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## Zone Failover

Zones allow you to spread your data store across various physical installation locations. The different locations can be anything from different physical buildings near each other, to different racks in the same building. The basic goal of spreading your store across locations is to guard against large-scale infrastructure disruptions, such as power outages or major storm damage, by placing the nodes in your store physically as far apart as possible.

Oracle NoSQL Database provides support for two kinds of zones. Primary zones contain nodes which can serve as masters or replicas. Zones are created as primary zones by default. Secondary zones contain nodes which can serve only as replicas. Secondary zones can be used to make a copy of the data available at a distant location, or to maintain an extra copy of the data to increase redundancy or read capacity.

Both types of zones require high throughput network connections to transmit the replication data required to keep replicas up-to-date. Failing to provide sufficient network capacity will result in nodes in poorly connected zones falling farther and farther behind. Locations connected by low throughput network connections are not suitable for use with zones.

For primary zones, in addition to a high throughput network, the network connections with other primary zones should provide highly reliable and low latency communication. These capabilities make it possible to perform master elections for quick master failovers, and to provide acknowledgments to meet write request timeout requirements. Primary zones are not, therefore, suitable for use with an unreliable or slow wide area network.

For secondary zones, the nodes do not participate in master elections or acknowledgments. For this reason, the system can tolerate reduced reliability or increased latency for connections between secondary and primary zones. The network connections still need to provide sufficient throughput to support replication, and must provide sufficient reliability that temporary interruptions do not interfere with network throughput.

If you deploy your store across multiple zones, then Oracle NoSQL Database tries to physically place at least one Replication Node from each shard in each zone. Whether Oracle NoSQL Database can do this depends on the number of shards in use in your store, the number of zones, the number of Replication Nodes, and the number of physical machines available in each zone. Still, Oracle NoSQL Database makes a best-effort to spread Replication Nodes across available zones. Doing so guards against losing entire shards should the zone become unavailable for any reason.

All of the failover descriptions covered here apply to zones. Failover works across zones in the same way as it does if all nodes are contained within a single zone. Zones offer you the ability for your data to remain available in the event of a large outage. However, read and write capability for any given shard is still gated by whether the remaining zone(s) constitute a majority node partition, and the durability and consistency policies in use for your store activities.