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| <section class="section" style="padding-top: 2rem;"> |
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| <p>在 pegasus 中，跨机房同步又被称为 <strong><em>热备份</em></strong>，或 <strong><em>duplication</em></strong>，简称 <strong><em>dup</em></strong>。这一功能的主要目的是保证 <strong>数据中心级别的可用性</strong>。当业务需要保证服务与数据能够容忍机房故障时，可以考虑使用此功能。</p> |
| |
| <p>此外，当 Pegasus 客户端在多机房分布时，时常会遇到跨机房访问 Pegasus 服务带来的高延时问题，这时我们可以将 Pegasus 的服务与客户端部署在相同的机房内，客户端可以只读写本地机房的服务，然后由热备份功能将写同步到各个机房上。这种做法既能保证各个机房都有完整数据，又能避免跨机房的延时开销。</p> |
| |
| <div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code> client client client |
| + + + |
| +---------v-------+ +--------v--------+ +------v-----------+ |
| | | | | | | |
| | pegasus-beijing <---> pegasus-tianjin <---> pegasus-shanghai | |
| | | | | | | |
| +----------^------+ +-----------------+ +---------^--------+ |
| | | |
| +------------------------------------------+ |
| </code></pre></div></div> |
| |
| <p>我们能够做到<strong>一主一备（single-master）</strong>，也能提供<strong>多机房多主（multi-master）</strong>，用户可以根据需要进行配置。</p> |
| |
| <p>这里需要注意的是，跨机房同步是<strong>异步</strong>的数据复制，并非完全实时。与单机房不同，该功能不提供跨机房 <em>read-after-write</em> 的一致性保证。目前在跨机房网络健康的环境下，对于备集群而言，数据延时大概在秒级左右，但具体表现与写入流量大小有关。经测试，写入小于1KB的数据的延时在1秒内，即 A 机房的写数据大概在1秒后会写入 B 机房。</p> |
| |
| <h2 id="操作上手">操作上手</h2> |
| |
| <p>假设我们有两个 pegasus 集群 <code class="language-plaintext highlighter-rouge">bjsrv-account</code> (源集群)和 <code class="language-plaintext highlighter-rouge">tjsrv-account</code>(目标集群)，分别位于北京与天津的两个机房内，表 <code class="language-plaintext highlighter-rouge">my_source_app</code> 由于存储了极其关键的用户帐号数据，需要能够在双集群保证可用，所以我们为它实施热备份：</p> |
| |
| <div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>#The cluster name is: bjsrv-account |
| #The cluster meta list is: ****** |
| |
| >./admin-cli -m ****** //use meta list to connect cluster |
| |
| >>> ls |
| app_id status app_name |
| 12 AVAILABLE my_source_app |
| |
| >>> use my_source_app |
| >>> dup add -c my_target_cluster -p |
| successfully add duplication [dupid: 1669972761] |
| |
| >>> dup list |
| [ |
| { |
| "dupid": 1692240106, |
| "status": "DS_LOG", |
| "remote": "tjsrv-account", |
| "create_ts": 1692240106066, |
| "fail_mode": "FAIL_SLOW" |
| } |
| ] |
| </code></pre></div></div> |
| |
| <p>通过 <code class="language-plaintext highlighter-rouge">dup add</code> 命令，bjsrv-account 集群的表 my_source_app 将会近实时地把数据复制到 tjsrv-account 上，这意味着，每一条在北京机房的写入，最终都一定会复制到天津机房。</p> |
| |
| <p>热备份使用日志异步复制的方式来实现跨集群的同步，可与 MySQL 的 binlog 复制和 HBase replication 类比。</p> |
| |
| <p>热备份功能<strong>以表为粒度</strong>，你可以只对集群内一部分表实施热备份。热备份的两集群的表名需要保持一致，但 partition 的个数不需要相同。例如用户可以建表如下：</p> |
| |
| <div class="language-sh highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c">## bjsrv-account</span> |
| <span class="o">>>></span> create my_source_app <span class="nt">-p</span> 128 |
| |
| <span class="c">## tjsrv-account</span> |
| <span class="o">>>></span> create my_source_app <span class="nt">-p</span> 32 |
| </code></pre></div></div> |
| |
| <h2 id="线上表开启热备份">线上表开启热备份</h2> |
| |
| <p>有时一个线上表可能在设计之初未考虑到跨机房同步的需求，而在服务一段时间后，才决定进行热备份。此时我们需要将源集群已有的全部数据复制到目的集群。因为是线上表，我们要求拷贝过程中：</p> |
| |
| <ol> |
| <li><strong>不可以停止服务</strong></li> |
| <li>拷贝过程中的<strong>写增量数据不能丢失</strong></li> |
| </ol> |
| |
| <p>面对这个需求，我们的操作思路是：</p> |
| |
| <ol> |
| <li>首先源集群<strong>保留从此刻开始的所有写增量</strong>（即WAL日志）</li> |
| <li>将源集群的全量快照（存量数据）移动到指定路径下，等待备集群(目标集群)对这些数据进行学习learn。</li> |
| <li>目标集群将存量数据学习完成后，利用学来的存量数据构建表。构建完成后，告知源集群进入WAL日志发送阶段。</li> |
| <li>此后源集群开启热备份，并复制此前堆积的写增量，发送到远端目标集群。</li> |
| </ol> |
| |
| <table> |
| <thead> |
| <tr> |
| <th>master cluster</th> |
| <th> </th> |
| <th>follower cluster</th> |
| <th> </th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td>meta</td> |
| <td>primary</td> |
| <td>primary</td> |
| <td>meta</td> |
| </tr> |
| <tr> |
| <td> </td> |
| <td>0.replica server在初始化时会进行周期性RPC的任务创建，进行replica server与meta之间的热备信息通信</td> |
| <td> </td> |
| <td> </td> |
| </tr> |
| <tr> |
| <td>0.收到replica server发送的RPC，汇总热备任务和进度，回复replica server</td> |
| <td> </td> |
| <td> </td> |
| <td> </td> |
| </tr> |
| <tr> |
| <td>1.发起添加表的热备任务的请求add_duplication，增加相关dup_info</td> |
| <td> </td> |
| <td> </td> |
| <td> </td> |
| </tr> |
| <tr> |
| <td>2.<strong>进入状态 DS_PREPARE</strong>，同步checkpoint点</td> |
| <td>3.得知meta上的新dup_info，创建replica_duplicator类，调用<strong>trigger_manual_emergency_checkpoint</strong>生成checkpoint。</td> |
| <td> </td> |
| <td> </td> |
| </tr> |
| <tr> |
| <td>4.得到replica server报告全部checkpoint生成完毕，开始创建备用集群的表create_follower_app_for_duplication</td> |
| <td>–> –>–> RPC_CM_CREATE_APP->–></td> |
| <td>–> –>–> –> 携带主表信息–> –> –> –></td> |
| <td>5.接到RPC_CM_CREATE_APP请求，开始创建表。duplicate_checkpoint</td> |
| </tr> |
| <tr> |
| <td> </td> |
| <td><– <– <– <– <– <– <– <– <– <– <– <– <-</td> |
| <td><– <– <– <–建表成功 返回ERR_OK <– <-</td> |
| <td>6.使用主表checkpoint初始化。发送拉取checkpoint的请求。底层调用nfs copy的方法async_duplicate_checkpoint_from_master_replica</td> |
| </tr> |
| <tr> |
| <td>7.接收到ERR_OK的返回，<strong>进入DS_APP状态</strong></td> |
| <td> </td> |
| <td> </td> |
| <td> </td> |
| </tr> |
| <tr> |
| <td>8.下一轮通讯中，在DS_APP状态下检查创建完成的表。无误后，<strong>进入DS_LOG状态</strong>check_follower_app_if_create_completed</td> |
| <td> </td> |
| <td> </td> |
| <td> </td> |
| </tr> |
| <tr> |
| <td> </td> |
| <td>9.replica server首次得知status已经切换到DS_LOG，开始热备plog部分数据start_dup_log</td> |
| <td> </td> |
| <td> </td> |
| </tr> |
| <tr> |
| <td> </td> |
| <td>10.load重放加载日志 ship打包发送</td> |
| <td>11.作为服务端接收ship的包，解包并根据具体包含的RPC类型处理pegasus_write_service::duplicate</td> |
| <td> </td> |
| </tr> |
| </tbody> |
| </table> |
| |
| <p>下面介绍给一张线上表开启具体的热备所需的步骤</p> |
| |
| <h3 id="执行步骤1-集群热备参数设置">执行步骤1 集群热备参数设置</h3> |
| |
| <p>主备集群两边的replication与duplication-group项下<strong>相关参数须保持一致</strong>。其中，主集群指同步数据的发送方，备集群指接收方。</p> |
| |
| <p>主集群配置示例：</p> |
| |
| <pre><code class="language-Shell">[replication] |
| duplication_enabled = true |
| duplicate_log_batch_bytes = 4096 # 0意味着不做batch处理，一般设置为4096即可，该配置可以通过admin-cli的server-config动态修改 |
| |
| [pegasus.clusters] |
| # 开启热备份的主集群必须配置备集群的具体meta server地址： |
| tjsrv-account = xxxxxxxxx |
| |
| # 热备份的两个集群需要登记源集群和目的集群的“cluster_id”： |
| [[duplication-group]] |
| bjsrv-account = 1 |
| tjsrv-account = 2 |
| </code></pre> |
| |
| <p>备集群配置示例：</p> |
| |
| <pre><code class="language-Shell">[replication] |
| duplication_enabled = true |
| duplicate_log_batch_bytes = 4096 # 0意味着不做batch处理，一般设置为4096即可，该配置可以通过admin-cli的server-config动态修改 |
| |
| [pegasus.clusters] |
| # 备集群的这一项可以不额外增加配置 |
| |
| # 热备份的两个集群需要登记源集群和目的集群的“cluster_id”： |
| [[duplication-group]] |
| bjsrv-account = 1 |
| tjsrv-account = 2 |
| </code></pre> |
| |
| <h3 id="执行步骤2-按需接入域名proxy系统-可选">执行步骤2 按需接入域名proxy系统 (可选)</h3> |
| |
| <p>跨机房热备的主要目的是提供机房级容灾，为了提供跨机房切换流量的能力，在内部使用中需要热备的业务必须接入meta-proxy。</p> |
| |
| <p>meta-proxy的逻辑是客户端访问proxy，proxy去zookeeper上找对应表的路径，获得一个真实的集群meta地址，然后再访问这个meta。meta-proxy在ZK上的路径配置是表级别的，所以要注意一个region内最好不要有不同业务的同名表。</p> |
| |
| <p>当然如果业务侧可以自己修改meta地址，是可以不接入域名proxy系统的。</p> |
| |
| <h3 id="执行步骤3-开启热备">执行步骤3 开启热备</h3> |
| |
| <p>在开启热备前，需要考虑好本次热备是同步表的全部数据(全量数据同步)还是只需要同步此刻开始(增量同步)。</p> |
| |
| <ol> |
| <li> |
| <p>如果进行表的全量数据拷贝，则需要拷贝的数据分为两部分，存量数据的checkpoint+增量写入的数据。在admin-cli中，使用dup add命令时增加<code class="language-plaintext highlighter-rouge">-p</code>参数，pegasus duplication即可生成checkpoint同步到备集群。需要注意的是，在这种情况下备集群不能存在同名表，在duplication逻辑中，备集群会使用主集群同步过来的checkpoint创建与主集群表名一致的新表，随后接收主集群同步过来的增量数据。</p> |
| </li> |
| <li> |
| <p>如果增量同步，不需要拷贝checkpoint（即仅同步增量数据），则需要确保备集群已经创建好同名表</p> |
| </li> |
| </ol> |
| |
| <div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code># 以admin-cli命令为例 |
| # dup add -c {集群名} -p {出现-p是全量同步，不带-p参数是增量同步} |
| |
| >> use my_source_app |
| >> dup add -c tjsrv-account -p |
| successfully add duplication [dupid: 1669972761] |
| </code></pre></div></div> |
| |
| <h3 id="执行步骤4-暂停重启删除一个热备任务">执行步骤4 暂停/重启/删除一个热备任务</h3> |
| |
| <div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code># 注意：仅在DS_LOG 阶段可以暂停 |
| >> dup pause/start/remove -d {dup的id，使用dup list 可以查看} |
| </code></pre></div></div> |
| |
| <p><strong>注：pause后，没有发送的日志持续堆积。remove后，没有发送的日志直接被清零</strong>。</p> |
| |
| <h2 id="热备份的可靠性">热备份的可靠性</h2> |
| |
| <h3 id="自动故障处理">自动故障处理</h3> |
| |
| <p>热备份是一个集成在ReplicaServer中的一个在线服务，因而我们对该功能的可靠性有较高的要求。 |
| 为应对在热备份过程中可能发生的各种故障，我们提供了几种故障处理的选项：</p> |
| |
| <ul> |
| <li> |
| <p><strong><em>fail-slow</em></strong>：在这种故障处理模式下，热备份对任何故障都会<strong>无限地重试</strong>。我们的运维人员需要对一些关键监控项设置报警，从而可以获知故障的发生。这是Pegasus的<strong>默认故障处理模式</strong>。</p> |
| </li> |
| <li> |
| <p><strong><em>fail-skip</em></strong>：遇到故障时，重试多次仍不成功后，直接跳过对当前这批数据的热备份，从而复制下一批数据。这适合那些<strong>可容忍数据丢失</strong>的业务场景。该选项通过数据丢失换得更好的可用性。</p> |
| </li> |
| </ul> |
| |
| <p>操作命令：</p> |
| |
| <div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>set_dup_fail_mode <app_name> <dupid> <slow|skip> |
| </code></pre></div></div> |
| |
| <h3 id="重要监控">重要监控</h3> |
| |
| <p>在热备份的运维中，我们建议观察几个核心监控，以持续留意服务情况：</p> |
| |
| <ul> |
| <li> |
| <p><code class="language-plaintext highlighter-rouge">collector*app.pegasus*app.stat.dup_failed_shipping_ops#<app_name></code>：有多少写复制RPC遇到失败。失败往往意味着远端集群或跨集群网络存在不可用。</p> |
| </li> |
| <li> |
| <p><code class="language-plaintext highlighter-rouge">replica*app.pegasus*dup.time_lag_ms@<app_name></code>：P99的数据复制延迟。即源集群的一条写过了多长时间才到达目的集群。</p> |
| </li> |
| <li> |
| <p><code class="language-plaintext highlighter-rouge">replica*app.pegasus*dup.lagging_writes@<app_name></code>：当前有多少写花费了过长的时间才到达目的集群。我们可以配置一个阈值，耗时超过该阈值的一条复制会被记录一次：</p> |
| |
| <div class="language-ini highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nn">[pegasus.server]</span> |
| <span class="py">dup_lagging_write_threshold_ms</span> <span class="p">=</span> <span class="s">10000</span> |
| </code></pre></div> </div> |
| </li> |
| <li> |
| <p><code class="language-plaintext highlighter-rouge">replica*eon.replica_stub*dup.pending_mutations_count</code>：当前有多少写堆积在源集群，且尚未复制。如果一切正常，该监控项会稳定维持在某个值上下。当热备份的某个环节出现故障时，往往会有大量的写堆积，该值会持续上涨。</p> |
| </li> |
| <li> |
| <p><code class="language-plaintext highlighter-rouge">replica*eon.replica_stub*dup.load_file_failed_count</code>：源集群读取日志文件的失败次数。日志文件的读取是热备份的关键环节，如果该环节因某种原因出现故障，则会导致热备份被阻塞。</p> |
| </li> |
| </ul> |
| |
| <h2 id="热备份的元信息">热备份的元信息</h2> |
| |
| <p>热备份的元信息会经由 MetaServer 持久化于 Zookeeper 上，其存储路径如下：</p> |
| |
| <div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code> <cluster_root> <app_id> <dupid> |
| | | | |
| | | | |
| [zk: 127.0.0.1:22181(CONNECTED) 0] get /pegasus/bjsrv-account/0.0.x.x/apps/1/duplication/1537336970 |
| |
| {"remote":"tjsrv-account","status":"DS_START","create_timestamp_ms":1537336970483} |
| </code></pre></div></div> |
| |
| <h2 id="热备相关配置项列表">热备相关配置项列表</h2> |
| |
| <div class="language-ini highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nn">[replication]</span> |
| <span class="c"># 如果遇到紧急情况想要手动关闭热备份，可以将该项设置为 false，默认为 true。 |
| </span> <span class="py">duplication_enabled</span> <span class="p">=</span> <span class="s">true</span> |
| |
| <span class="nn">[pegasus.clusters]</span> |
| <span class="c"># 开启热备份的集群必须配置目的集群的具体地址： |
| </span> <span class="py">tjsrv-account</span> <span class="p">=</span> <span class="s">127.0.0.1:51601,127.0.0.1:51601</span> |
| |
| <span class="nn">[pegasus.server]</span> |
| <span class="py">dup_lagging_write_threshold_ms</span> <span class="p">=</span> <span class="s">10000</span> |
| |
| <span class="c"># 热备份的两个集群需要登记源集群和目的集群的“cluster_id”： |
| </span><span class="nn">[duplication-group]</span> |
| <span class="py">bjsrv-account</span> <span class="p">=</span> <span class="s">1</span> |
| <span class="py">tjsrv-account</span> <span class="p">=</span> <span class="s">2</span> |
| |
| </code></pre></div></div> |
| |
| <p>我们在每条数据前都会加上 <code class="language-plaintext highlighter-rouge">timestamp+cluster_id</code> 的前缀，timestamp 即数据写到 pegasus 的时间戳，cluster_id 即上面 duplication-group 中所配置的，bjsrv-account集群的cluster_id 为 1，tjsrv-account集群的 cluster_id 为 2。</p> |
| |
| <p>cluster_id 的作用是：一旦出现写冲突，例如 bjsrv-account 和 tjsrv-account 同时写 key <code class="language-plaintext highlighter-rouge">"user_1"</code>，系统首先会检查两次写的时间戳，以时间戳大的为最终值。当极罕见地遇到时间戳相同的情况时，以 cluster_id 大的为最终值。使用这种机制我们可以保证两集群的最终值一定相同。</p> |
| |
| <h2 id="known-limitations">Known Limitations</h2> |
| |
| <ul> |
| <li>热备份暂时不建议两机房同时写一份数据。在我们的业务经验看来，通常这是可以接受的。用户可以将数据均分在两个不同的机房内，热备份能保证当任一机房宕机，只有数秒的数据丢失（假设机房之间网络稳定）。</li> |
| </ul> |
| |
| </div> |
| </section> |
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