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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 monitors 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: 100ms, controls how often to perform the more expensive part of host score calculation.dynamic_snitch_reset_interval: 10m, 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.
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.
Cloud-based snitches
These snitches are used in cloud environment for various cloud vendors. All cloud-based snitch implementations are currently extending AbstractCloudMetadataServiceSnitch (which in turn extends AbstractNetworkTopologySnitch).
Each cloud-based snitch has its own way how to resolve what rack and datacenter a respective node belongs to. AbstractCloudMetadataServiceSnitch encapsulates the most common apparatus for achieving it. All cloud-based snitches are calling an HTTP service, specific to a respective cloud. The constructor of AbstractCloudMetadataServiceSnitch accepts implementations of AbstractCloudMetadataServiceConnector which implement a method apiCall which, by default, executes an HTTP GET request against a predefined HTTP URL, sending no HTTP headers, and it expects a response with HTTP code 200. It is possible to send various HTTP headers as part of the request if an implementator wants that.
Currently, the only implementation of AbstractCloudMetadataServiceConnector is DefaultCloudMetadataServiceConnector. If a user has a need to override the behavior of AbstractCloudMetadataServiceConnector, a user is welcome to do so by implementing its own connector and propagating it to the constructor of AbstractCloudMetadataServiceSnitch.
All cloud-based snitches are accepting these properties in cassandra-rackdc.properties:
metadata_url
URL of cloud service to retrieve topology information from, this is cloud-specific.
metadata_request_timeout
Default value of 30s (30 seconds) sets connect timeout upon calls by apiCall. In other words, request against metadata_url will time out if no response arrives in that period.
dc_suffix
A string, by default empty, which will be appended to resolved datacenter.
In-built cloud-based snitches are:
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).
For Ec2 snitches, since CASSANDRA-16555, it is possible to choose version of AWS IMDS. By default, IMDSv2 is used. The version of IMDS is driven by property ec2_metadata_type and can be of value either v1 or v2. It is possible to specify custom URL of IMDS by ec2_metadata_url (or by metadata_url) which is by default [169.254.169.254](http://169.254.169.254) and then a query against /latest/meta-data/placement/availability-zone endpoint is executed.
IMDSv2 is secured by a token which needs to be fetched from IDMSv2 first, and it has to be passed in a header for the actual queries to IDMSv2. Ec2Snitch and Ec2MultiRegionSnitch are doing this automatically. The only configuration parameter exposed to a user is ec2_metadata_token_ttl_seconds which is by default set to 21600. TTL has to be an integer from the range [30, 21600].
AlibabaCloudSnitch
A snitch that assumes an ECS region is a DC and an ECS availability_zone is a rack. This information is available in the config for the node. the format of the zone-id is like cn-hangzhou-a where cn means China, hangzhou means the Hangzhou region, a means the az id. We use cn-hangzhou as the dc, and a as the zone-id. metadata_url for this snitch is, by default, [100.100.100.200/](http://100.100.100.200/) and it will execute an HTTP request against endpoint /latest/meta-data/zone-id.
AzureSnitch
Azure Snitch will resolve datacenter and rack by calling /metadata/instance/compute?api-version=%s&format=json endpoint against metadata_url of [169.254.169.254](http://169.254.169.254) returning the response in JSON format for, by default, API version 2021-12-13. The version of API is configurable via property azure_api_version in cassandra-rackdc.properties. A datacenter is resolved from location field of the response and a rack is resolved by looking into zone field first. When zone is not set, or it is an empty string, it will look into platformFaultDomain field. Such resolved value is prepended by rack- string.
GoogleCloudSnitch
Google snitch will resolve datacenter and rack by calling /computeMetadata/v1/instance/zone endpoint against metadata_url of [metadata.google.internal](http://metadata.google.internal).
