1999 seems such a long time ago now, but that's when many of us first faced the need to deploy Active Directory (AD). For many companies, the need arose because Microsoft replaced the Exchange Directory Store (DS), used for Exchange 4.0 to 5.5, with Active Directory in Exchange 2000. The DS was very much like AD.
In an outage that reminded many of the June 2014 problems that affected Exchange Online users in the U.S. for seven hours, the Azure Active Directory infrastructure serving Office 365 and other Microsoft cloud services in Europe experienced major problems on December 3. The effect of the outage was not uniform and appeared to be confined to Northern and Western Europe.