There are 2 levels of high availability, application level and database level.

On the application level, a customer can deploy multiple instances of the DualShield servers to achieve high availability. After adding and configuring multiple instances of the DualShield servers in your network, you can configure failover and balance the load across all instances of the DualShield servers. To do so, you must place a load-balancer in front of the cluster of multiple instances of the DualShield servers.

On the database level, high availability is achieved by the SQL server used by the DualShield server. DualShield supports MySQL, Microsoft SQL and Oracle SQL. All of those SQL servers support high availability.

Generally speaking, there are 2 types of SQL high availability clusters, namely Mirroring Cluster and Replication Cluster.

SQL Mirroring Cluster

In a Mirroring Cluster, there is an active SQL database server and a passive database server. When the active database server is down for some reason, the cluster will fail over to the passive database server, and the passive database will become the active database server. 

In a Mirroring Cluster, only one database server is active at one time and the database operation is served from the active server only. In principle, data change in the active database is copied to the passive database in real time. 

SQL Replication Cluster

In a Replication Clusters, there are multiple SQL database servers and all of them are active at one time. Data change in any database is replicated to the other databases in real time. 

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