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    a snitch determines which data centers and racks nodes belong to. snitches inform cassandra about the network topology so that requests are routed efficiently and allows cassandra to distribute replicas by grouping machines into data centers and racks.

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    Snitch

    Cassandra Documentation

    Version:

    4.1

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    Dynamic snitching Snitch classes Edit

    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: 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.

    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.

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    स्रोत : cassandra.apache.org

    Snitches

    A snitch determines which datacenters and racks nodes belong to.

    Snitches

    A snitch determines which datacenters and racks nodes belong to. They inform Cassandra about the network topology so that requests are routed efficiently and allows Cassandra to distribute replicas by grouping machines into datacenters and racks. Specifically, the replication strategy places the replicas based on the information provided by the new snitch. All nodes must return to the same rack and datacenter. Cassandra does its best not to have more than one replica on the same rack (which is not necessarily a physical location).

    Note: If you change snitches, you may need to perform additional steps because the snitch affects where replicas are placed. See Switching snitches.

    स्रोत : docs.datastax.com

    Snitches in Cassandra

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    Snitches in Cassandra

    Last Updated : 16 Jan, 2022

    In this article, we will discuss the Snitch and its types and Understanding how to configure the cassandra-topology.properties and cassandra-rackdc.properties files helps configure data centers and clusters.

    Snitches :

    In Cassandra Snitch is very useful and snitch is also helps in keep record to avoid storing multiple replicas of data on the same rack. In Cassandra, it is very important aspects to avoid multiple replica. In replication strategy we assign number of replica and also we define the data-center. This information is very helpful for snitch to identify the node and which rack belong to.

    In Cassandra, snitch job is to determine which data centers and racks it should use to read data from and write data to. In Cassandra, all snitch are dynamic by default.

    Types of Snitches:SimpleSnitch:

    In Cassandra, It is default snitch and good for development environments. It is unaware of datacenters or racks and also is not look for Cassandra-topologies.properties file and therefore is unusable for multi-datacenter environments.

    GossipingPropertyFileSnitch:

    In Cassandra, it is very important file snitch also recommends by datastax for production usage. This snitch also look for the Cassandra-topologies.properties file to identify the cluster information such that which data center and rack belong to then we configure in the cassandra-rackdc.properties file to the rest of the nodes using gossip.

    We can configure the GossipingPropertyFileSnitch by editing the Cassandra-topologies.properties file.

    Let’s have a look. dc=DC1 rack=RACK1 prefer_local=true

    Here, we are using dc and rack which refers to datacenter and rack and prefer_local=true refers to communicate with local IP address while it is not communicating in multiple data center in order to limit the network bandwidth usage.

    Ec2Snitch:

    It is important snitch for deployments and it is a simple snitch for Amazon EC2 deployments where all nodes are in a single region. In Ec2Snitch region name refers to data center and availability zone refers to rack in a cluster.

    Ec2MultiRegionSnitch:

    In Cassandra, this snitch we use where the clusters span multiple regions and Ec2MultiRegionSnitch for Amazon EC2-based clusters.

    GoogleCloudSnitch:

    In Cassandra, it is the snitch for a Cassandra deployment on the Google Cloud Platform (GCP) across a single or multiple regions. It is the snitch which supports GCP (Google Cloud Plateform).

    RackInferringSnitch:

    In this snitch we find out the location by rack and datacenter. In this snitch the 3rd and 4th octets of IP address for example 10.40.08.230 corresponds to rack and datacenter. This is very useful snitch for writing custom snitch classes.

    PropertyFileSnitch:

    This snitch uses the cassandra-topology.properties file and we must define the nodes information by which we can Determines the closeness of the nodes.

    We can identify nodes information based on the datacenter and rack which they belong to. To determine the closeness of the nodes The PropertyFileSnitch used the network definitions from the cassandra-topology.properties file.

    CloudstackSnitch:

    It is the snitch which based on cloud and It is snitch for an Apache Cloudstack-based cluster.

    Now, let’s understand the cassandra-topology.properties and the cassandra-rackdc.properties Files.

    Understanding the cassandra-topology.properties and the cassandra-rackdc.properties Files: 

    It contains the topology of entire cluster and the information of the cassandra-topology.properties and the cassandra-rackdc.properties Files.

    Let’s take an example.

    dc = DC1 rack = RAC1 rack= RAC2

    In below given example DC1 and DC2 are two physical datacenters and there are two rack for each of them. In Cassandra The PropertyFileSnitch uses the properties file which is cassandra-topologies.properties file to identify the cluster’s node. If we don’t identify cluster’s nodes in the cassandra-topologies.properties file then database assumes that data are in default datacenter and rack.

    # datacenter One

    10.40.08.230 = DC1:RAC1

    10.30.11.231 = DC1:RAC1

    10.54.06.232 = DC1:RAC1

    130.40.20.106 = DC1:RAC2

    130.41.21.229 = DC1:RAC2

    130.42.29.111 = DC1:RAC2

    # datacenter Two

    100.46.12.120 = DC2:RAC1

    100.60.13.201 = DC2:RAC1

    100.24.35.184 = DC2:RAC1

    30.22.20.110 = DC2:RAC2

    30.35.21.210 = DC2:RAC2

    30.27.20.231 = DC2:RAC2

    The Cassandra-topologies.properties file is very important to update while we are going to add or delete nodes from cluster then to make aware that the nodes belong to which datacenter and rack. from performance perspective it is very important to keep record that information to Cassandra.

    Here, we are going to describe the cassandra-rackdc.properties file: In above given example we are using following information of node.

    Let’s have a look. dc=DC1 rack=RAC1

    Note:

    There are following snitch types look up the cassandra-rackdc.properties file for identifying the nodes cluster information such that which data center and which rack belong to.

    Let’s have a look.

    स्रोत : www.geeksforgeeks.org

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