Why are transactions useful?
Before examining how MongoDB treats transactions and its technical requirements, let's examine what transactions are, when they're needed, and the benefits and constraints they bring to your applications.
Formally, transactions are logical units of work. Every transaction is self-contained and defined; performed within a database management system against a database, and generally composed of multiple databases read and/or write operations.
From a practical standpoint, a transaction can be viewed as a sequence of database operations, considered complete only when every operation within the transaction has been executed successfully.
There are numerous situations in which your databases may need to perform read and write operations to and from multiple documents. These documents may reside in the same collection, or they may belong to different collections. These operations often have the constraint that they should all either succeed...
