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| <li class="toctree-l2 current"><a class="current reference internal" href="#">Hints</a><ul> |
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| <li class="toctree-l3"><a class="reference internal" href="#configuring-hints-at-runtime-with-nodetool">Configuring Hints at Runtime with <code class="docutils literal notranslate"><span class="pre">nodetool</span></code></a></li> |
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| |
| <div class="section" id="hints"> |
| <span id="id1"></span><h1>Hints<a class="headerlink" href="#hints" title="Permalink to this headline">¶</a></h1> |
| <p>Hinting is a data repair technique applied during write operations. When |
| replica nodes are unavailable to accept a mutation, either due to failure or |
| more commonly routine maintenance, coordinators attempting to write to those |
| replicas store temporary hints on their local filesystem for later application |
| to the unavailable replica. Hints are an important way to help reduce the |
| duration of data inconsistency. Coordinators replay hints quickly after |
| unavailable replica nodes return to the ring. Hints are best effort, however, |
| and do not guarantee eventual consistency like <a class="reference internal" href="repair.html#repair"><span class="std std-ref">anti-entropy repair</span></a> does.</p> |
| <p>Hints are useful because of how Apache Cassandra replicates data to provide |
| fault tolerance, high availability and durability. Cassandra <a class="reference internal" href="../architecture/dynamo.html#consistent-hashing-token-ring"><span class="std std-ref">partitions |
| data across the cluster</span></a> using consistent |
| hashing, and then replicates keys to multiple nodes along the hash ring. To |
| guarantee availability, all replicas of a key can accept mutations without |
| consensus, but this means it is possible for some replicas to accept a mutation |
| while others do not. When this happens an inconsistency is introduced.</p> |
| <p>Hints are one of the three ways, in addition to read-repair and |
| full/incremental anti-entropy repair, that Cassandra implements the eventual |
| consistency guarantee that all updates are eventually received by all replicas. |
| Hints, like read-repair, are best effort and not an alternative to performing |
| full repair, but they do help reduce the duration of inconsistency between |
| replicas in practice.</p> |
| <div class="section" id="hinted-handoff"> |
| <h2>Hinted Handoff<a class="headerlink" href="#hinted-handoff" title="Permalink to this headline">¶</a></h2> |
| <p>Hinted handoff is the process by which Cassandra applies hints to unavailable |
| nodes.</p> |
| <p>For example, consider a mutation is to be made at <code class="docutils literal notranslate"><span class="pre">Consistency</span> <span class="pre">Level</span></code> |
| <code class="docutils literal notranslate"><span class="pre">LOCAL_QUORUM</span></code> against a keyspace with <code class="docutils literal notranslate"><span class="pre">Replication</span> <span class="pre">Factor</span></code> of <code class="docutils literal notranslate"><span class="pre">3</span></code>. |
| Normally the client sends the mutation to a single coordinator, who then sends |
| the mutation to all three replicas, and when two of the three replicas |
| acknowledge the mutation the coordinator responds successfully to the client. |
| If a replica node is unavailable, however, the coordinator stores a hint |
| locally to the filesystem for later application. New hints will be retained for |
| up to <code class="docutils literal notranslate"><span class="pre">max_hint_window_in_ms</span></code> of downtime (defaults to <code class="docutils literal notranslate"><span class="pre">3</span> <span class="pre">hours</span></code>). If the |
| unavailable replica does return to the cluster before the window expires, the |
| coordinator applies any pending hinted mutations against the replica to ensure |
| that eventual consistency is maintained.</p> |
| <div class="figure" id="id2"> |
| <img alt="Hinted Handoff Example" src="../_images/hints.svg" /><p class="caption"><span class="caption-text">Hinted Handoff in Action</span></p> |
| </div> |
| <ul class="simple"> |
| <li>(<code class="docutils literal notranslate"><span class="pre">t0</span></code>): The write is sent by the client, and the coordinator sends it |
| to the three replicas. Unfortunately <code class="docutils literal notranslate"><span class="pre">replica_2</span></code> is restarting and cannot |
| receive the mutation.</li> |
| <li>(<code class="docutils literal notranslate"><span class="pre">t1</span></code>): The client receives a quorum acknowledgement from the coordinator. |
| At this point the client believe the write to be durable and visible to reads |
| (which it is).</li> |
| <li>(<code class="docutils literal notranslate"><span class="pre">t2</span></code>): After the write timeout (default <code class="docutils literal notranslate"><span class="pre">2s</span></code>), the coordinator decides |
| that <code class="docutils literal notranslate"><span class="pre">replica_2</span></code> is unavailable and stores a hint to its local disk.</li> |
| <li>(<code class="docutils literal notranslate"><span class="pre">t3</span></code>): Later, when <code class="docutils literal notranslate"><span class="pre">replica_2</span></code> starts back up it sends a gossip message |
| to all nodes, including the coordinator.</li> |
| <li>(<code class="docutils literal notranslate"><span class="pre">t4</span></code>): The coordinator replays hints including the missed mutation |
| against <code class="docutils literal notranslate"><span class="pre">replica_2</span></code>.</li> |
| </ul> |
| <p>If the node does not return in time, the destination replica will be |
| permanently out of sync until either read-repair or full/incremental |
| anti-entropy repair propagates the mutation.</p> |
| <div class="section" id="application-of-hints"> |
| <h3>Application of Hints<a class="headerlink" href="#application-of-hints" title="Permalink to this headline">¶</a></h3> |
| <p>Hints are streamed in bulk, a segment at a time, to the target replica node and |
| the target node replays them locally. After the target node has replayed a |
| segment it deletes the segment and receives the next segment. This continues |
| until all hints are drained.</p> |
| </div> |
| <div class="section" id="storage-of-hints-on-disk"> |
| <h3>Storage of Hints on Disk<a class="headerlink" href="#storage-of-hints-on-disk" title="Permalink to this headline">¶</a></h3> |
| <p>Hints are stored in flat files in the coordinator node’s |
| <code class="docutils literal notranslate"><span class="pre">$CASSANDRA_HOME/data/hints</span></code> directory. A hint includes a hint id, the target |
| replica node on which the mutation is meant to be stored, the serialized |
| mutation (stored as a blob) that couldn’t be delivered to the replica node, the |
| mutation timestamp, and the Cassandra version used to serialize the mutation. |
| By default hints are compressed using <code class="docutils literal notranslate"><span class="pre">LZ4Compressor</span></code>. Multiple hints are |
| appended to the same hints file.</p> |
| <p>Since hints contain the original unmodified mutation timestamp, hint application |
| is idempotent and cannot overwrite a future mutation.</p> |
| </div> |
| <div class="section" id="hints-for-timed-out-write-requests"> |
| <h3>Hints for Timed Out Write Requests<a class="headerlink" href="#hints-for-timed-out-write-requests" title="Permalink to this headline">¶</a></h3> |
| <p>Hints are also stored for write requests that time out. The |
| <code class="docutils literal notranslate"><span class="pre">write_request_timeout_in_ms</span></code> setting in <code class="docutils literal notranslate"><span class="pre">cassandra.yaml</span></code> configures the |
| timeout for write requests.</p> |
| <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>write_request_timeout_in_ms: 2000 |
| </pre></div> |
| </div> |
| <p>The coordinator waits for the configured amount of time for write requests to |
| complete, at which point it will time out and generate a hint for the timed out |
| request. The lowest acceptable value for <code class="docutils literal notranslate"><span class="pre">write_request_timeout_in_ms</span></code> is 10 ms.</p> |
| </div> |
| </div> |
| <div class="section" id="configuring-hints"> |
| <h2>Configuring Hints<a class="headerlink" href="#configuring-hints" title="Permalink to this headline">¶</a></h2> |
| <p>Hints are enabled by default as they are critical for data consistency. The |
| <code class="docutils literal notranslate"><span class="pre">cassandra.yaml</span></code> configuration file provides several settings for configuring |
| hints:</p> |
| <p>Table 1. Settings for Hints</p> |
| <table border="1" class="docutils"> |
| <colgroup> |
| <col width="37%" /> |
| <col width="36%" /> |
| <col width="26%" /> |
| </colgroup> |
| <tbody valign="top"> |
| <tr class="row-odd"><td>Setting</td> |
| <td>Description</td> |
| <td>Default Value</td> |
| </tr> |
| <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">hinted_handoff_enabled</span></code></td> |
| <td>Enables/Disables hinted handoffs</td> |
| <td><code class="docutils literal notranslate"><span class="pre">true</span></code></td> |
| </tr> |
| <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">hinted_handoff_disabled_datacenters</span></code></td> |
| <td><p class="first">A list of data centers that do not perform |
| hinted handoffs even when handoff is |
| otherwise enabled. |
| Example:</p> |
| <blockquote class="last"> |
| <div><div class="highlight-yaml notranslate"><div class="highlight"><pre><span></span><span class="nt">hinted_handoff_disabled_datacenters</span><span class="p">:</span> |
| <span class="p p-Indicator">-</span> <span class="l l-Scalar l-Scalar-Plain">DC1</span> |
| <span class="p p-Indicator">-</span> <span class="l l-Scalar l-Scalar-Plain">DC2</span> |
| </pre></div> |
| </div> |
| </div></blockquote> |
| </td> |
| <td><code class="docutils literal notranslate"><span class="pre">unset</span></code></td> |
| </tr> |
| <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">max_hint_window_in_ms</span></code></td> |
| <td>Defines the maximum amount of time (ms) |
| a node shall have hints generated after it |
| has failed.</td> |
| <td><code class="docutils literal notranslate"><span class="pre">10800000</span></code> # 3 hours</td> |
| </tr> |
| <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">hinted_handoff_throttle_in_kb</span></code></td> |
| <td>Maximum throttle in KBs per second, per |
| delivery thread. This will be reduced |
| proportionally to the number of nodes in |
| the cluster. |
| (If there are two nodes in the cluster, |
| each delivery thread will use the maximum |
| rate; if there are 3, each will throttle |
| to half of the maximum,since it is expected |
| for two nodes to be delivering hints |
| simultaneously.)</td> |
| <td><code class="docutils literal notranslate"><span class="pre">1024</span></code></td> |
| </tr> |
| <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">max_hints_delivery_threads</span></code></td> |
| <td>Number of threads with which to deliver |
| hints; Consider increasing this number when |
| you have multi-dc deployments, since |
| cross-dc handoff tends to be slower</td> |
| <td><code class="docutils literal notranslate"><span class="pre">2</span></code></td> |
| </tr> |
| <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">hints_directory</span></code></td> |
| <td>Directory where Cassandra stores hints.</td> |
| <td><code class="docutils literal notranslate"><span class="pre">$CASSANDRA_HOME/data/hints</span></code></td> |
| </tr> |
| <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">hints_flush_period_in_ms</span></code></td> |
| <td>How often hints should be flushed from the |
| internal buffers to disk. Will <em>not</em> |
| trigger fsync.</td> |
| <td><code class="docutils literal notranslate"><span class="pre">10000</span></code></td> |
| </tr> |
| <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">max_hints_file_size_in_mb</span></code></td> |
| <td>Maximum size for a single hints file, in |
| megabytes.</td> |
| <td><code class="docutils literal notranslate"><span class="pre">128</span></code></td> |
| </tr> |
| <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">hints_compression</span></code></td> |
| <td>Compression to apply to the hint files. |
| If omitted, hints files will be written |
| uncompressed. LZ4, Snappy, and Deflate |
| compressors are supported.</td> |
| <td><code class="docutils literal notranslate"><span class="pre">LZ4Compressor</span></code></td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| <div class="section" id="configuring-hints-at-runtime-with-nodetool"> |
| <h2>Configuring Hints at Runtime with <code class="docutils literal notranslate"><span class="pre">nodetool</span></code><a class="headerlink" href="#configuring-hints-at-runtime-with-nodetool" title="Permalink to this headline">¶</a></h2> |
| <p><code class="docutils literal notranslate"><span class="pre">nodetool</span></code> provides several commands for configuring hints or getting hints |
| related information. The nodetool commands override the corresponding |
| settings if any in <code class="docutils literal notranslate"><span class="pre">cassandra.yaml</span></code> for the node running the command.</p> |
| <p>Table 2. Nodetool Commands for Hints</p> |
| <table border="1" class="docutils"> |
| <colgroup> |
| <col width="43%" /> |
| <col width="57%" /> |
| </colgroup> |
| <tbody valign="top"> |
| <tr class="row-odd"><td>Command</td> |
| <td>Description</td> |
| </tr> |
| <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">nodetool</span> <span class="pre">disablehandoff</span></code></td> |
| <td>Disables storing and delivering hints</td> |
| </tr> |
| <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">nodetool</span> <span class="pre">disablehintsfordc</span></code></td> |
| <td>Disables storing and delivering hints to a |
| data center</td> |
| </tr> |
| <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">nodetool</span> <span class="pre">enablehandoff</span></code></td> |
| <td>Re-enables future hints storing and |
| delivery on the current node</td> |
| </tr> |
| <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">nodetool</span> <span class="pre">enablehintsfordc</span></code></td> |
| <td>Enables hints for a data center that was |
| previously disabled</td> |
| </tr> |
| <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">nodetool</span> <span class="pre">getmaxhintwindow</span></code></td> |
| <td>Prints the max hint window in ms. New in |
| Cassandra 4.0.</td> |
| </tr> |
| <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">nodetool</span> <span class="pre">handoffwindow</span></code></td> |
| <td>Prints current hinted handoff window</td> |
| </tr> |
| <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">nodetool</span> <span class="pre">pausehandoff</span></code></td> |
| <td>Pauses hints delivery process</td> |
| </tr> |
| <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">nodetool</span> <span class="pre">resumehandoff</span></code></td> |
| <td>Resumes hints delivery process</td> |
| </tr> |
| <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">nodetool</span> |
| <span class="pre">sethintedhandoffthrottlekb</span></code></td> |
| <td>Sets hinted handoff throttle in kb |
| per second, per delivery thread</td> |
| </tr> |
| <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">nodetool</span> <span class="pre">setmaxhintwindow</span></code></td> |
| <td>Sets the specified max hint window in ms</td> |
| </tr> |
| <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">nodetool</span> <span class="pre">statushandoff</span></code></td> |
| <td>Status of storing future hints on the |
| current node</td> |
| </tr> |
| <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">nodetool</span> <span class="pre">truncatehints</span></code></td> |
| <td>Truncates all hints on the local node, or |
| truncates hints for the endpoint(s) |
| specified.</td> |
| </tr> |
| </tbody> |
| </table> |
| <div class="section" id="make-hints-play-faster-at-runtime"> |
| <h3>Make Hints Play Faster at Runtime<a class="headerlink" href="#make-hints-play-faster-at-runtime" title="Permalink to this headline">¶</a></h3> |
| <p>The default of <code class="docutils literal notranslate"><span class="pre">1024</span> <span class="pre">kbps</span></code> handoff throttle is conservative for most modern |
| networks, and it is entirely possible that in a simple node restart you may |
| accumulate many gigabytes hints that may take hours to play back. For example if |
| you are ingesting <code class="docutils literal notranslate"><span class="pre">100</span> <span class="pre">Mbps</span></code> of data per node, a single 10 minute long |
| restart will create <code class="docutils literal notranslate"><span class="pre">10</span> <span class="pre">minutes</span> <span class="pre">*</span> <span class="pre">(100</span> <span class="pre">megabit</span> <span class="pre">/</span> <span class="pre">second)</span> <span class="pre">~=</span> <span class="pre">7</span> <span class="pre">GiB</span></code> of data |
| which at <code class="docutils literal notranslate"><span class="pre">(1024</span> <span class="pre">KiB</span> <span class="pre">/</span> <span class="pre">second)</span></code> would take <code class="docutils literal notranslate"><span class="pre">7.5</span> <span class="pre">GiB</span> <span class="pre">/</span> <span class="pre">(1024</span> <span class="pre">KiB</span> <span class="pre">/</span> <span class="pre">second)</span> <span class="pre">=</span> |
| <span class="pre">2.03</span> <span class="pre">hours</span></code> to play back. The exact math depends on the load balancing strategy |
| (round robin is better than token aware), number of tokens per node (more |
| tokens is better than fewer), and naturally the cluster’s write rate, but |
| regardless you may find yourself wanting to increase this throttle at runtime.</p> |
| <p>If you find yourself in such a situation, you may consider raising |
| the <code class="docutils literal notranslate"><span class="pre">hinted_handoff_throttle</span></code> dynamically via the |
| <code class="docutils literal notranslate"><span class="pre">nodetool</span> <span class="pre">sethintedhandoffthrottlekb</span></code> command.</p> |
| </div> |
| <div class="section" id="allow-a-node-to-be-down-longer-at-runtime"> |
| <h3>Allow a Node to be Down Longer at Runtime<a class="headerlink" href="#allow-a-node-to-be-down-longer-at-runtime" title="Permalink to this headline">¶</a></h3> |
| <p>Sometimes a node may be down for more than the normal <code class="docutils literal notranslate"><span class="pre">max_hint_window_in_ms</span></code>, |
| (default of three hours), but the hardware and data itself will still be |
| accessible. In such a case you may consider raising the |
| <code class="docutils literal notranslate"><span class="pre">max_hint_window_in_ms</span></code> dynamically via the <code class="docutils literal notranslate"><span class="pre">nodetool</span> <span class="pre">setmaxhintwindow</span></code> |
| command added in Cassandra 4.0 (<a class="reference external" href="https://issues.apache.org/jira/browse/CASSANDRA-11720">CASSANDRA-11720</a>). |
| This will instruct Cassandra to continue holding hints for the down |
| endpoint for a longer amount of time.</p> |
| <p>This command should be applied on all nodes in the cluster that may be holding |
| hints. If needed, the setting can be applied permanently by setting the |
| <code class="docutils literal notranslate"><span class="pre">max_hint_window_in_ms</span></code> setting in <code class="docutils literal notranslate"><span class="pre">cassandra.yaml</span></code> followed by a rolling |
| restart.</p> |
| </div> |
| </div> |
| <div class="section" id="monitoring-hint-delivery"> |
| <h2>Monitoring Hint Delivery<a class="headerlink" href="#monitoring-hint-delivery" title="Permalink to this headline">¶</a></h2> |
| <p>Cassandra 4.0 adds histograms available to understand how long it takes to deliver |
| hints which is useful for operators to better identify problems (<a class="reference external" href="https://issues.apache.org/jira/browse/CASSANDRA-13234">CASSANDRA-13234</a>).</p> |
| <p>There are also metrics available for tracking <span class="xref std std-ref">Hinted Handoff</span> |
| and <a class="reference internal" href="metrics.html#hintsservice-metrics"><span class="std std-ref">Hints Service</span></a> metrics.</p> |
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