What is the difference between DirSync DC and Operational DC?
DirSync DC is a DC that ARS Administration Service uses to communicate with AD. By default, Administration Service selects any available (nearest) DC for a managed domain. This is a per-Serivce per-domain setting. To change this setting, use DirSync Servers tab, either on the managed domain object property sheet (located in the Configuration/Server Configuration/Managed Domains/ folder), or service configuration object property sheet (located in the Configuration/Server Configuration/EDM Servers/ folder).
The DirSync DC is used by Service to load directory data, receive change notification events from AD, lookup information in AD and execute other AD operations. Plus, Service uses DirSync DC by default for all client operations - operations, requested by a particular connected client, until client specified it's own Operational DC.
Operational DC is a DC, specified by client application that is used by Service to execute AD operations, requested by that client. User can set Operational DC for a domain in Snap-in (All Tasks -> Change Operational DC command on the domain object), or in Web Interface (Change Operational DC command on the domain object). User can select "Any writable Domain Controller" option on this dialog, which means usage of DirSync DC for that client. Both Snap-in and Web Interface stores Operational DC between sessions (Snap-in in MSC file, Web Interface in the browser cookie).
When DirSync DC becomes unavailable, Service selects another DC, if "use any available DC (default)" or "use any available DC from this site" options are used. When Operational DC becomes unavailable, Snap-in and Web Interface display an error message and provides user an ability to select another DC.