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| <div class="section" id="snitch"> |
| <span id="id1"></span><h1>Snitch<a class="headerlink" href="#snitch" title="Permalink to this headline">¶</a></h1> |
| <p>In cassandra, the snitch has two functions:</p> |
| <ul class="simple"> |
| <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 class="section" id="dynamic-snitching"> |
| <h2>Dynamic snitching<a class="headerlink" href="#dynamic-snitching" title="Permalink to this headline">¶</a></h2> |
| <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 class="docutils literal notranslate"><span class="pre">cassandra.yaml</span></code>:</p> |
| <ul class="simple"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">dynamic_snitch</span></code>: whether the dynamic snitch should be enabled or disabled.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">dynamic_snitch_update_interval_in_ms</span></code>: controls how often to perform the more expensive part of host score |
| calculation.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">dynamic_snitch_reset_interval_in_ms</span></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 class="docutils literal notranslate"><span class="pre">dynamic_snitch_badness_threshold:</span></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 class="section" id="snitch-classes"> |
| <h2>Snitch classes<a class="headerlink" href="#snitch-classes" title="Permalink to this headline">¶</a></h2> |
| <p>The <code class="docutils literal notranslate"><span class="pre">endpoint_snitch</span></code> parameter in <code class="docutils literal notranslate"><span class="pre">cassandra.yaml</span></code> should be set to the class that implements |
| <code class="docutils literal notranslate"><span class="pre">IEndPointSnitch</span></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> |
| <dl class="simple"> |
| <dt>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 class="docutils literal notranslate"><span class="pre">cassandra-topology.properties</span></code> exists, |
| it is used as a fallback, allowing migration from the PropertyFileSnitch.</p> |
| </dd> |
| <dt>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>PropertyFileSnitch</dt><dd><p>Proximity is determined by rack and data center, which are explicitly configured in |
| <code class="docutils literal notranslate"><span class="pre">cassandra-topology.properties</span></code>.</p> |
| </dd> |
| <dt>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 class="reference external" 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>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 class="docutils literal notranslate"><span class="pre">storage_port</span></code> or <code class="docutils literal notranslate"><span class="pre">ssl_storage_port</span></code> on the public IP firewall |
| (For intra-Region traffic, Cassandra will switch to the private IP after establishing a connection).</p> |
| </dd> |
| <dt>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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