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9.1.2 High availability configuration types

There are two types of highly available data services: - A scalable data service spreads an application across multiple nodes to create a single, logical service. Scalable services leverage the number of nodes and processors in the entire cluster on which they run. In case of a failure this solution means a loss in capacity.

- A failover data service runs an application on only one primary node in the cluster at a time. Other nodes might run other applications, but each application runs on only a single node. If a primary node fails, the applications running on the failed node failover to another node and continue running. We can further divide this category into two subcategories:

Active/Active mode, where two services reside in two nodes that are configured as mutual failover.

Active/Passive mode, where one node is configured as the primary to run the service, while the other node is configured as a hot standby.

The configuration for both modes is very similar. The advantage of the Active/Active mode configuration is lower hardware cost. However, the service performance is reduced when a failover occurs. The advantage of the Active/Passive mode configuration is steady performance, but redundant hardware is needed. Furthermore, the Active/Active mode configuration might have twice as many interruptions as the Active/Passive mode configuration, because a failure in any of two nodes can cause a failover.

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