Virtual node (vnode) configuration

Virtual nodes simplify many tasks in DataStax Enterprise (DSE), such as eliminating the need to determine the partition range, calculate and assign tokens, rebalance the cluster when adding or removing nodes, and replace dead nodes. For a complete description of virtual nodes and how they work, see Virtual nodes.

Guidelines for using virtual nodes

DSE requires the same token architecture on all nodes in a datacenter.

All the nodes must be either vnode-enabled or use a single-token architecture. Across the entire cluster, datacenter architecture can vary.

For example, a single cluster with:
- A transaction-only datacenter running OLTP.
- A single-token architecture search datacenter (no vnodes).
- An analytics datacenter with vnodes.
DataStax recommends using 8 vnodes (tokens).

Restriction: DataStax recommends not using vnodes with DSE Search. However, if you decide to use vnodes with DSE Search, do not use more than 8 vnodes and ensure that you configure the allocate_tokens_for_local_replication_factor option in the cassandra.yaml configuration file to match your environment.

Using 8 vnodes distributes the workload between systems with a ~10% variance and has minimal impact on performance.
Ensure the correct vnode configuration with cassandra.yaml settings:
- When you add a vnode to an existing cluster or set up nodes in a new datacenter, set the target replication factor (RF) of keyspaces in the datacenter with the allocate_tokens_for_local_replication_factor option.
- The allocation algorithm distributes the token ranges proportionately using the num_tokens settings.
  
  All systems in the datacenter should have the same num_tokens settings unless the systems performance varies between systems.
  
  The allocation algorithm efficiently balances the workload using fewer tokens; when systems are added to a datacenter, the algorithm maintains the balance. Using a higher number of tokens more evenly distributes the workload, but also significantly increases token management overhead.
  
  Set the number of vnode tokens based on the workload distribution requirements of your datacenter:
  
  Replication factor 4 vnode (tokens) 8 vnode (tokens) 64 vnode (tokens) 128 vnode (tokens)
  
  2
  
  ~17.5%
  
  ~12.5%
  
  ~3%
  
  ~1%
  
  3
  
  ~14%
  
  ~10%
  
  ~2%
  
  ~1%
  
  5
  
  ~11%
  
  ~7%
  
  ~1%
  
  ~1%

Replication factor	4 vnode (tokens)	8 vnode (tokens)	64 vnode (tokens)	128 vnode (tokens)
2	~17.5%	~12.5%	~3%	~1%
3	~14%	~10%	~2%	~1%
5	~11%	~7%	~1%	~1%

Add nodes to the cluster one at a time.

When adding multiple nodes to the cluster using the allocation algorithm, ensure that nodes are added one at a time. If nodes are added concurrently, the algorithm assigns the same tokens to different nodes.

Enable vnodes

In the cassandra.yaml file:

Uncomment num_tokens and set the required number of tokens.
Recommended: To use the allocation algorithm, uncomment the allocate_tokens_for_local_replication_factor option and set it to the target replication factor for the keyspaces in the datacenter. If the replication varies, alternate between the replication factor (RF) settings.
Comment out the initial_token option or leave it unset.

To upgrade existing clusters to vnodes, see Enable virtual nodes on an existing production cluster.

Disable vnodes

If you do not use vnodes, you must make sure that each node is responsible for roughly an equal amount of data. To ensure that each node is responsible for an equal amount of data, assign each node an initial_token value and calculate the tokens for each datacenter as described in Generate tokens.

In the cassandra.yaml file:

Comment out the num_tokens and allocate_tokens_for_local_replication_factor options.
Uncomment the initial_token option and set it to 1 or to the value of a generated token for a multi-node cluster.

Virtual node (vnode) configuration

Guidelines for using virtual nodes

Enable vnodes

Disable vnodes

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