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| <div class="section" id="time-window-compactionstrategy"> |
| <span id="twcs"></span><h1>Time Window CompactionStrategy<a class="headerlink" href="#time-window-compactionstrategy" title="Permalink to this headline">¶</a></h1> |
| <p><code class="docutils literal notranslate"><span class="pre">TimeWindowCompactionStrategy</span></code> (TWCS) is designed specifically for workloads where it’s beneficial to have data on |
| disk grouped by the timestamp of the data, a common goal when the workload is time-series in nature or when all data is |
| written with a TTL. In an expiring/TTL workload, the contents of an entire SSTable likely expire at approximately the |
| same time, allowing them to be dropped completely, and space reclaimed much more reliably than when using |
| <code class="docutils literal notranslate"><span class="pre">SizeTieredCompactionStrategy</span></code> or <code class="docutils literal notranslate"><span class="pre">LeveledCompactionStrategy</span></code>. The basic concept is that |
| <code class="docutils literal notranslate"><span class="pre">TimeWindowCompactionStrategy</span></code> will create 1 sstable per file for a given window, where a window is simply calculated |
| as the combination of two primary options:</p> |
| <dl class="simple"> |
| <dt><code class="docutils literal notranslate"><span class="pre">compaction_window_unit</span></code> (default: DAYS)</dt><dd><p>A Java TimeUnit (MINUTES, HOURS, or DAYS).</p> |
| </dd> |
| <dt><code class="docutils literal notranslate"><span class="pre">compaction_window_size</span></code> (default: 1)</dt><dd><p>The number of units that make up a window.</p> |
| </dd> |
| <dt><code class="docutils literal notranslate"><span class="pre">unsafe_aggressive_sstable_expiration</span></code> (default: false)</dt><dd><p>Expired sstables will be dropped without checking its data is shadowing other sstables. This is a potentially |
| risky option that can lead to data loss or deleted data re-appearing, going beyond what |
| <cite>unchecked_tombstone_compaction</cite> does for single sstable compaction. Due to the risk the jvm must also be |
| started with <cite>-Dcassandra.unsafe_aggressive_sstable_expiration=true</cite>.</p> |
| </dd> |
| </dl> |
| <p>Taken together, the operator can specify windows of virtually any size, and <cite>TimeWindowCompactionStrategy</cite> will work to |
| create a single sstable for writes within that window. For efficiency during writing, the newest window will be |
| compacted using <cite>SizeTieredCompactionStrategy</cite>.</p> |
| <p>Ideally, operators should select a <code class="docutils literal notranslate"><span class="pre">compaction_window_unit</span></code> and <code class="docutils literal notranslate"><span class="pre">compaction_window_size</span></code> pair that produces |
| approximately 20-30 windows - if writing with a 90 day TTL, for example, a 3 Day window would be a reasonable choice |
| (<code class="docutils literal notranslate"><span class="pre">'compaction_window_unit':'DAYS','compaction_window_size':3</span></code>).</p> |
| <div class="section" id="timewindowcompactionstrategy-operational-concerns"> |
| <h2>TimeWindowCompactionStrategy Operational Concerns<a class="headerlink" href="#timewindowcompactionstrategy-operational-concerns" title="Permalink to this headline">¶</a></h2> |
| <p>The primary motivation for TWCS is to separate data on disk by timestamp and to allow fully expired SSTables to drop |
| more efficiently. One potential way this optimal behavior can be subverted is if data is written to SSTables out of |
| order, with new data and old data in the same SSTable. Out of order data can appear in two ways:</p> |
| <ul class="simple"> |
| <li><p>If the user mixes old data and new data in the traditional write path, the data will be comingled in the memtables |
| and flushed into the same SSTable, where it will remain comingled.</p></li> |
| <li><p>If the user’s read requests for old data cause read repairs that pull old data into the current memtable, that data |
| will be comingled and flushed into the same SSTable.</p></li> |
| </ul> |
| <p>While TWCS tries to minimize the impact of comingled data, users should attempt to avoid this behavior. Specifically, |
| users should avoid queries that explicitly set the timestamp via CQL <code class="docutils literal notranslate"><span class="pre">USING</span> <span class="pre">TIMESTAMP</span></code>. Additionally, users should run |
| frequent repairs (which streams data in such a way that it does not become comingled).</p> |
| </div> |
| <div class="section" id="changing-timewindowcompactionstrategy-options"> |
| <h2>Changing TimeWindowCompactionStrategy Options<a class="headerlink" href="#changing-timewindowcompactionstrategy-options" title="Permalink to this headline">¶</a></h2> |
| <p>Operators wishing to enable <code class="docutils literal notranslate"><span class="pre">TimeWindowCompactionStrategy</span></code> on existing data should consider running a major compaction |
| first, placing all existing data into a single (old) window. Subsequent newer writes will then create typical SSTables |
| as expected.</p> |
| <p>Operators wishing to change <code class="docutils literal notranslate"><span class="pre">compaction_window_unit</span></code> or <code class="docutils literal notranslate"><span class="pre">compaction_window_size</span></code> can do so, but may trigger |
| additional compactions as adjacent windows are joined together. If the window size is decrease d (for example, from 24 |
| hours to 12 hours), then the existing SSTables will not be modified - TWCS can not split existing SSTables into multiple |
| windows.</p> |
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