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## Determining Your Store's Configuration

A store consists of a number of Storage Nodes. Each Storage Node can host one or more Replication Nodes, based on its storage capacity. The term `topology` is used to describe the distribution of Replication Nodes. A topology is derived from the number and capacity of available Storage Nodes, the number of partitions in the store, and the replication factors of the store's zones. Topology layouts are also governed by a set of rules that maximize the availability of the store.

All topologies must adhere to the following rules:

1. Each Replication Node from the same shard must reside on a different Storage Node. This rule prevents a single Storage Node failure causing multiple points of failure for a single shard.

2. The number of Replication Nodes assigned to a Storage Node must be less than or equal to the capacity of Storage Nodes.

3. A zone must have one or more Replication Nodes from each shard.

4. A valid Arbiter Node distribution is one in which the Arbiter Node is hosted on a Storage Node that does not contain other members of its shard.

The store’s initial configuration, or topology, is set when you create the store. Over time, it can be necessary to change the store topology. There are several reasons for such a change:

1. You need to replace or upgrade an existing Storage Node.

2. You need to increase read throughput. This is done by increasing the replication factor and creating more copies of the store's data which can be used to service read only requests.

3. You need to increase write throughput. Since each shard has a single master node, distributing the data in the store over a larger number of shards provides the store with more nodes to execute write operations.

You change the store's configuration by changing the number or capacity of available Storage Nodes, or the replication factor of a zone. To change from one configuration to another, you either create a new initial topology, or `clone` an existing topology and modify it into your target topology. You then deploy this target topology.

Note:

Deploying a target topology can be a lengthy operation. Plus, the time required scales with the amount of data to move. During the deployment, the system updates the topology at each step. Because of that, the store passes through intermediate topologies which you did not explicitly create.

This chapter discusses how to make configuration or topological changes to a store. It also describes how to deploy a topology enabled with Arbiter Nodes.

Note:

Do not make configuration changes while a taking a snapshot, or take a snapshot when changing the configuration. Before making configuration changes, we recommend you first create a snapshot as a backup. For additional information on creating snapshots, see Taking a Snapshot.