# 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 alternatives:

* Nodes are all on the same rack (can be updated without downtime)
* Nodes are on different racks (requires full shutdown)

To check which node is on which rack, use `nodetool status`

Note that if the Replication Factor (RF) of the relevant Keyspace is equal to the number of nodes, regardless of the number of racks, for example, RF=3, nodes=3, you can use the first procedure without downtime.

## All nodes are on the same rack

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

Alter the following:

* Keyspace created by the user.
* System: `system_distributed`, `system_traces`.

For example:

Before

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

ALTER Command

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

After

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

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

## Nodes are on different racks

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://opensource.docs.scylladb.com/stable/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.