Identity Synchronization for Windows takes many precautions to ensure that you do not lose user change events— even when components become temporarily unavailable. Identity Synchronization for Windows’ reliability is similar to the TCP network protocol. TCP guarantees that even over a lossy and intermittently connected network, it will eventually deliver all data in order. Data sent during a temporary network outage is queued while the network is down and re-delivered once connectivity is restored. Identity Synchronization for Windows will eventually detect and apply user change events if one of the following components becomes temporarily unavailable:
Active Directory Domain Controller
Windows NT Primary Domain Controller
If one of these components is not available, Identity Synchronization for Windows will delay synchronization until the affected component is available without losing any changes (even to passwords). This version of Identity Synchronization for Windows does not support Sun Cluster or other true, high-availability solutions. Because Identity Synchronization for Windows is a behind-the-scenes application that users do not interact with directly, high availability is not usually required. If you ever experience a catastrophic failure, you can re-install Identity Synchronization for Windows components and use the idsync resync command to re-synchronize all directory sources.
In most situations, when a component is unavailable, the program queues synchronization events and applies them only when the component becomes available. There are two exceptions to this process:
If the preferred Directory Server is unavailable, then the Directory Server Connector will apply changes to one of the available secondary servers from the MMR topology.
While the Active Directory Connector can communicate with a single Active Directory domain controller only, the Directory Server Plug-in can fail between all Active Directory domain controllers while performing on-demand password synchronization. This point is where failover is most important— if the Directory Server Plug-in cannot contact an Active Directory domain controller to verify a user's new password, the user cannot log into Directory Server.