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| <article class="doc"> |
| <h1 class="page">Hints</h1> |
| <div id="preamble"> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <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 href="repair.html" class="page"><code>anti-entropy repair</code></a> does.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Hints are useful because of how Apache Cassandra replicates data to |
| provide fault tolerance, high availability and durability. Cassandra |
| <a href="../architecture/dynamo.html#consistent-hashing-using-a-token-ring" class="page"><code>partitions data across the cluster</code></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> |
| </div> |
| <div class="paragraph"> |
| <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> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="hinted-handoff"><a class="anchor" href="#hinted-handoff"></a><a class="link" href="#hinted-handoff">Hinted Handoff</a></h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Hinted handoff is the process by which Cassandra applies hints to |
| unavailable nodes.</p> |
| </div> |
| <div class="paragraph"> |
| <p>For example, consider a mutation is to be made at <code>Consistency Level</code> |
| <code>LOCAL_QUORUM</code> against a keyspace with <code>Replication Factor</code> of <code>3</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>max_hint_window_in_ms</code> of downtime (defaults to <code>3 hours</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> |
| <div class="imageblock"> |
| <div class="content"> |
| <img src="../_images/hints.svg" alt="Hinted Handoff in Action"> |
| </div> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>(<code>t0</code>): The write is sent by the client, and the coordinator sends it |
| to the three replicas. Unfortunately <code>replica_2</code> is restarting and |
| cannot receive the mutation.</p> |
| </li> |
| <li> |
| <p>(<code>t1</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).</p> |
| </li> |
| <li> |
| <p>(<code>t2</code>): After the write timeout (default <code>2s</code>), the coordinator |
| decides that <code>replica_2</code> is unavailable and stores a hint to its local |
| disk.</p> |
| </li> |
| <li> |
| <p>(<code>t3</code>): Later, when <code>replica_2</code> starts back up it sends a gossip |
| message to all nodes, including the coordinator.</p> |
| </li> |
| <li> |
| <p>(<code>t4</code>): The coordinator replays hints including the missed mutation |
| against <code>replica_2</code>.</p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <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> |
| <div class="sect2"> |
| <h3 id="application-of-hints"><a class="anchor" href="#application-of-hints"></a><a class="link" href="#application-of-hints">Application of Hints</a></h3> |
| <div class="paragraph"> |
| <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> |
| <div class="sect2"> |
| <h3 id="storage-of-hints-on-disk"><a class="anchor" href="#storage-of-hints-on-disk"></a><a class="link" href="#storage-of-hints-on-disk">Storage of Hints on Disk</a></h3> |
| <div class="paragraph"> |
| <p>Hints are stored in flat files in the coordinator node’s |
| <code>$CASSANDRA_HOME/data/hints</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>LZ4Compressor</code>. Multiple hints are appended to the same hints file.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Since hints contain the original unmodified mutation timestamp, hint |
| application is idempotent and cannot overwrite a future mutation.</p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="hints-for-timed-out-write-requests"><a class="anchor" href="#hints-for-timed-out-write-requests"></a><a class="link" href="#hints-for-timed-out-write-requests">Hints for Timed Out Write Requests</a></h3> |
| <div class="paragraph"> |
| <p>Hints are also stored for write requests that time out. The |
| <code>write_request_timeout_in_ms</code> setting in <code>cassandra.yaml</code> configures the |
| timeout for write requests.</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlightjs highlight"><code class="language-none hljs" data-lang="none">write_request_timeout_in_ms: 2000</code></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <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>write_request_timeout_in_ms</code> is 10 ms.</p> |
| </div> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="configuring-hints"><a class="anchor" href="#configuring-hints"></a><a class="link" href="#configuring-hints">Configuring Hints</a></h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Hints are enabled by default as they are critical for data consistency. |
| The <code>cassandra.yaml</code> configuration file provides several settings for |
| configuring hints:</p> |
| </div> |
| <div class="paragraph"> |
| <p>Table 1. Settings for Hints</p> |
| </div> |
| <table class="tableblock frame-all grid-all stretch"> |
| <colgroup> |
| <col style="width: 38%;"> |
| <col style="width: 36%;"> |
| <col style="width: 26%;"> |
| </colgroup> |
| <thead> |
| <tr> |
| <th class="tableblock halign-left valign-top">Setting</th> |
| <th class="tableblock halign-left valign-top">Description</th> |
| <th class="tableblock halign-left valign-top">Default Value</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>hinted_handoff_enabled</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Enables/Disables hinted handoffs</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>true</code></p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>hinted_handoff_disabled_datacenters</code></p></td> |
| <td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph"> |
| <p>A list of data centers that do not perform hinted handoffs even when |
| handoff is otherwise enabled. Example:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlightjs highlight"><code class="language-yaml hljs" data-lang="yaml">hinted_handoff_disabled_datacenters: |
| - DC1 |
| - DC2</code></pre> |
| </div> |
| </div></div></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>unset</code></p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>max_hint_window_in_ms</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Defines the maximum amount of time (ms) a node |
| shall have hints generated after it has failed.</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>10800000</code> # 3 hours</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>hinted_handoff_throttle_in_kb</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">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.)</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>1024</code></p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>max_hints_delivery_threads</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">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</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>2</code></p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>hints_directory</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Directory where Cassandra stores hints.</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>$CASSANDRA_HOME/data/hints</code></p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>hints_flush_period_in_ms</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">How often hints should be flushed from the |
| internal buffers to disk. Will <em>not</em> trigger fsync.</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>10000</code></p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>max_hints_file_size_in_mb</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Maximum size for a single hints file, in |
| megabytes.</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>128</code></p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>hints_compression</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Compression to apply to the hint files. If |
| omitted, hints files will be written uncompressed. LZ4, Snappy, and |
| Deflate compressors are supported.</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>LZ4Compressor</code></p></td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="configuring-hints-at-runtime-with-nodetool"><a class="anchor" href="#configuring-hints-at-runtime-with-nodetool"></a><a class="link" href="#configuring-hints-at-runtime-with-nodetool">Configuring Hints at Runtime with <code>nodetool</code></a></h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p><code>nodetool</code> provides several commands for configuring hints or getting |
| hints related information. The nodetool commands override the |
| corresponding settings if any in <code>cassandra.yaml</code> for the node running |
| the command.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Table 2. Nodetool Commands for Hints</p> |
| </div> |
| <table class="tableblock frame-all grid-all stretch"> |
| <colgroup> |
| <col style="width: 43%;"> |
| <col style="width: 57%;"> |
| </colgroup> |
| <thead> |
| <tr> |
| <th class="tableblock halign-left valign-top">Command</th> |
| <th class="tableblock halign-left valign-top">Description</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>nodetool disablehandoff</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Disables storing and delivering hints</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>nodetool disablehintsfordc</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Disables storing and delivering hints to |
| a data center</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>nodetool enablehandoff</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Re-enables future hints storing and delivery |
| on the current node</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>nodetool enablehintsfordc</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Enables hints for a data center that was |
| previously disabled</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>nodetool getmaxhintwindow</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Prints the max hint window in ms. New in |
| Cassandra 4.0.</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>nodetool handoffwindow</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Prints current hinted handoff window</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>nodetool pausehandoff</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Pauses hints delivery process</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>nodetool resumehandoff</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Resumes hints delivery process</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>nodetool sethintedhandoffthrottlekb</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Sets hinted handoff throttle in |
| kb per second, per delivery thread</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>nodetool setmaxhintwindow</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Sets the specified max hint window in ms</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>nodetool statushandoff</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Status of storing future hints on the current |
| node</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>nodetool truncatehints</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Truncates all hints on the local node, or |
| truncates hints for the endpoint(s) specified.</p></td> |
| </tr> |
| </tbody> |
| </table> |
| <div class="sect2"> |
| <h3 id="make-hints-play-faster-at-runtime"><a class="anchor" href="#make-hints-play-faster-at-runtime"></a><a class="link" href="#make-hints-play-faster-at-runtime">Make Hints Play Faster at Runtime</a></h3> |
| <div class="paragraph"> |
| <p>The default of <code>1024 kbps</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>100 Mbps</code> of data per node, |
| a single 10 minute long restart will create |
| <code>10 minutes * (100 megabit / second) ~= 7 GiB</code> of data which at |
| <code>(1024 KiB / second)</code> would take |
| <code>7.5 GiB / (1024 KiB / second) = 2.03 hours</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> |
| </div> |
| <div class="paragraph"> |
| <p>If you find yourself in such a situation, you may consider raising the |
| <code>hinted_handoff_throttle</code> dynamically via the |
| <code>nodetool sethintedhandoffthrottlekb</code> command.</p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="allow-a-node-to-be-down-longer-at-runtime"><a class="anchor" href="#allow-a-node-to-be-down-longer-at-runtime"></a><a class="link" href="#allow-a-node-to-be-down-longer-at-runtime">Allow a Node to be Down Longer at Runtime</a></h3> |
| <div class="paragraph"> |
| <p>Sometimes a node may be down for more than the normal |
| <code>max_hint_window_in_ms</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>max_hint_window_in_ms</code> dynamically via the |
| <code>nodetool setmaxhintwindow</code> command added in Cassandra 4.0 |
| (<a 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> |
| </div> |
| <div class="paragraph"> |
| <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>max_hint_window_in_ms</code> setting in <code>cassandra.yaml</code> followed |
| by a rolling restart.</p> |
| </div> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="monitoring-hint-delivery"><a class="anchor" href="#monitoring-hint-delivery"></a><a class="link" href="#monitoring-hint-delivery">Monitoring Hint Delivery</a></h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <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 href="https://issues.apache.org/jira/browse/CASSANDRA-13234">CASSANDRA-13234</a>).</p> |
| </div> |
| <div class="paragraph"> |
| <p>There are also metrics available for tracking |
| <code>Hinted Handoff <handoff-metrics></code> and |
| <code>Hints Service <hintsservice-metrics></code> metrics.</p> |
| </div> |
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