Simply put, directory synchronization is the synchronization between two different directories. Directories hold things like users, information, groups, contacts, and more, and directory synchronization allows those objects and identities to be referred to in another directory. Usually, directory synchronization works like a one-way street, allowing this information to flow in one direction. Azure D Connect however is a little bit different as it works more like a two-way street. 

Formerly known as Windows Azure Active Directory Synchronization or DirSync, Azure AD Connect is Microsoft's tool to synchronize your current on-premises directory with Azure AD and allows information to move between directories in both directions. Because of this functionality, Azure AD provides organizations a bunch of different benefits and features, like hybrid identity, single sign-on, multi-factor authentication, AD policies, and more. But let's take a deeper look at how it functions with Azure AD. Starting off, it's important to understand that every object within Azure AD is required to have a single source of authority.

Simply put, you have to have a main authority when managing those objects. When Azure AD Connect is being used to synchronize your directories, your on-premises active directory is the source of authority with objects. These objects are mastered within that on-premises directory by using the management tools for the on-premises AD. The only exception to this rule is licensing, which is all mastered on the cloud within Microsoft 365. Once synchronized from the cloud, that authority is transferred from the cloud to the on-premises active directory,  and as such, the cloud objects become read-only. But what happens when you create or change objects within your on-premises active directory? 

Well, when you create a new user, group, contact objects in the on-premises directory, they're also added into Microsoft 365. When you modify existing users, groups, or contact objects, in your on-premises active directory, those changes are synchronized to Microsoft 365. Likewise, when you delete or disable any of those within your on-premises active directory, they are also deleted or disabled within Microsoft 365. It's important to note, that any changes made in the on-premises active directory do not affect licensing, and as such licenses will still need to be assigned to the object synchronized within Microsoft 365. Now that we know the basics of how Azure AD Connect functions, let's get into different troubleshooting steps.