Durability

Oracle NoSQL Database provides a range of durability policies that specify what guarantees the system makes after a crash. At one extreme, applications can request that write requests block until the record has been written to stable storage on all copies. This has obvious performance and availability implications, but ensures that if the application successfully writes data, that data will persist and can be recovered even if all the copies become temporarily unavailable due to multiple simultaneous failures. At the other extreme, applications can request that write operations return as soon as the system has recorded the existence of the write, even if the data is not persistent anywhere. Such a policy provides the best write performance, but provides no durability guarantees.

The following illustration depicts the range of durability policies that can be used by an application that makes use of Oracle NoSQL Database:

By specifying when the database writes records to disk and what fraction of the copies of the record must be persistent (none, all, or a simple majority), applications can enforce a wide range of durability policies.

Quorum

Operations that modify data in Oracle NoSQL Database require that at least a simple majority of primary nodes be available to form a quorum in the shard that stores the specified key.

Quorum is the minimum number of primary nodes required in a shard, or in the set of admin nodes, to permit electing a master to support write operations. To form a quorum requires that a minimum number of primary nodes represent a majority in the group.

Note:

Secondary nodes are not counted when computing the quorum.

Consider the following example using a store with four zones. Zones 1, 2, and 3 are primary zones with replication factor 1, and zone 4 is a secondary zone with replication factor 1. The number of primary nodes in each shard is 3, which is the sum of the replication factors for the primary zones. In a group of 3 nodes, 2 is the smallest number of nodes that represent a majority, so the quorum is 2. The secondary nodes in zone 4 have no impact on the quorum.

In general, to compute the quorum, first determine the primary replication factor, which is the sum of the replication factors of all primary zones. The quorum value must be one greater than half of the primary replication factor, rounding down when computing the half.

For example, for primary replication factor of 1, the quorum is 1. For primary replication factor of 5 the quorum is 3. For primary replication factor of 6, the quorum is 4.