# Update Topology Strategy From Simple to Network

The following procedure specifies how to update the replication strategy from `SimpleStrategy` to `NetworkTopologyStrategy`.
Note that `SimpleStrategy` is **not** recommended for production usage, and it is strongly advised to create new clusters with `NetworkTopologyStrategy`.

In case you are using `SimpleStrategy`, there are two possible procedures:

* **Procedure with no downtime**: This procedure can be used if any of the following conditions are met:
  * All nodes are on the same rack
  * Regardless of the number of racks, the Replication Factor (RF) of the `SimpleStrategy` keyspace is not changing, and the RF is also equal to the number of cluster nodes. For example, if the RF of a keyspace is 3 and the cluster has 3 nodes, and if the RF is not changing, then the number of racks does not matter.
* **Procedure with downtime**: This procedure should be used if any of the following conditions are met:
  * Cluster nodes are on multiple racks, and the Replication Factor (RF) of the `SimpleStrategy` keyspace is not changing, and the RF is **not** equal to the number of nodes in the cluster.
  * Cluster nodes are on multiple racks, and the Replication Factor (RF) of the `SimpleStrategy` keyspace is changing.

To check which node is on which rack, use `nodetool status` (or `sctool status` from the Scylla Manager node)

## Check keyspaces

Check if the following keyspaces use `SimpleStrategy`:

* Any user-created keyspace.

  To check the replication strategy of a specific user-created keyspace:
  ```cql
  DESCRIBE KEYSPACE mykeyspace;
  ```

  Or to check all user-created keyspaces:
  ```cql
  DESCRIBE SCHEMA;
  ```
* Certain system keyspaces, including `system_distributed`, `system_traces`, `system_audit`.

  To check the replication strategy of the above system keyspaces:
  ```cql
  DESCRIBE KEYSPACE system_distributed;
  DESCRIBE KEYSPACE system_traces;
  DESCRIBE KEYSPACE system_audit;
  ```

If any of the above keyspaces uses `SimpleStrategy`, use the relevant procedure on this page to switch it to `NetworkTopologyStategy`.

## Procedure with no downtime

Alter each Keyspace replication to use `class : NetworkTopologyStrategy`.

For example:

Before

```cql
DESCRIBE KEYSPACE mykeyspace;
CREATE KEYSPACE mykeyspace WITH replication = { 'class' : 'SimpleStrategy', 'replication_factor': '3'};
```

ALTER Command

```cql
ALTER KEYSPACE mykeyspace WITH replication = { 'class' : 'NetworkTopologyStrategy', '<existing_dc>' : 3};
```

After

```cql
DESCRIBE KEYSPACE mykeyspace;
CREATE KEYSPACE mykeyspace WITH replication = { 'class' : 'NetworkTopologyStrategy', '<existing_dc>' : 3};
```

To complete the process, you need to [change the snitch](https://docs.scylladb.com/manual/master/operating-scylla/procedures/config-change/switch-snitch.md), edit the
`cassandra-rackdc.properties` file, and set the preferred data-center name.

## Procedure with downtime

This is a more complex scenario, as the new strategy may select different replicas depending on whether the nodes are on different racks.
To fix that, you will need a **full shutdown of the cluster**.

1. Stop all traffic to the cluster. **A failure to stop the traffic can cause data loss.**
2. Run [full repair on the cluster](https://docs.scylladb.com/manual/master/operating-scylla/procedures/maintenance/repair.md)
3. Alter the strategy as detailed above
4. Run a second full repair
5. Run `nodetool cleanup` on each node
6. Resume traffic

To complete the process, you need to change the Snitch, edit the `cassandra-rackdc.properties` file, and set the preferred data-center name.