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| <article class="doc"> |
| <h1 class="page">Snitch</h1> |
| <div id="preamble"> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>In cassandra, the snitch has two functions:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>it teaches Cassandra enough about your network topology to route |
| requests efficiently.</p> |
| </li> |
| <li> |
| <p>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).</p> |
| </li> |
| </ul> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="dynamic-snitching"><a class="anchor" href="#dynamic-snitching"></a>Dynamic snitching</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>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 <code>cassandra.yaml</code>:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><code>dynamic_snitch</code>: whether the dynamic snitch should be enabled or |
| disabled.</p> |
| </li> |
| <li> |
| <p><code>dynamic_snitch_update_interval_in_ms</code>: controls how often to perform |
| the more expensive part of host score calculation.</p> |
| </li> |
| <li> |
| <p><code>dynamic_snitch_reset_interval_in_ms</code>: if set greater than zero, this |
| will allow 'pinning' of replicas to hosts in order to increase cache |
| capacity.</p> |
| </li> |
| <li> |
| <p><code>dynamic_snitch_badness_threshold:</code>: 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.</p> |
| </li> |
| </ul> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="snitch-classes"><a class="anchor" href="#snitch-classes"></a>Snitch classes</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>The <code>endpoint_snitch</code> parameter in <code>cassandra.yaml</code> should be set to the |
| class that implements <code>IEndPointSnitch</code> 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:</p> |
| </div> |
| <div class="dlist"> |
| <dl> |
| <dt class="hdlist1">GossipingPropertyFileSnitch</dt> |
| <dd> |
| <p>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 <code>cassandra-topology.properties</code> exists, it is used as a fallback, |
| allowing migration from the PropertyFileSnitch.</p> |
| </dd> |
| <dt class="hdlist1">SimpleSnitch</dt> |
| <dd> |
| <p>Treats Strategy order as proximity. This can improve cache locality |
| when disabling read repair. Only appropriate for single-datacenter |
| deployments.</p> |
| </dd> |
| <dt class="hdlist1">PropertyFileSnitch</dt> |
| <dd> |
| <p>Proximity is determined by rack and data center, which are explicitly |
| configured in <code>cassandra-topology.properties</code>.</p> |
| </dd> |
| <dt class="hdlist1">Ec2Snitch</dt> |
| <dd> |
| <p>Appropriate for EC2 deployments in a single Region, or in multiple |
| regions with inter-region VPC enabled (available since the end of |
| 2017, see |
| <a href="https://aws.amazon.com/about-aws/whats-new/2017/11/announcing-support-for-inter-region-vpc-peering/">AWS |
| announcement</a>). 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.</p> |
| </dd> |
| <dt class="hdlist1">Ec2MultiRegionSnitch</dt> |
| <dd> |
| <p>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 <code>storage_port</code> or <code>ssl_storage_port</code> |
| on the public IP firewall (For intra-Region traffic, Cassandra will |
| switch to the private IP after establishing a connection).</p> |
| </dd> |
| <dt class="hdlist1">RackInferringSnitch</dt> |
| <dd> |
| <p>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.</p> |
| </dd> |
| </dl> |
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