Configuring Database Mirroring in Asynchronous mode
All the transaction log records are sent asynchronously to the mirrored server during the process of the configuration of database mirroring in asynchronous mode. The primary database server doesn’t wait for the acknowledgment of the receipt of the transaction log records before the confirmation of committing to the SAP application from the mirrored server. This refers to a long latency period in transmission of the records and the confirmation does in no way impact the overall response times on the principal server.
Asynchronous mirroring does not ensure that all transactions committed on the principal server have been saved to the mirrored server. This refers to that during the case of a crash of the principal servers the last few committed transactions which take place on the principal server may be lost. Therefore, with the asynchronous mirroring, automatic failover is not possible. In this configuration, a failover to the mirror server should be manually initiated.
The asynchronous mirroring in disaster recovery scenarios incase:
- The small loss of the last few transactions can be tolerated.
- The database is generally mirrored to a remote site over longer distances, thus resulting in very long latency periods.
At present, most of the SAP customers utilize log shipping under alike conditions. Using asynchronous mirroring instead is an effective alternative which improves the overall transactional consistency and this is extremely important for providing a mirrored image of the database which is very close to the principal image of the database. The main disadvantage compared to log shipping is that changes on the principle side are all broadcasted within split seconds and thus they get applied to the mirror database. Therefore, by utilizing the database mirroring, all the errors which are caused by humans can in no way be rectified by causing a delay in restoration of the transaction log records, and all this possible by utilizing the log shipping.
Generally a manual failover can be started at any point in time. SAP processes would be reconnecting to the mirror server. However all the transactions which are open and running would be rolled back. Hence, if the manual failover occurs which is controlled and planned, then its is suggested to close down the SAP applications and force the failover to the be manually mirrored and then restart the applications for more details read next How to Configure SAP Applications for Database Mirroring.