Availability Despite the Failure of One Element in a Replica Set

A main goal for TimesTen Scaleout is to provide access to the data even if there are failures.

When k >= 2, the data contained within a replica set is available as long as at least one element in the replica set is up. If an element in the replica set goes down and then recovers, then the element is automatically re-synchronized with another element in its replica set.

Note:

If k = 1, any element failure results in the replica set being down because the replica set contains only a single element. See Recovering When the Replica Set Has a Permanently Failed Element for details on recovery when an element permanently fails when k = 1.

The following example shows a grid where k = 3. Two replica sets are created, each with three elements in the replica set. The element on the host4.instance1 data instance fails. TimesTen Scaleout automatically re-connects to one of the other available elements in the replica set to continue executing the transaction. In this example, TimesTen Scaleout automatically re-connects to the element within the host3.instance1 data instance. While the element on the host4.instance1 data instance is unavailable or in the middle of recovering, the element on the host3.instance1 data instance handles all transactions for the replica set. Once the element on the host4.instance1 data instance recovers, all elements in the replica set can handle transactions.

Figure 13-1 K-Safety Reacts to One Data Instance Failure

Description of "Figure 13-1 K-Safety Reacts to One Data Instance Failure"

Multiple failures in different replica sets do not result in loss of functionality, as long as there is at least one element up in each replica set. You may lose data if an entire replica set fails.

The following example shows a grid where k = 3 with two replica sets. In this example, the elements in the host4.instance1, host6.instance1, and host8.instance1 data instances fail. However, your transactions continue to run since there is at least one element available in each replica set.

Figure 13-2 K-Safety Reacts to Multiple Data Instance Failures

Description of "Figure 13-2 K-Safety Reacts to Multiple Data Instance Failures"