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## 11 Consistency Guarantees

A Oracle NoSQL Database store is built from some number of computers (generically referred to as nodes) that are working together using a network. All data in your store is first written to a master node. The master node then copies that data to other nodes in the store. Nodes which are not master nodes are referred to as replicas.

Because of the nature of distributed systems, there is a possibility that, at any given moment, a write operation that was performed on the master node will not yet have been performed on some other node in the store.

Consistency, then, is the policy describing whether it is possible for a row on Node A to be different from the same row on Node B.

When there is a high likelihood that a row stored on one node is identical to the same row stored on another node, we say that we have a high consistency guarantee. Likewise, a low consistency guarantee means that there is a good possibility that a row on one node differs in some way from the same row stored on another node.

You can control how high you want your consistency guarantee to be. Note that the trade-off in setting a high consistency guarantee is that your store's read performance might not be as high as if you use a low consistency guarantee.

There are several different forms of consistency guarantees that you can use. They are described in the following sections.

Note that by default, Oracle NoSQL Database uses the lowest possible consistency possible.