The xDB was introduced in Sitecore 7.5, and is the scalable alternative to the single SQL analytics database of old. The xDB introduces a dependency on MongoDB , but you can also opt for Sitecore’s xDB in the cloud service.
An overview of all xDB components and how they fit together, including some example scaling diagrams.
An in-depth look at session state and what role it plays in the xDB.
A visitor to Sitecore is known as ‘contact’, and their information is stored in the MongoDB analytics database, which is the data collection component of the xDB.
A contact facet is a property of a contact, such as their name or job title. The default list can easily be extended.
How to create an engagement plan, and what happens to the tree structure in the background.
How to make sure that you engagement plan works as expected - debugging with the xDB.