rollover period length - For how long is the rollover valid, i.e. how many periods from the past can be used.
3. Rollover usage mode - Should free units be used first subtracted from surplus or from the current SubscriptionBundle.
4. Internall order of the surplus - Are the 'newer' or 'older' free units used first.

Design and implementation

The starting point for the design, was the fact, that a subscription bundle needs to work in two contexts:

• in the original - when a billing record has the charge date between start and end date of the subscription bundle
• as a container of surplus available for other subscription_bundles.
  In the rating process, it should be a responsibility of the bundle to fetch and update both original and additional, used for surplus subscription bundles.

SubscriptionBundle

The existing SubscriptionBundle_, has been extended with methods defined by RolloverSurplus interface. These are:

• getSurplus - returns amount of units, that can be used by another bundles,
• updateSurplus - updates the surplus with a given value, returns remnant, that is above the maximal surplus value - VALUE_3,
• save - saves the object.

A subscription_bundle that is used by rollover bundles, utilizes all four value parameters:

• VALUE_1 - number of units inside bundle,
• VALUE_2 - number of units used,
• VALUE_3 - number of units, that can be rolled over,
• VALUE_4 - number of used rollover units.

Updating SubscriptionBundle

Standard update

When a SubscriptionBundle object is used in the normal way, the used units are accumulated on VALUE_2. When VALUE_2 is greater than VALUE_1 - VALUE_3, then VALUE_4 gets a new value calculated in the following way VALUE_2 - (VALUE_1 - VALUE_3). If VALUE_4 is greater than VALUE_3 then VALUE_3 is set.

Update through rollover

The usage is aggregated together on VALUE_2 and VALUE_4 until the maximum, defined by VALUE_3 is reached.

Bundle

If a bundle is supposed to be a rollover bundle, it is a requirement for it to implement the RolloverBundle interface. The interface has five methods and two enumeration types:

• getSurplusContainter - returns a SurplusContainter object, that wraps surplus for a given SubscriptionBundle. The SurplusContainter is described in the following paragraph,
• getMaxTransferableAmount - returns the maximum amount of units, that can be transferred as surplus from a SubscriptionBundle,
• getSurplusPeriodOrder - returns one of the values of SURPLUS_PERIOD_ORDER, which indicates, whether SubscriptionBundle used for surplus should be ordered from newer to older or the other way around,
• getSurplusUsageMode - returns one of the values of SURPLUS_PERIOD_ORDER and is used to define, whether surplus should be used before or after SubscriptionBundle,
• getAmountOfRolloverPeriods - returns an amount of previous periods, from which surplus can be used
• SURPLUS_PERIOD_ORDER - an enum with two values: NEWER_FIRST and OLDER_FIRST used by getSurplusPeriodOrder,
• SURPLUS_USAGE_MODE - - an enum with two values: USE_SURPLUS_BEFORE_BUNDLE and USE_SURPLUS_AFTER_BUNDLE used by getSurplusUsageMode,
  Currently, the only Bundle, that implements the RolloverBundle interface is RestrictActive. This means, that DataBundle, DurationBundle, EventBundle and ForwardBundle have a possibility, when properly configured, to behave like a RolloverBundle.

One thing is worth to point out: a new method, for calculating the amount of free units on a SubscriptionBundle, has been introduced on the Bundle_ class . It is called getFreeUnits. This has happened, because the classical VALUE_1 - VALUE_2 is no longer valid, when surplus is used.

SurplusContainer

SurplusContainer is a wrapper object, that encapsulates a list of RolloverSurplus objects. It has two methods:

• getSurplusValue returns sum of surplus for the whole period,
• updateSurplus updates the whole surplus, through looping through the list of RolloverSurplus; returns the remainder from the last RolloverSurplus.updateSurplus.
  The class doesn't have a default constructor and can be only constructed by SurplusContainter(List<RolloverSurplus> surplusList). The list can be contructed with a helper class SurplusContainerFetcher.

Configuration

In order to configure DataBundle, DurationBundle, EventBundle or ForwardBundle to use rollover functionality following parameters need to be set up: