Adding a New Data Center Into an Existing ScyllaDB Cluster¶

Prerequisites¶

1. Log in to one of the nodes in the cluster, collect the following info from the node:

   • cluster_name - grep cluster_name /etc/scylla/scylla.yaml

   • seeds - grep seeds: /etc/scylla/scylla.yaml

   • endpoint_snitch - grep endpoint_snitch /etc/scylla/scylla.yaml

   • ScyllaDB version - scylla --version

   • Authenticator - grep authenticator /etc/scylla/scylla.yaml

2. On all client applications, switch the consistency level to LOCAL_* (LOCAL_ONE, LOCAL_QUORUM,etc.) to prevent the coordinators from accessing the data center you’re adding.

3. Install the new clean ScyllaDB nodes (See Clean Data from Nodes below) on the new datacenter, see Getting Started for further instructions, create as many nodes that you need. Follow the ScyllaDB install procedure up to scylla.yaml configuration phase. In the case that the node starts during the installation process follow these instructions.

Updating the cluster topology requires at least a quorum of nodes in a cluster to be available. If the quorum is lost, it must be restored before you change the cluster topology. See Handling Node Failures for details.

You can check the status of the nodes in the cluster using the nodetool status command.

Clean Data from Nodes¶

sudo rm -rf /var/lib/scylla/data
sudo find /var/lib/scylla/commitlog -type f -delete
sudo find /var/lib/scylla/hints -type f -delete
sudo find /var/lib/scylla/view_hints -type f -delete

Add New DC¶

Procedure

1. For each node in the existing data-center(s) edit the scylla.yaml file to use either

   • Ec2MultiRegionSnitch - for AWS cloud-based, multi-data-center deployments.

   • GossipingPropertyFileSnitch - for bare metal and cloud (other than AWS) deployments.

2. Set the DC and Rack Names. In the cassandra-rackdc.properties file edit the parameters listed below. The file can be found under /etc/scylla/.

   Ec2MultiRegionSnitch

   Ec2MultiRegionSnitch gives each DC and rack default names, the region name defined as datacenter name, and availability zones are defined as racks within a datacenter.

   For example:

   A node in the us-east-1 region, us-east is the data center name, and 1 is the rack location.

   To change the DC names, do the following:

   Edit the cassandra-rackdc.properties file with the preferred datacenter name. The file can be found under /etc/scylla/

   The dc_suffix defines a suffix added to the datacenter name as described below.

   For example:

   Region us-east

   dc_suffix=_1_scylla will be us-east_1_scylla

   Or

   Region us-west

   dc_suffix=_1_scylla will be us-west_1_scylla

   GossipingPropertyFileSnitch

   • dc - Set the datacenter name

   • rack - Set the rack name

   For example:

   # cassandra-rackdc.properties
   #
   # The lines may include white spaces at the beginning and the end.
   # The rack and data center names may also include white spaces.
   # All trailing and leading white spaces will be trimmed.
   #
   dc=thedatacentername
   rack=therackname
   # prefer_local=<false | true>
   # dc_suffix=<Data Center name suffix, used by EC2SnitchXXX snitches>
   

3. In the existing datacenter(s), restart the ScyllaDB nodes one by one.

   Supported OSDocker

   sudo systemctl restart scylla-server
   

4. For each node in the new datacenter edit the scylla.yaml file parameters listed below, the file can be found under /etc/scylla/.

   • cluster_name - Set the selected cluster_name.

   • seeds - IP address of an existing node (or nodes).

   • listen_address - IP address that ScyllaDB used to connect to the other ScyllaDB nodes in the cluster.

   • endpoint_snitch - Set the selected snitch.

   • rpc_address - Address for CQL client connections.

   The parameters seeds, cluster_name and endpoint_snitch need to match the existing cluster.

5. In the new datacenter, set the DC and Rack Names (see step number three for more details).

6. In the new datacenter, start the ScyllaDB nodes one by one.

   Supported OSDocker

   sudo systemctl start scylla-server
   

7. Verify that the nodes were added to the cluster using nodetool status.

   For example:

   $ nodetool status
   
   Datacenter: US-DC
   =========================
   Status=Up/Down
   |/ State=Normal/Leaving/Joining/Moving
   --   Address         Load            Tokens  Owns            Host ID                                 Rack
   UN   54.191.2.121    120.97 KB       256     ?               c84b80ea-cb60-422b-bc72-fa86ede4ac2e    RACK1
   UN   54.191.72.56    109.54 KB       256     ?               129087eb-9aea-4af6-92c6-99fdadb39c33    RACK1
   UN   54.187.25.99    104.94 KB       256     ?               0540c7d7-2622-4f1f-a3f0-acb39282e0fc    RACK1
   
   Datacenter: ASIA-DC
   =======================
   Status=Up/Down
   |/ State=Normal/Leaving/Joining/Moving
   --   Address         Load            Tokens  Owns            Host ID                                 Rack
   UN   54.160.174.243  109.54 KB       256     ?               c7686ffd-7a5b-4124-858e-df2e61130aaa    RACK1
   UN   54.235.9.159    109.75 KB       256     ?               39798227-9f6f-4868-8193-08570856c09a    RACK1
   UN   54.146.228.25   128.33 KB       256     ?               7a4957a1-9590-4434-9746-9c8a6f796a0c    RACK1
   

1. When all nodes are up and running ALTER the following Keyspaces in the new nodes:

   • Keyspace created by the user (which needed to replicate to the new DC).

   • System: system_distributed, system_traces, for example, replicate the data to three nodes in the new DC.

   • audit - if enabled - replicate the data to three nodes in the new DC.

   For example:

   Before

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

   ALTER Command

   ALTER KEYSPACE mykeyspace WITH replication = { 'class' : 'NetworkTopologyStrategy', '<exiting_dc>' : 3, <new_dc> : 3};
   ALTER KEYSPACE system_distributed WITH replication = { 'class' : 'NetworkTopologyStrategy', '<exiting_dc>' : 3, <new_dc> : 3};
   ALTER KEYSPACE system_traces WITH replication = { 'class' : 'NetworkTopologyStrategy', '<exiting_dc>' : 3, <new_dc> : 3};
   

   After

   DESCRIBE KEYSPACE mykeyspace;
   CREATE KEYSPACE mykeyspace WITH REPLICATION = {'class’: 'NetworkTopologyStrategy', <exiting_dc>:3, <new_dc>: 3};
   CREATE KEYSPACE system_distributed WITH replication = { 'class' : 'NetworkTopologyStrategy', '<exiting_dc>' : 3, <new_dc> : 3};
   CREATE KEYSPACE system_traces WITH replication = { 'class' : 'NetworkTopologyStrategy', '<exiting_dc>' : 3, <new_dc> : 3};
   

2. Run nodetool rebuild on each node in the new datacenter, specify the existing datacenter name in the rebuild command.

   For example:

   nodetool rebuild <existing_data_center_name>

   The rebuild ensures that the new nodes that were just added to the cluster will recognize the existing datacenters in the cluster.

3. Run a full cluster repair, using nodetool repair -pr on each node, or using ScyllaDB Manager ad-hoc repair

4. If you are using ScyllaDB Monitoring, update the monitoring stack to monitor it. If you are using ScyllaDB Manager, make sure you install the Manager Agent and Manager can access the new DC.

Configure the Client not to Connect to the New DC¶

This procedure will help your clients not to connect to the DC you just added.

The example below is for clients using the Java driver. Modify this example to suit your needs.

1. One way to prevent clients from connecting to the new DC is to temporarily restrict them to only use the nodes which are in the same DC as the clients. This can be done by ensuring the operations are performed with CL=LOCAL_* (for example LOCAL_QUORUM) and using DCAwareRoundRobinPolicy created by calling withLocalDc("dcLocalToTheClient) and withUsedHostsPerRemoteDc(0) on DCAwareRoundRobinPolicy.Builder.

   Example of DCAwareRoundRobinPolicy creation:

   variable = DCAwareRoundRobinPolicy.builder()
       .withLocalDc("<name of DC local to the client being configured>")
       .withUsedHostsPerRemoteDc(0)
       .build();
   

2. Once the new DC is operational you can remove “withUsedHostsPerRemoteDc(0)” from the configuration and change CL to the previous value.