ScyllaDB Docs ScyllaDB Manual ScyllaDB for Administrators Procedures Cluster Management Procedures Update Topology Strategy From Simple to Network

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

To check the replication strategy of the above system keyspaces:
```
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

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

ALTER Command

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

After

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

To complete the process, you need to change the snitch, 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.

Stop all traffic to the cluster. A failure to stop the traffic can cause data loss.
Run full repair on the cluster
Alter the strategy as detailed above
Run a second full repair
Run nodetool cleanup on each node
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.

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