The primary server maintains the active copy of the database file and transaction logs. This server accepts active connections and processes database transactions. All transactions are sent to the mirror server by the primary server.

The mirror server receives all transactions that have been applied to the primary server directly from the primary server. The mirror server becomes the primary server if failover occurs.

The MobiLink server establishes an ODBC connection with the primary server and processes the upload stream from the MobiLink client. It generates the download stream to be sent to the clients. The MobiLink server always connects to the server acting as primary, never to the mirror server.

Data synchronization is initiated when the MobiLink client establishes a connection with the remote database and the MobiLink server. The MobiLink client must scan the active transaction log and offline transaction logs. However, in a high availability environment it is not always known which database is acting as the primary server and where the active and offline transaction logs exist. If offline transaction logs are associated with the database, you must specify -e dir=path-name when starting the MobiLink client. See OfflineDirectory (dir) extended option.

The MobiLink client generates the upload stream from the remote database's transaction log to send to the MobiLink server. The client then processes and applies the download stream.

To prevent your high availability data synchronization system from having a single point of failure, it is recommended that you use multiple MobiLink servers and load balancers. In this case, each component of the data synchronization environment is redundant and the system functions even if hardware or software failures occur in the environment. For more information about using multiple MobiLink servers, see Run the MobiLink server in a server farm and Separately licensed components.

Similarly, to avoid a single point of failure at the remote database, an implementation using an external procedure, such as the xp_cmdshell system procedure, to call the MobiLink client can be triggered via a scheduled event that is stored in both the primary and mirror databases.

• xp_cmdshell system procedure
• SQL Anywhere high availability
• MobiLink server technology