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| </section> |
| <section class="section" style="padding-top: 2rem;"> |
| <div class="content"> |
| <h1 id="原理">原理</h1> |
| <p>Pegasus支持TTL（Time-To-Live）功能，即在写入数据的时候，可以指定数据的过期时间。数据一旦过期，便对用户不可见了，通过get/multiGet等接口都不再能访问到数据。</p> |
| |
| <p>用户通过<code class="language-plaintext highlighter-rouge">ttl_seconds</code>参数来设置TTL，表示从当前时间开始计算，多少秒之后数据过期。如果为0，则表示不设置TTL，即数据永不过期。</p> |
| |
| <p>这是怎么实现的呢？数据会被立即从磁盘上删除吗？下面来讲讲TTL的实现原理。</p> |
| |
| <p>简单来说，Pegasus的TTL是通过在写数据时记录数据的过期时间，在查询时对过期时间进行检查来实现的。如下图：</p> |
| |
| <p><img src="/assets/images/pegasus-ttl.png" alt="pegasus-ttl.png" class="img-responsive" /></p> |
| |
| <p><strong>写入过程</strong></p> |
| <ul> |
| <li>在写入数据时，用户在客户端通过<code class="language-plaintext highlighter-rouge">ttl_seconds</code>参数设置TTL时间，客户端先计算数据的过期时间<code class="language-plaintext highlighter-rouge">ExpireTime = CurrentTime + ttl_seconds</code>，然后通过RPC将数据和<code class="language-plaintext highlighter-rouge">ExpireTime</code>一起传到ReplicaServer端。</li> |
| <li>ReplicaServer收到写请求后，经过各种处理（包括写WAL、replication复制等），最后将数据存储到RocksDB中。在存储value的时候，会将<code class="language-plaintext highlighter-rouge">ExpireTime</code>放在value头部。</li> |
| </ul> |
| |
| <p><strong>读取过程</strong></p> |
| <ul> |
| <li>用户通过客户端查询指定key对应的value数据</li> |
| <li>ReplicaServer收到读请求后，先从RocksDB获取到key对应的value，然后从value头部提取出<code class="language-plaintext highlighter-rouge">ExpireTime</code>： |
| <ul> |
| <li>如果ExpireTime == 0，表示数据没有设置TTL，是有效的。</li> |
| <li>如果ExpireTime > 0，表示数据设置了TTL，则进一步比较： |
| <ul> |
| <li>如果ExpireTime > now，则数据没有过期，返回value中的用户数据</li> |
| <li>如果ExpireTime <= now，则数据已经过期，返回<code class="language-plaintext highlighter-rouge">NotFound</code></li> |
| </ul> |
| </li> |
| </ul> |
| </li> |
| </ul> |
| |
| <p><strong>数据删除</strong></p> |
| <ul> |
| <li>数据过期后，并不是立即从RocksDB中删除，而是通过<a href="https://github.com/facebook/rocksdb/wiki/Compaction">compaction</a>来进行垃圾回收的。</li> |
| <li>Pegasus使用了自定义的RocksDB <a href="https://github.com/facebook/rocksdb/wiki/Compaction-Filter">CompactionFilter</a>，使其在compaction过程中检查数据value头部的<code class="language-plaintext highlighter-rouge">ExpireTime</code>，如果已经过期，则将数据丢弃，它将不会出现在新生成的文件中。</li> |
| <li>因为过期数据的垃圾回收过程是异步的，与compaction的执行时机和频率有关，所以数据过期与数据删除通常不是同时发生的，唯一能保证的是数据删除肯定发生在数据过期之后。</li> |
| <li>已过期但未删除的数据依然会占用据磁盘空间。</li> |
| </ul> |
| |
| <h1 id="接口">接口</h1> |
| <p>我们在客户端和Shell工具都提供了设置和查询TTL的接口。</p> |
| |
| <p>以Pegasus Java Client为例，获取TTL的接口有：</p> |
| <ul> |
| <li><a href="/zh/clients/java-client#ttl">ttl</a> |
| 设置TTL的接口有：</li> |
| <li><a href="/zh/clients/java-client#set">set</a></li> |
| <li><a href="/zh/clients/java-client#batchset">batchSet</a></li> |
| <li><a href="/zh/clients/java-client#multiset">multiSet</a></li> |
| <li><a href="/zh/clients/java-client#batchmultiset">batchMultiSet</a></li> |
| <li><a href="/zh/clients/java-client#incr">incr</a>（从v1.11.1版本开始）</li> |
| <li><a href="/zh/clients/java-client#checkandset">checkAndSet</a></li> |
| </ul> |
| |
| <p>Shell工具中以下命令可以查询/设置TTL：</p> |
| <ul> |
| <li><a href="/zh/docs/tools/shell/#ttl">ttl</a></li> |
| <li><a href="/zh/docs/tools/shell/#set">set</a></li> |
| <li><a href="/zh/docs/tools/shell/#multi_set">multi_set</a></li> |
| </ul> |
| |
| <h1 id="表级ttl">表级TTL</h1> |
| <p>从v1.11.2版本开始，Pegasus支持表级TTL功能。</p> |
| |
| <h2 id="实现原理">实现原理</h2> |
| <ul> |
| <li>用户在<a href="/zh/administration/table-env">Table环境变量</a>中设置<code class="language-plaintext highlighter-rouge">default_ttl</code>环境变量</li> |
| <li>MetaServer将环境变量异步地同步到到各个ReplicaServer，使该表的每个replica都获取到该环境变量</li> |
| <li>replica获得环境变量后，解析获得<code class="language-plaintext highlighter-rouge">default_ttl</code>配置，并立即生效。此后： |
| <ul> |
| <li>用户新写入的数据，如果<code class="language-plaintext highlighter-rouge">ExpireTime</code> = 0，则将数据的实际<code class="language-plaintext highlighter-rouge">ExpireTime</code>设置为<code class="language-plaintext highlighter-rouge">default_ttl</code></li> |
| <li>RocksDB在进行compaction的时候，如果compact输入文件的原数据<strong>没有</strong><code class="language-plaintext highlighter-rouge">ExpireTime</code>，则将compact输出文件的新数据的<code class="language-plaintext highlighter-rouge">ExpireTime</code>设置为<code class="language-plaintext highlighter-rouge">default_ttl</code></li> |
| <li>由于后台compaction执行时机的不确定性，未设置TTL的数据被设置TTL为<code class="language-plaintext highlighter-rouge">default_ttl</code>的时机也是是不确定的</li> |
| <li>如果想快速设置所有数据的TTL，则可以执行<a href="/zh/administration/manual-compact">Manual Compact</a>。那么所有数据都会被compaction处理，未设置TTL的数据都会被被设置TTL为<code class="language-plaintext highlighter-rouge">default_ttl</code></li> |
| </ul> |
| </li> |
| </ul> |
| |
| <h2 id="应用场景">应用场景</h2> |
| <ul> |
| <li>数据表占用的磁盘空间越来越大。用户想降低磁盘空间占用，或通过清理数据来提升查询性能，降低磁盘、CPU等资源消耗</li> |
| <li>数据表中的所有数据或部分数据没有设置TTL</li> |
| <li>未设置TTL的数据的有效性跟写入时间相关，比如写入时间超过一个月的数据就不再会有查询需求了，可以丢弃 |
| 同时满足以上3个条件的场景，就可以通过<code class="language-plaintext highlighter-rouge">表级TTL</code>和<code class="language-plaintext highlighter-rouge">Manual Compact</code>的功能实现清理磁盘释放资源的目的。</li> |
| </ul> |
| |
| <h1 id="通过ttl计算数据写入时间">通过TTL计算数据写入时间</h1> |
| <p>如果数据写入时设置了TTL，则可以通过TTL计算出数据写入的时间。 |
| 由于：</p> |
| <div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>ExpireTime = InsertTime + TTLSeconds = now + TTLRemainingSeconds |
| </code></pre></div></div> |
| <p>因此：</p> |
| <div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>InsertTime = now + TTLRemainingSeconds - TTLSeconds |
| </code></pre></div></div> |
| <p>其中：</p> |
| <ul> |
| <li>now：执行Shell ttl命令时的时间。</li> |
| <li>TTLRemainingSeconds：通过<a href="/zh/overview/shell#ttl">Shell的ttl命令</a>获取。</li> |
| <li>TTLSeconds：用户写入数据时设置的TTL。</li> |
| </ul> |
| |
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