This document discusses billing systems for telecommunications providers and how MongoDB was used to build a billing system for Golan Telecom. It describes how data from different sources like call records, SMS records, and data usage were originally stored in a relational database, requiring complex SQL queries. MongoDB allowed consolidating the different record types into a single collection, making aggregation and billing calculations simpler to implement and more performant. Sample documents show how call, SMS, and data records from various systems are structured and stored in MongoDB to power the billing solution.