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| <div class="section" id="adding-replacing-moving-and-removing-nodes"> |
| <span id="topology-changes"></span><h1>Adding, replacing, moving and removing nodes<a class="headerlink" href="#adding-replacing-moving-and-removing-nodes" title="Permalink to this headline">¶</a></h1> |
| <div class="section" id="bootstrap"> |
| <h2>Bootstrap<a class="headerlink" href="#bootstrap" title="Permalink to this headline">¶</a></h2> |
| <p>Adding new nodes is called “bootstrapping”. The <code class="docutils literal notranslate"><span class="pre">num_tokens</span></code> parameter will define the amount of virtual nodes |
| (tokens) the joining node will be assigned during bootstrap. The tokens define the sections of the ring (token ranges) |
| the node will become responsible for.</p> |
| <div class="section" id="token-allocation"> |
| <h3>Token allocation<a class="headerlink" href="#token-allocation" title="Permalink to this headline">¶</a></h3> |
| <p>With the default token allocation algorithm the new node will pick <code class="docutils literal notranslate"><span class="pre">num_tokens</span></code> random tokens to become responsible |
| for. Since tokens are distributed randomly, load distribution improves with a higher amount of virtual nodes, but it |
| also increases token management overhead. The default of 256 virtual nodes should provide a reasonable load balance with |
| acceptable overhead.</p> |
| <p>On 3.0+ a new token allocation algorithm was introduced to allocate tokens based on the load of existing virtual nodes |
| for a given keyspace, and thus yield an improved load distribution with a lower number of tokens. To use this approach, |
| the new node must be started with the JVM option <code class="docutils literal notranslate"><span class="pre">-Dcassandra.allocate_tokens_for_keyspace=<keyspace></span></code>, where |
| <code class="docutils literal notranslate"><span class="pre"><keyspace></span></code> is the keyspace from which the algorithm can find the load information to optimize token assignment for.</p> |
| <div class="section" id="manual-token-assignment"> |
| <h4>Manual token assignment<a class="headerlink" href="#manual-token-assignment" title="Permalink to this headline">¶</a></h4> |
| <p>You may specify a comma-separated list of tokens manually with the <code class="docutils literal notranslate"><span class="pre">initial_token</span></code> <code class="docutils literal notranslate"><span class="pre">cassandra.yaml</span></code> parameter, and |
| if that is specified Cassandra will skip the token allocation process. This may be useful when doing token assignment |
| with an external tool or when restoring a node with its previous tokens.</p> |
| </div> |
| </div> |
| <div class="section" id="range-streaming"> |
| <h3>Range streaming<a class="headerlink" href="#range-streaming" title="Permalink to this headline">¶</a></h3> |
| <p>After the tokens are allocated, the joining node will pick current replicas of the token ranges it will become |
| responsible for to stream data from. By default it will stream from the primary replica of each token range in order to |
| guarantee data in the new node will be consistent with the current state.</p> |
| <p>In the case of any unavailable replica, the consistent bootstrap process will fail. To override this behavior and |
| potentially miss data from an unavailable replica, set the JVM flag <code class="docutils literal notranslate"><span class="pre">-Dcassandra.consistent.rangemovement=false</span></code>.</p> |
| </div> |
| <div class="section" id="resuming-failed-hanged-bootstrap"> |
| <h3>Resuming failed/hanged bootstrap<a class="headerlink" href="#resuming-failed-hanged-bootstrap" title="Permalink to this headline">¶</a></h3> |
| <p>On 2.2+, if the bootstrap process fails, it’s possible to resume bootstrap from the previous saved state by calling |
| <code class="docutils literal notranslate"><span class="pre">nodetool</span> <span class="pre">bootstrap</span> <span class="pre">resume</span></code>. If for some reason the bootstrap hangs or stalls, it may also be resumed by simply |
| restarting the node. In order to cleanup bootstrap state and start fresh, you may set the JVM startup flag |
| <code class="docutils literal notranslate"><span class="pre">-Dcassandra.reset_bootstrap_progress=true</span></code>.</p> |
| <p>On lower versions, when the bootstrap proces fails it is recommended to wipe the node (remove all the data), and restart |
| the bootstrap process again.</p> |
| </div> |
| <div class="section" id="manual-bootstrapping"> |
| <h3>Manual bootstrapping<a class="headerlink" href="#manual-bootstrapping" title="Permalink to this headline">¶</a></h3> |
| <p>It’s possible to skip the bootstrapping process entirely and join the ring straight away by setting the hidden parameter |
| <code class="docutils literal notranslate"><span class="pre">auto_bootstrap:</span> <span class="pre">false</span></code>. This may be useful when restoring a node from a backup or creating a new data-center.</p> |
| </div> |
| </div> |
| <div class="section" id="removing-nodes"> |
| <h2>Removing nodes<a class="headerlink" href="#removing-nodes" title="Permalink to this headline">¶</a></h2> |
| <p>You can take a node out of the cluster with <code class="docutils literal notranslate"><span class="pre">nodetool</span> <span class="pre">decommission</span></code> to a live node, or <code class="docutils literal notranslate"><span class="pre">nodetool</span> <span class="pre">removenode</span></code> (to any |
| other machine) to remove a dead one. This will assign the ranges the old node was responsible for to other nodes, and |
| replicate the appropriate data there. If decommission is used, the data will stream from the decommissioned node. If |
| removenode is used, the data will stream from the remaining replicas.</p> |
| <p>No data is removed automatically from the node being decommissioned, so if you want to put the node back into service at |
| a different token on the ring, it should be removed manually.</p> |
| </div> |
| <div class="section" id="moving-nodes"> |
| <h2>Moving nodes<a class="headerlink" href="#moving-nodes" title="Permalink to this headline">¶</a></h2> |
| <p>When <code class="docutils literal notranslate"><span class="pre">num_tokens:</span> <span class="pre">1</span></code> it’s possible to move the node position in the ring with <code class="docutils literal notranslate"><span class="pre">nodetool</span> <span class="pre">move</span></code>. Moving is both a |
| convenience over and more efficient than decommission + bootstrap. After moving a node, <code class="docutils literal notranslate"><span class="pre">nodetool</span> <span class="pre">cleanup</span></code> should be |
| run to remove any unnecessary data.</p> |
| </div> |
| <div class="section" id="replacing-a-dead-node"> |
| <h2>Replacing a dead node<a class="headerlink" href="#replacing-a-dead-node" title="Permalink to this headline">¶</a></h2> |
| <p>In order to replace a dead node, start cassandra with the JVM startup flag |
| <code class="docutils literal notranslate"><span class="pre">-Dcassandra.replace_address_first_boot=<dead_node_ip></span></code>. Once this property is enabled the node starts in a hibernate |
| state, during which all the other nodes will see this node to be DOWN (DN), however this node will see itself as UP |
| (UN). Accurate replacement state can be found in <code class="docutils literal notranslate"><span class="pre">nodetool</span> <span class="pre">netstats</span></code>.</p> |
| <p>The replacing node will now start to bootstrap the data from the rest of the nodes in the cluster. A replacing node will |
| only receive writes during the bootstrapping phase if it has a different ip address to the node that is being replaced. |
| (See CASSANDRA-8523 and CASSANDRA-12344)</p> |
| <p>Once the bootstrapping is complete the node will be marked “UP”.</p> |
| <div class="admonition note"> |
| <p class="admonition-title">Note</p> |
| <p>If any of the following cases apply, you <strong>MUST</strong> run repair to make the replaced node consistent again, since |
| it missed ongoing writes during/prior to bootstrapping. The <em>replacement</em> timeframe refers to the period from when the |
| node initially dies to when a new node completes the replacement process.</p> |
| <ol class="arabic simple"> |
| <li><p>The node is down for longer than <code class="docutils literal notranslate"><span class="pre">max_hint_window_in_ms</span></code> before being replaced.</p></li> |
| <li><p>You are replacing using the same IP address as the dead node <strong>and</strong> replacement takes longer than <code class="docutils literal notranslate"><span class="pre">max_hint_window_in_ms</span></code>.</p></li> |
| </ol> |
| </div> |
| </div> |
| <div class="section" id="monitoring-progress"> |
| <h2>Monitoring progress<a class="headerlink" href="#monitoring-progress" title="Permalink to this headline">¶</a></h2> |
| <p>Bootstrap, replace, move and remove progress can be monitored using <code class="docutils literal notranslate"><span class="pre">nodetool</span> <span class="pre">netstats</span></code> which will show the progress |
| of the streaming operations.</p> |
| </div> |
| <div class="section" id="cleanup-data-after-range-movements"> |
| <h2>Cleanup data after range movements<a class="headerlink" href="#cleanup-data-after-range-movements" title="Permalink to this headline">¶</a></h2> |
| <p>As a safety measure, Cassandra does not automatically remove data from nodes that “lose” part of their token range due |
| to a range movement operation (bootstrap, move, replace). Run <code class="docutils literal notranslate"><span class="pre">nodetool</span> <span class="pre">cleanup</span></code> on the nodes that lost ranges to the |
| joining node when you are satisfied the new node is up and working. If you do not do this the old data will still be |
| counted against the load on that node.</p> |
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