Thursday, August 25, 2011

Billing in cloud computing - StratosLive billing explained

StratosLive comes with its own billing system to bill the tenants for using the cloud. Lets have a look at the bill generation process of StratosLive. Shown below is a simple diagram to explain the base entities involved in bill generation.

There can be multiple billing engines in your environment. In StratosLive there are two of them. One is for scheduled bill generation (monthly invoice) and the other is for on-demand bill generation (interim invoice). Both these engines use a single billing database. Each of these engines have a set of handlers to be executed. These handlers can be configured via the billing-config.xml. These handlers are executed by the engine at the time of bill generation. Shown below is segment from the billing-config.xml



What are the data used in bill generation?

  • Subscription details – subscriptions of the user for the duration considered in the bill generation. There is a subscription entry for each usage plan which the user was subscribed to during the billing period
  • Usage details – Usage details of the user for the month of bill generation. This includes bandwidth usage (webapp bandwidth, service bandwidth and registry bandwidth) and registry storage of the user.
  • Pricing strategy – This defines how users are going to be charged for subscription and overusage. Shown below is a sample xml file which defines the pricing strategy.





When does bill generation happen?

As I mentioned previously, monthly invoice is generated by a scheduled task. This is achieved via Synapse scheduled tasks. When the billing manager is started (i.e. when the server starts), bill generation task is scheduled for the billing engine. This task scheduling can be configured via the billing-config.xml.

Lets go through the bill generation step-by-step. Billing engine for scheduled bill generation executes the following set of handlers.

1.SubscriptionFeedingHandler – Retrieves the subscription entries relevant to the bill generation period. For each subscription, relevant user's usage data also retrieved. Subscription entries are added to the BillingEngineContext to be used by other handlers.

2.SubscriptionTreeBuildingHandler – Goes through all the subscription in the context and arrange the subscriptions of same customer together. Creates invoices and assigns the subscriptions to them. Fills some data of the invoice such as invoice dates, payments falling under the billing period.

3.RuleHandler – Rules are used to calculate the subscription fees. Rule handler executes the rules over the billing engine context. This will assign a subscription fee for all the subscriptions in the contex.

4.InvoiceCalculationHandler – Over usage charges are calculated by this handler according to the pricing strategy define in a xml file. Total cost, total payments an carried forward values are also calculated. At this moment, there is a complete invoice for all the customers in the memory.

5.Finalizing Handler – This handler takes care of storing the invoices in the billing database.

6.EmailSendingHandler - After generating and storing the invoices, customers have to be notified. It is done by the email sending handler. When this is done you receive an email notifying you about the invoice.

Diagram below gives an idea on what happens in the first four handlers.

Scheduled vs. On-demand bill generation

These two operations are carried out by two billing engines

Scheduled generation executes two additional handlers than the on-demand generation. They are the finalizing handler and the email sending handler. When the invoice is generated on-demand, it is not stored in the database (because it is only for presenting to the user). No email notification is necessary because the invoice shown in the UI. That is the reason for FinalizingHandler and EmailSendingHandler not needed for on-demand bill generation.


The whole bill generation process is execution of set of handlers. If you want to add something new to the flow, you can simply write a handler. Then you need to mention that handler in the billing-config.xml for it to be executed by the engine.

Billing component will be further improved to calculate all the costs via rules. Then it will be super easy to make changes in cost items and pricing because only what we need is to edit the rule file. This avoids any code level change when it comes to changes in calculating the cost which is a huge advantage.

1 comment:

  1. Nice post. I was checking constantly this blog and I am impressed! Extremely helpful information specially the last part I care for such info a lot. time and billing software