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| <div class="section" id="find-the-misbehaving-nodes"> |
| <h1>Find The Misbehaving Nodes<a class="headerlink" href="#find-the-misbehaving-nodes" title="Permalink to this headline">¶</a></h1> |
| <p>The first step to troubleshooting a Cassandra issue is to use error messages, |
| metrics and monitoring information to identify if the issue lies with the |
| clients or the server and if it does lie with the server find the problematic |
| nodes in the Cassandra cluster. The goal is to determine if this is a systemic |
| issue (e.g. a query pattern that affects the entire cluster) or isolated to a |
| subset of nodes (e.g. neighbors holding a shared token range or even a single |
| node with bad hardware).</p> |
| <p>There are many sources of information that help determine where the problem |
| lies. Some of the most common are mentioned below.</p> |
| <div class="section" id="client-logs-and-errors"> |
| <h2>Client Logs and Errors<a class="headerlink" href="#client-logs-and-errors" title="Permalink to this headline">¶</a></h2> |
| <p>Clients of the cluster often leave the best breadcrumbs to follow. Perhaps |
| client latencies or error rates have increased in a particular datacenter |
| (likely eliminating other datacenter’s nodes), or clients are receiving a |
| particular kind of error code indicating a particular kind of problem. |
| Troubleshooters can often rule out many failure modes just by reading the error |
| messages. In fact, many Cassandra error messages include the last coordinator |
| contacted to help operators find nodes to start with.</p> |
| <p>Some common errors (likely culprit in parenthesis) assuming the client has |
| similar error names as the Datastax <a class="reference internal" href="../getting_started/drivers.html#client-drivers"><span class="std std-ref">drivers</span></a>:</p> |
| <ul class="simple"> |
| <li><code class="docutils literal notranslate"><span class="pre">SyntaxError</span></code> (<strong>client</strong>). This and other <code class="docutils literal notranslate"><span class="pre">QueryValidationException</span></code> |
| indicate that the client sent a malformed request. These are rarely server |
| issues and usually indicate bad queries.</li> |
| <li><code class="docutils literal notranslate"><span class="pre">UnavailableException</span></code> (<strong>server</strong>): This means that the Cassandra |
| coordinator node has rejected the query as it believes that insufficent |
| replica nodes are available. If many coordinators are throwing this error it |
| likely means that there really are (typically) multiple nodes down in the |
| cluster and you can identify them using <a class="reference internal" href="use_nodetool.html#nodetool-status"><span class="std std-ref">nodetool status</span></a> If only a single coordinator is throwing this error it may |
| mean that node has been partitioned from the rest.</li> |
| <li><code class="docutils literal notranslate"><span class="pre">OperationTimedOutException</span></code> (<strong>server</strong>): This is the most frequent |
| timeout message raised when clients set timeouts and means that the query |
| took longer than the supplied timeout. This is a <em>client side</em> timeout |
| meaning that it took longer than the client specified timeout. The error |
| message will include the coordinator node that was last tried which is |
| usually a good starting point. This error usually indicates either |
| aggressive client timeout values or latent server coordinators/replicas.</li> |
| <li><code class="docutils literal notranslate"><span class="pre">ReadTimeoutException</span></code> or <code class="docutils literal notranslate"><span class="pre">WriteTimeoutException</span></code> (<strong>server</strong>): These |
| are raised when clients do not specify lower timeouts and there is a |
| <em>coordinator</em> timeouts based on the values supplied in the <code class="docutils literal notranslate"><span class="pre">cassandra.yaml</span></code> |
| configuration file. They usually indicate a serious server side problem as |
| the default values are usually multiple seconds.</li> |
| </ul> |
| </div> |
| <div class="section" id="metrics"> |
| <h2>Metrics<a class="headerlink" href="#metrics" title="Permalink to this headline">¶</a></h2> |
| <p>If you have Cassandra <a class="reference internal" href="../operating/metrics.html#monitoring-metrics"><span class="std std-ref">metrics</span></a> reporting to a |
| centralized location such as <a class="reference external" href="https://graphiteapp.org/">Graphite</a> or |
| <a class="reference external" href="https://grafana.com/">Grafana</a> you can typically use those to narrow down |
| the problem. At this stage narrowing down the issue to a particular |
| datacenter, rack, or even group of nodes is the main goal. Some helpful metrics |
| to look at are:</p> |
| <div class="section" id="errors"> |
| <h3>Errors<a class="headerlink" href="#errors" title="Permalink to this headline">¶</a></h3> |
| <p>Cassandra refers to internode messaging errors as “drops”, and provided a |
| number of <a class="reference internal" href="../operating/metrics.html#dropped-metrics"><span class="std std-ref">Dropped Message Metrics</span></a> to help narrow |
| down errors. If particular nodes are dropping messages actively, they are |
| likely related to the issue.</p> |
| </div> |
| <div class="section" id="latency"> |
| <h3>Latency<a class="headerlink" href="#latency" title="Permalink to this headline">¶</a></h3> |
| <p>For timeouts or latency related issues you can start with <a class="reference internal" href="../operating/metrics.html#table-metrics"><span class="std std-ref">Table |
| Metrics</span></a> by comparing Coordinator level metrics e.g. |
| <code class="docutils literal notranslate"><span class="pre">CoordinatorReadLatency</span></code> or <code class="docutils literal notranslate"><span class="pre">CoordinatorWriteLatency</span></code> with their associated |
| replica metrics e.g. <code class="docutils literal notranslate"><span class="pre">ReadLatency</span></code> or <code class="docutils literal notranslate"><span class="pre">WriteLatency</span></code>. Issues usually show |
| up on the <code class="docutils literal notranslate"><span class="pre">99th</span></code> percentile before they show up on the <code class="docutils literal notranslate"><span class="pre">50th</span></code> percentile or |
| the <code class="docutils literal notranslate"><span class="pre">mean</span></code>. While <code class="docutils literal notranslate"><span class="pre">maximum</span></code> coordinator latencies are not typically very |
| helpful due to the exponentially decaying reservoir used internally to produce |
| metrics, <code class="docutils literal notranslate"><span class="pre">maximum</span></code> replica latencies that correlate with increased <code class="docutils literal notranslate"><span class="pre">99th</span></code> |
| percentiles on coordinators can help narrow down the problem.</p> |
| <p>There are usually three main possibilities:</p> |
| <ol class="arabic simple"> |
| <li>Coordinator latencies are high on all nodes, but only a few node’s local |
| read latencies are high. This points to slow replica nodes and the |
| coordinator’s are just side-effects. This usually happens when clients are |
| not token aware.</li> |
| <li>Coordinator latencies and replica latencies increase at the |
| same time on the a few nodes. If clients are token aware this is almost |
| always what happens and points to slow replicas of a subset of token |
| ranges (only part of the ring).</li> |
| <li>Coordinator and local latencies are high on many nodes. This usually |
| indicates either a tipping point in the cluster capacity (too many writes or |
| reads per second), or a new query pattern.</li> |
| </ol> |
| <p>It’s important to remember that depending on the client’s load balancing |
| behavior and consistency levels coordinator and replica metrics may or may |
| not correlate. In particular if you use <code class="docutils literal notranslate"><span class="pre">TokenAware</span></code> policies the same |
| node’s coordinator and replica latencies will often increase together, but if |
| you just use normal <code class="docutils literal notranslate"><span class="pre">DCAwareRoundRobin</span></code> coordinator latencies can increase |
| with unrelated replica node’s latencies. For example:</p> |
| <ul class="simple"> |
| <li><code class="docutils literal notranslate"><span class="pre">TokenAware</span></code> + <code class="docutils literal notranslate"><span class="pre">LOCAL_ONE</span></code>: should always have coordinator and replica |
| latencies on the same node rise together</li> |
| <li><code class="docutils literal notranslate"><span class="pre">TokenAware</span></code> + <code class="docutils literal notranslate"><span class="pre">LOCAL_QUORUM</span></code>: should always have coordinator and |
| multiple replica latencies rise together in the same datacenter.</li> |
| <li><code class="docutils literal notranslate"><span class="pre">TokenAware</span></code> + <code class="docutils literal notranslate"><span class="pre">QUORUM</span></code>: replica latencies in other datacenters can |
| affect coordinator latencies.</li> |
| <li><code class="docutils literal notranslate"><span class="pre">DCAwareRoundRobin</span></code> + <code class="docutils literal notranslate"><span class="pre">LOCAL_ONE</span></code>: coordinator latencies and unrelated |
| replica node’s latencies will rise together.</li> |
| <li><code class="docutils literal notranslate"><span class="pre">DCAwareRoundRobin</span></code> + <code class="docutils literal notranslate"><span class="pre">LOCAL_QUORUM</span></code>: different coordinator and replica |
| latencies will rise together with little correlation.</li> |
| </ul> |
| </div> |
| <div class="section" id="query-rates"> |
| <h3>Query Rates<a class="headerlink" href="#query-rates" title="Permalink to this headline">¶</a></h3> |
| <p>Sometimes the <a class="reference internal" href="../operating/metrics.html#table-metrics"><span class="std std-ref">Table</span></a> query rate metrics can help |
| narrow down load issues as “small” increase in coordinator queries per second |
| (QPS) may correlate with a very large increase in replica level QPS. This most |
| often happens with <code class="docutils literal notranslate"><span class="pre">BATCH</span></code> writes, where a client may send a single <code class="docutils literal notranslate"><span class="pre">BATCH</span></code> |
| query that might contain 50 statements in it, which if you have 9 copies (RF=3, |
| three datacenters) means that every coordinator <code class="docutils literal notranslate"><span class="pre">BATCH</span></code> write turns into 450 |
| replica writes! This is why keeping <code class="docutils literal notranslate"><span class="pre">BATCH</span></code>’s to the same partition is so |
| critical, otherwise you can exhaust significant CPU capacitity with a “single” |
| query.</p> |
| </div> |
| </div> |
| <div class="section" id="next-step-investigate-the-node-s"> |
| <h2>Next Step: Investigate the Node(s)<a class="headerlink" href="#next-step-investigate-the-node-s" title="Permalink to this headline">¶</a></h2> |
| <p>Once you have narrowed down the problem as much as possible (datacenter, rack |
| , node), login to one of the nodes using SSH and proceed to debug using |
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