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In contrast, federated databases do not have a single database image, which means they are
multiple independent databases. Data must either be distributed across participating databases
(for large, frequently updated transaction tables) or replicated (for reference data tables that
can be accessed equally from every node). Dividing data entities across databases requires
creating distributed partition views, which are distinct on every node.
??? High.availability:.Before going into details of the availability framework for cluster
systems, it seems valuable to understand the differences between availability within
a single node and within a cluster. In a single node, database system availability
refers to the ability to survive various application and operation failures within the
database instance. In the worst case of the failure of the node, availability refers to
the ability to recover the database to a transaction consistent state as fast as possible.
For a cluster system, aside from handling failure scenarios in a single node, it needs
to handle failure scenarios associated with a node, a group of nodes, or a network,
while providing required performance.
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