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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>
 </div>
 </div>


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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>
	</div>
	</div>


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