Snitch

In cassandra, the snitch has two functions:

  • it teaches Cassandra enough about your network topology to route requests efficiently.
  • it allows Cassandra to spread replicas around your cluster to avoid correlated failures. It does this by grouping machines into “datacenters” and “racks.” Cassandra will do its best not to have more than one replica on the same “rack” (which may not actually be a physical location).

Dynamic snitching

The dynamic snitch monitor read latencies to avoid reading from hosts that have slowed down. The dynamic snitch is configured with the following properties on cassandra.yaml:

  • dynamic_snitch: whether the dynamic snitch should be enabled or disabled.
  • dynamic_snitch_update_interval_in_ms: controls how often to perform the more expensive part of host score calculation.
  • dynamic_snitch_reset_interval_in_ms: if set greater than zero, this will allow ‘pinning’ of replicas to hosts in order to increase cache capacity.
  • dynamic_snitch_badness_threshold:: The badness threshold will control how much worse the pinned host has to be before the dynamic snitch will prefer other replicas over it. This is expressed as a double which represents a percentage. Thus, a value of 0.2 means Cassandra would continue to prefer the static snitch values until the pinned host was 20% worse than the fastest.

Snitch classes

The endpoint_snitch parameter in cassandra.yaml should be set to the class that implements IEndPointSnitch which will be wrapped by the dynamic snitch and decide if two endpoints are in the same data center or on the same rack. Out of the box, Cassandra provides the snitch implementations:

GossipingPropertyFileSnitch
This should be your go-to snitch for production use. The rack and datacenter for the local node are defined in cassandra-rackdc.properties and propagated to other nodes via gossip. If cassandra-topology.properties exists, it is used as a fallback, allowing migration from the PropertyFileSnitch.
SimpleSnitch
Treats Strategy order as proximity. This can improve cache locality when disabling read repair. Only appropriate for single-datacenter deployments.
PropertyFileSnitch
Proximity is determined by rack and data center, which are explicitly configured in cassandra-topology.properties.
Ec2Snitch
Appropriate for EC2 deployments in a single Region, or in multiple regions with inter-region VPC enabled (available since the end of 2017, see AWS announcement). Loads Region and Availability Zone information from the EC2 API. The Region is treated as the datacenter, and the Availability Zone as the rack. Only private IPs are used, so this will work across multiple regions only if inter-region VPC is enabled.
Ec2MultiRegionSnitch
Uses public IPs as broadcast_address to allow cross-region connectivity (thus, you should set seed addresses to the public IP as well). You will need to open the storage_port or ssl_storage_port on the public IP firewall (For intra-Region traffic, Cassandra will switch to the private IP after establishing a connection).
RackInferringSnitch
Proximity is determined by rack and data center, which are assumed to correspond to the 3rd and 2nd octet of each node’s IP address, respectively. Unless this happens to match your deployment conventions, this is best used as an example of writing a custom Snitch class and is provided in that spirit.