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K-safety is a measure of fault tolerance in the database cluster. The value K represents the number of replicas of the data in the database that exist in the database cluster. These replicas allow other nodes to take over for failed nodes, allowing the database to continue running while still ensuring data integrity. If more than K nodes in the database fail, some of the data in the database may become unavailable. In that case, the database is considered unsafe and automatically shuts down.
It is possible for an HP Vertica database to have more than K nodes fail and still continue running safely, because the database continues to run as long as every data segment is available on at least one functioning cluster node. Potentially, up to half the nodes in a database with a K-safety level of 1 could fail without causing the database to shut down. As long as the data on each failed node is available from another active node, the database continues to run.
Note: If half or more of the nodes in the database cluster fail, the database will automatically shut down even if all of the data in the database is technically available from replicas. This behavior prevents issues due to network partitioning.
In HP Vertica, the value of K can be zero (0), one (1), or two (2). If a database that has a K-safety of one (K=1) loses a node, the database continues to run normally. Potentially, the database could continue running if additional nodes fail, as long as at least one other node in the cluster has a copy of the failed node's data. Increasing K-safety to 2 ensures that HP Vertica can run normally if any two nodes fail. When the failed node or nodes return and successfully recover, they can participate in database operations again.
The physical schema design must meet certain requirements. To create designs that are K-safe, HP recommends using the Database Designer.
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