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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.
Alter each Keyspace replication to use class : NetworkTopologyStrategy.
Alter the following:
Keyspace created by the user.
System: system_distributed, system_traces.
For example:
Before
DESCRIBE KEYSPACE mykeyspace;
CREATE KEYSPACE mykespace WITH replication = { 'class' : 'SimpleStrategy', 'replication_factor': '3'};
ALTER Command
ALTER KEYSPACE mykespace WITH replication = { 'class' : 'NetworkTopologyStrategy', '<existing_dc>' : 3};
After
DESCRIBE KEYSPACE mykeyspace;
CREATE KEYSPACE mykespace 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.
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.
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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