This section describes how T-Server supports the hot-standby high-availability (HA) synchronization.
The following figure shows the process of a successful detection of T-Server synchronization. The primary T-Server is assumed to have successfully completed the switch synchronization.
Primary T-Server Still in Start-up Phase
If the primary T-Server is still in the process of switch synchronization when it receives a Backup Readymessage from the backup T-Server, the primary T-Server sends the Full Sync Done message immediately. This allows the backup T-Server to send an EventLinkConnected message and become available. The Management Layer then sets the backup T-Server as the new primary, and vice versa. Once the old primary T-Server finishes switch synchronization, it initiates T-Server synchronization with the new primary T-Server as shown in the table above.
Primary T-Server's Link with the Switch is Down
If the primary T-Server has lost communication with the switch when it receives a Backup Ready message from the other T-Server, it sends the Full Sync Done message immediately. It can be assumed to have lost synchronization with the switch itself and there is no guarantee that it will recover communication with the link, which the backup T-Server currently has.
Backup T-Server Fails During Synchronization
If the backup T-Server fails while waiting for synchronization, the primary T-Server stops the synchronization process.
Primary T-Server Fails During Synchronization
If the primary T-Server fails while waiting for synchronization, then the backup T-Server sends an EventLinkConnected message immediately.
Call Synchronization Between T-Servers
An integral part of T-Server synchronization is the synchronization of the Connection IDs of the calls between T-Servers. It is the Connection IDs of calls created by the backup T-Server during the switch synchronization phase that differ from those in the primary T-Server—those created afterwards are synchronized by the normal HA mechanism. When the primary T-Server receives the Backup Ready message from the backup T-Server, it tags all current calls. Once all tagged calls have been released, the primary T-Server can be certain that the Connection IDs for all current calls have been synchronized with the backup T-Server because they were created after the backup T-Server completed its startup phase. If no further T-Server synchronization is required, the primary T-Server sends the Full Sync Done message to the backup T-Server.