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<h1 class="page__title" itemprop="headline">ServiceComb Pack 0.4.0 Cluster : Implementation
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<p>Now you can run multiple Alpha server in High Availability (HA) mode which support to run the event scanning in the master node only. You can also turn off event table scanning for some nodes with parameter <code class="language-plaintext highlighter-rouge">alpha.event.scanner.enabled=false</code> . However, transaction compensation will fail when the node that enabled the event table scan is crashed.
We implemented database-based distributed lock in version 0.4.0 version, event scanning only runs on the master node in the cluster, When the master node is down, other nodes in the cluster will elect a new master node.</p>
<h2 id="quick-starts">Quick Starts</h2>
<p>Enable cluster support by parameter <code class="language-plaintext highlighter-rouge">alpha.cluster.master.enabled=true</code>.</p>
<ul>
<li>Start Two Alphas.</li>
</ul>
<div class="language-bash highlighter-rouge"><div class="highlight"><pre class="highlight"><code>java <span class="nt">-jar</span> alpha-server-0.4.0-SNAPSHOT-exec.jar <span class="se">\</span>
<span class="nt">--server</span>.port<span class="o">=</span>8090 <span class="se">\</span>
<span class="nt">--alpha</span>.server.port<span class="o">=</span>8080 <span class="se">\</span>
<span class="nt">--spring</span>.datasource.url<span class="o">=</span><span class="s2">"jdbc:postgresql://127.0.0.1:5432/saga?useSSL=false"</span> <span class="se">\</span>
<span class="nt">--spring</span>.datasource.username<span class="o">=</span>saga-user <span class="se">\</span>
<span class="nt">--spring</span>.datasource.password<span class="o">=</span>saga-password <span class="se">\</span>
<span class="nt">--alpha</span>.cluster.master.enabled<span class="o">=</span><span class="nb">true</span>
</code></pre></div></div>
<div class="language-bash highlighter-rouge"><div class="highlight"><pre class="highlight"><code>java <span class="nt">-jar</span> alpha-server-0.4.0-SNAPSHOT-exec.jar <span class="se">\</span>
<span class="nt">--server</span>.port<span class="o">=</span>8091 <span class="se">\</span>
<span class="nt">--alpha</span>.server.port<span class="o">=</span>8081 <span class="se">\</span>
<span class="nt">--spring</span>.datasource.url<span class="o">=</span><span class="s2">"jdbc:postgresql://127.0.0.1:5432/saga?useSSL=false"</span> <span class="se">\</span>
<span class="nt">--spring</span>.datasource.username<span class="o">=</span>saga-user <span class="se">\</span>
<span class="nt">--spring</span>.datasource.password<span class="o">=</span>saga-password <span class="se">\</span>
<span class="nt">--alpha</span>.cluster.master.enabled<span class="o">=</span><span class="nb">true</span>
</code></pre></div></div>
<ul>
<li>Who Is Master</li>
</ul>
<p><code class="language-plaintext highlighter-rouge">Master Node</code> This alpha is the master</p>
<div class="language-bash highlighter-rouge"><div class="highlight"><pre class="highlight"><code>01:31:07.032 <span class="o">[</span>pool-3-thread-1] INFO org.apache.servicecomb.pack.alpha.server.cluster.master.ClusterLockService - Master Node
</code></pre></div></div>
<p><code class="language-plaintext highlighter-rouge">Slave Node</code> This alpha is the slave</p>
<div class="language-bash highlighter-rouge"><div class="highlight"><pre class="highlight"><code>01:31:31.059 <span class="o">[</span>pool-3-thread-1] INFO org.apache.servicecomb.pack.alpha.server.cluster.master.ClusterLockService - Slave Node
</code></pre></div></div>
<ul>
<li>Switch Master Nodes</li>
</ul>
<p>After the master node is off line, other nodes in the cluster will elect a new master node</p>
<h2 id="how-to-make-event-scanning-only-run-on-the-master-node">How to make event scanning only run on the master node</h2>
<p>Event scanning is implemented by <code class="language-plaintext highlighter-rouge">EventScanner.java</code> , initialize it in <code class="language-plaintext highlighter-rouge">AlphaConfig.java</code> , you can enable it by setting the parameter ‘alpha.event.scanner.enabled=true’, it will be instantiated EventScanner. The variable nodeStatus is the node type(Master or Slave). Later, I will introduce how <code class="language-plaintext highlighter-rouge">nodeStatus</code> is initialized.</p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nd">@Bean</span>
<span class="nc">TxConsistentService</span> <span class="nf">txConsistentService</span><span class="o">(</span>
<span class="nd">@Value</span><span class="o">(</span><span class="s">"${alpha.event.pollingInterval:500}"</span><span class="o">)</span> <span class="kt">int</span> <span class="n">eventPollingInterval</span><span class="o">,</span>
<span class="nd">@Value</span><span class="o">(</span><span class="s">"${alpha.event.scanner.enabled:true}"</span><span class="o">)</span> <span class="kt">boolean</span> <span class="n">eventScannerEnabled</span><span class="o">,</span>
<span class="nc">ScheduledExecutorService</span> <span class="n">scheduler</span><span class="o">,</span>
<span class="nc">TxEventRepository</span> <span class="n">eventRepository</span><span class="o">,</span>
<span class="nc">CommandRepository</span> <span class="n">commandRepository</span><span class="o">,</span>
<span class="nc">TxTimeoutRepository</span> <span class="n">timeoutRepository</span><span class="o">,</span>
<span class="nc">OmegaCallback</span> <span class="n">omegaCallback</span><span class="o">)</span> <span class="o">{</span>
<span class="k">if</span> <span class="o">(</span><span class="n">eventScannerEnabled</span><span class="o">)</span> <span class="o">{</span>
<span class="k">new</span> <span class="nf">EventScanner</span><span class="o">(</span><span class="n">scheduler</span><span class="o">,</span>
<span class="n">eventRepository</span><span class="o">,</span> <span class="n">commandRepository</span><span class="o">,</span> <span class="n">timeoutRepository</span><span class="o">,</span>
<span class="n">omegaCallback</span><span class="o">,</span> <span class="n">eventPollingInterval</span><span class="o">,</span> <span class="n">nodeStatus</span><span class="o">).</span><span class="na">run</span><span class="o">();</span>
<span class="no">LOG</span><span class="o">.</span><span class="na">info</span><span class="o">(</span><span class="s">"Starting the EventScanner."</span><span class="o">);</span>
<span class="o">}</span>
<span class="nc">TxConsistentService</span> <span class="n">consistentService</span> <span class="o">=</span> <span class="k">new</span> <span class="nc">TxConsistentService</span><span class="o">(</span><span class="n">eventRepository</span><span class="o">);</span>
<span class="k">return</span> <span class="n">consistentService</span><span class="o">;</span>
<span class="o">}</span>
</code></pre></div></div>
<p>The pollEvents method of <code class="language-plaintext highlighter-rouge">EventScanner.java</code> for scanning events, If the master node <code class="language-plaintext highlighter-rouge">nodeStatus.isMaster()</code> returns true.</p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">private</span> <span class="kt">void</span> <span class="nf">pollEvents</span><span class="o">()</span> <span class="o">{</span>
<span class="n">scheduler</span><span class="o">.</span><span class="na">scheduleWithFixedDelay</span><span class="o">(</span>
<span class="o">()</span> <span class="o">-&gt;</span> <span class="o">{</span>
<span class="c1">// only pull the events when working in the master mode</span>
<span class="k">if</span><span class="o">(</span><span class="n">nodeStatus</span><span class="o">.</span><span class="na">isMaster</span><span class="o">()){</span>
<span class="n">updateTimeoutStatus</span><span class="o">();</span>
<span class="n">findTimeoutEvents</span><span class="o">();</span>
<span class="n">abortTimeoutEvents</span><span class="o">();</span>
<span class="n">saveUncompensatedEventsToCommands</span><span class="o">();</span>
<span class="n">compensate</span><span class="o">();</span>
<span class="n">updateCompensatedCommands</span><span class="o">();</span>
<span class="n">deleteDuplicateSagaEndedEvents</span><span class="o">();</span>
<span class="n">updateTransactionStatus</span><span class="o">();</span>
<span class="o">}</span>
<span class="o">},</span>
<span class="mi">0</span><span class="o">,</span>
<span class="n">eventPollingInterval</span><span class="o">,</span>
<span class="no">MILLISECONDS</span><span class="o">);</span>
<span class="o">}</span>
</code></pre></div></div>
<p>Construct NodeStatus in <code class="language-plaintext highlighter-rouge">AlphaConfig.java</code> by the following to ensure that the event scan will work regardless of whether you have configured the <code class="language-plaintext highlighter-rouge">alpha.cluster.master.enabled</code> parameter, Here you can see the node is a slave just after startup when cluster mode is enabled. Later, I will introduce how to switch to master.</p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code> <span class="nd">@Bean</span>
<span class="nc">NodeStatus</span> <span class="nf">nodeStatus</span> <span class="o">(){</span>
<span class="k">if</span><span class="o">(</span><span class="n">masterEnabled</span><span class="o">){</span>
<span class="k">return</span> <span class="k">new</span> <span class="nf">NodeStatus</span><span class="o">(</span><span class="nc">NodeStatus</span><span class="o">.</span><span class="na">TypeEnum</span><span class="o">.</span><span class="na">SLAVE</span><span class="o">);</span>
<span class="o">}</span><span class="k">else</span><span class="o">{</span>
<span class="k">return</span> <span class="k">new</span> <span class="nf">NodeStatus</span><span class="o">(</span><span class="nc">NodeStatus</span><span class="o">.</span><span class="na">TypeEnum</span><span class="o">.</span><span class="na">MASTER</span><span class="o">);</span>
<span class="o">}</span>
<span class="o">}</span>
<span class="nd">@Autowired</span>
<span class="nc">NodeStatus</span> <span class="n">nodeStatus</span><span class="o">;</span>
</code></pre></div></div>
<p><code class="language-plaintext highlighter-rouge">ClusterLockService.java</code> is in charge of node state switching, It periodically perform lock preemption and set the node as a master after successful preemption.</p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code> <span class="nd">@Autowired</span>
<span class="nc">LockProvider</span> <span class="n">lockProvider</span><span class="o">;</span>
<span class="o">...</span>
<span class="o">...</span>
<span class="nd">@Scheduled</span><span class="o">(</span><span class="n">cron</span> <span class="o">=</span> <span class="s">"0/1 * * * * ?"</span><span class="o">)</span>
<span class="kd">public</span> <span class="kt">void</span> <span class="nf">masterCheck</span><span class="o">()</span> <span class="o">{</span>
<span class="k">if</span> <span class="o">(</span><span class="n">applicationReady</span><span class="o">)</span> <span class="o">{</span>
<span class="k">this</span><span class="o">.</span><span class="na">locker</span> <span class="o">=</span> <span class="n">lockProvider</span><span class="o">.</span><span class="na">lock</span><span class="o">(</span><span class="k">this</span><span class="o">.</span><span class="na">getMasterLock</span><span class="o">());</span>
<span class="k">if</span> <span class="o">(</span><span class="k">this</span><span class="o">.</span><span class="na">locker</span><span class="o">.</span><span class="na">isPresent</span><span class="o">())</span> <span class="o">{</span>
<span class="k">if</span> <span class="o">(!</span><span class="k">this</span><span class="o">.</span><span class="na">locked</span><span class="o">)</span> <span class="o">{</span>
<span class="k">this</span><span class="o">.</span><span class="na">locked</span> <span class="o">=</span> <span class="kc">true</span><span class="o">;</span>
<span class="n">nodeStatus</span><span class="o">.</span><span class="na">setTypeEnum</span><span class="o">(</span><span class="nc">NodeStatus</span><span class="o">.</span><span class="na">TypeEnum</span><span class="o">.</span><span class="na">MASTER</span><span class="o">);</span>
<span class="no">LOG</span><span class="o">.</span><span class="na">info</span><span class="o">(</span><span class="s">"Master Node"</span><span class="o">);</span>
<span class="o">}</span>
<span class="c1">//Keep locked</span>
<span class="o">}</span> <span class="k">else</span> <span class="o">{</span>
<span class="k">if</span> <span class="o">(</span><span class="k">this</span><span class="o">.</span><span class="na">locked</span> <span class="o">||</span> <span class="o">!</span><span class="n">lockExecuted</span><span class="o">)</span> <span class="o">{</span>
<span class="n">locked</span> <span class="o">=</span> <span class="kc">false</span><span class="o">;</span>
<span class="n">nodeStatus</span><span class="o">.</span><span class="na">setTypeEnum</span><span class="o">(</span><span class="nc">NodeStatus</span><span class="o">.</span><span class="na">TypeEnum</span><span class="o">.</span><span class="na">SLAVE</span><span class="o">);</span>
<span class="no">LOG</span><span class="o">.</span><span class="na">info</span><span class="o">(</span><span class="s">"Slave Node"</span><span class="o">);</span>
<span class="o">}</span>
<span class="o">}</span>
<span class="n">lockExecuted</span> <span class="o">=</span> <span class="kc">true</span><span class="o">;</span>
<span class="o">}</span>
<span class="o">}</span>
</code></pre></div></div>
<h2 id="lock-architecture">Lock architecture</h2>
<p>As you can see from the previous description, in the <code class="language-plaintext highlighter-rouge">masterCheck</code> method of <code class="language-plaintext highlighter-rouge">ClusterLockService.java</code>, get a lock by <code class="language-plaintext highlighter-rouge">this.locker = lockProvider.lock(this.getMasterLock());</code> and determine if the lock was successful.</p>
<p><code class="language-plaintext highlighter-rouge">LockProvider.java</code> is an interface. Currently, we provide an implementation based on JDBC. The package structure and class dependencies are as follows:</p>
<p><img src="/assets/images/2019-04-03-servicecomb-pack-cluster/cluster-package-structure.png" alt="cluster-package-structure" /></p>
<p>Dependencies are as follows</p>
<p><img src="/assets/images/2019-04-03-servicecomb-pack-cluster/class-dependency-1.png" alt="class-dependency-1" /></p>
<ul>
<li>
<p>LockProvider.java</p>
<p>The interface defines the lock methods</p>
</li>
<li>
<p>LockProviderPersistence.java</p>
<p>The interface defines the following three persistence methods</p>
<ul>
<li>initLock Create a lock, try to lock and return whether the lock was successful</li>
<li>updateLock Update the lock, lock it again and return it successfully (the interface design of the update lock is designed for non-long connection lock design, for example, for locking according to a fixed period)</li>
<li>unLock</li>
</ul>
</li>
<li>
<p>AbstractLockProvider.java</p>
<p>Implemented the <code class="language-plaintext highlighter-rouge">LockProvider.java</code> interface and called the internal <code class="language-plaintext highlighter-rouge">LockProviderPersistence.java</code> interface for locking operations</p>
</li>
</ul>
<h2 id="jdbc-implementation">JDBC implementation</h2>
<blockquote>
<p>I like to run with you, the same rhythm, the same heartbeat, I feel very good, when you fall behind, I lead everyone to run together</p>
</blockquote>
<ul>
<li>JDBC class diagram</li>
</ul>
<p><img src="/assets/images/2019-04-03-servicecomb-pack-cluster/class-dependency-2.png" alt="class-dependency-2" /></p>
<ul>
<li>
<p>JdbcLockPersistence.java</p>
<p>Implement <code class="language-plaintext highlighter-rouge">LockProviderPersistence.java</code> interface operation database</p>
</li>
<li>
<p>JdbcLockProvider.java</p>
<p>Inherit the abstract class <code class="language-plaintext highlighter-rouge">AbstractLockProvider.java</code>, passing the <code class="language-plaintext highlighter-rouge">LockProviderPersistence.java</code> interface implementation class <code class="language-plaintext highlighter-rouge">JdbcLockPersistence</code> in the constructor.</p>
</li>
<li>
<p>LockProviderJdbcConfiguration.java</p>
<p>Declare a LockProvider Bean with @Configuration</p>
</li>
<li>
<p>JPA</p>
<p>MasterLockEntityRepository.java、SpringMasterLockRepository.java、org.apache.servicecomb.pack.alpha.server.cluster.master.provider.jdbc.jpa.*</p>
</li>
<li>
<p>Create master_lock Table</p>
</li>
</ul>
<div class="language-sql highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">CREATE</span> <span class="k">TABLE</span> <span class="n">IF</span> <span class="k">NOT</span> <span class="k">EXISTS</span> <span class="n">master_lock</span> <span class="p">(</span>
<span class="n">serviceName</span> <span class="nb">varchar</span><span class="p">(</span><span class="mi">36</span><span class="p">)</span> <span class="k">not</span> <span class="k">NULL</span><span class="p">,</span>
<span class="n">expireTime</span> <span class="nb">datetime</span> <span class="k">NOT</span> <span class="k">NULL</span> <span class="k">DEFAULT</span> <span class="k">CURRENT_TIMESTAMP</span><span class="p">,</span>
<span class="n">lockedTime</span> <span class="nb">datetime</span> <span class="k">NOT</span> <span class="k">NULL</span> <span class="k">DEFAULT</span> <span class="k">CURRENT_TIMESTAMP</span><span class="p">,</span>
<span class="n">instanceId</span> <span class="nb">varchar</span><span class="p">(</span><span class="mi">255</span><span class="p">)</span> <span class="k">not</span> <span class="k">NULL</span><span class="p">,</span>
<span class="k">PRIMARY</span> <span class="k">KEY</span> <span class="p">(</span><span class="n">serviceName</span><span class="p">)</span>
<span class="p">)</span> <span class="n">ENGINE</span><span class="o">=</span><span class="n">InnoDB</span> <span class="k">DEFAULT</span> <span class="n">CHARSET</span><span class="o">=</span><span class="n">utf8</span><span class="p">;</span>
</code></pre></div></div>
<ul>
<li>serviceName Service name, this field takes the value <code class="language-plaintext highlighter-rouge">$ {spring.application.name}</code></li>
<li>expireTime Lock expiration time, this field takes value lockedTime + 5s</li>
<li>lockedTime Last lock time</li>
<li>instanceId Service instance ID, this field takes the value <code class="language-plaintext highlighter-rouge">$ {alpha.server.host}]:$ {alpha.server.port}</code></li>
</ul>
<h2 id="lock-processing">Lock processing</h2>
<p>The process of locking/updating a lock is a cyclically repeating action, as follows:</p>
<ul>
<li>
<p><code class="language-plaintext highlighter-rouge">ClusterLockService.java</code> calls the lock method of <code class="language-plaintext highlighter-rouge">LockProvider.java</code> interface once per second</p>
</li>
<li>
<p>The lock method in the abstract class <code class="language-plaintext highlighter-rouge">AbstractLockProvider.java</code> will attempt to lock the iniLock method of <code class="language-plaintext highlighter-rouge">JdbcLockPersistence.java</code>, and the locked SQL implementation is defined in <code class="language-plaintext highlighter-rouge">MasterLockEntityRepository.java</code></p>
<ul>
<li>If the table is empty then insert a record and return the lock successfully</li>
<li>Exception caught and lock failed if there are records with the same service name field in the table</li>
</ul>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code> <span class="nd">@Transactional</span>
<span class="nd">@Modifying</span>
<span class="nd">@Query</span><span class="o">(</span><span class="n">value</span> <span class="o">=</span> <span class="s">"INSERT INTO master_lock "</span>
<span class="o">+</span> <span class="s">"(serviceName, expireTime, lockedTime, instanceId) "</span>
<span class="o">+</span> <span class="s">"VALUES "</span>
<span class="o">+</span> <span class="s">"(?1, ?2, ?3, ?4)"</span><span class="o">,</span> <span class="n">nativeQuery</span> <span class="o">=</span> <span class="kc">true</span><span class="o">)</span>
<span class="kt">int</span> <span class="nf">initLock</span><span class="o">(</span>
<span class="nd">@Param</span><span class="o">(</span><span class="s">"serviceName"</span><span class="o">)</span> <span class="nc">String</span> <span class="n">serviceName</span><span class="o">,</span>
<span class="nd">@Param</span><span class="o">(</span><span class="s">"expireTime"</span><span class="o">)</span> <span class="nc">Date</span> <span class="n">expireTime</span><span class="o">,</span>
<span class="nd">@Param</span><span class="o">(</span><span class="s">"lockedTime"</span><span class="o">)</span> <span class="nc">Date</span> <span class="n">lockedTime</span><span class="o">,</span>
<span class="nd">@Param</span><span class="o">(</span><span class="s">"instanceId"</span><span class="o">)</span> <span class="nc">String</span> <span class="n">instanceId</span><span class="o">);</span>
</code></pre></div> </div>
</li>
<li>
<p>If the initLock lock fails, try to update the lock by calling the updateLock method in <code class="language-plaintext highlighter-rouge">MasterLockEntityRepository.java</code>.</p>
<ul>
<li>If the record instanceId of the same service name is the same as the instance instanceId, the update succeeds and the lock is successfully returned.</li>
<li>If the record expireTime of the same service name is less than the current lock time, the update is successful and the lock is successful.</li>
</ul>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code> <span class="nd">@Transactional</span>
<span class="nd">@Modifying</span><span class="o">(</span><span class="n">clearAutomatically</span> <span class="o">=</span> <span class="kc">true</span><span class="o">)</span>
<span class="nd">@Query</span><span class="o">(</span><span class="s">"UPDATE org.apache.servicecomb.pack.alpha.server.cluster.master.provider.jdbc.jpa.MasterLock t "</span>
<span class="o">+</span> <span class="s">"SET t.expireTime = :expireTime"</span>
<span class="o">+</span> <span class="s">",t.lockedTime = :lockedTime "</span>
<span class="o">+</span> <span class="s">",t.instanceId = :instanceId "</span>
<span class="o">+</span> <span class="s">"WHERE t.serviceName = :serviceName AND (t.expireTime &lt;= :lockedTime OR t.instanceId = :instanceId)"</span><span class="o">)</span>
<span class="kt">int</span> <span class="nf">updateLock</span><span class="o">(</span>
<span class="nd">@Param</span><span class="o">(</span><span class="s">"serviceName"</span><span class="o">)</span> <span class="nc">String</span> <span class="n">serviceName</span><span class="o">,</span>
<span class="nd">@Param</span><span class="o">(</span><span class="s">"lockedTime"</span><span class="o">)</span> <span class="nc">Date</span> <span class="n">lockedTime</span><span class="o">,</span>
<span class="nd">@Param</span><span class="o">(</span><span class="s">"expireTime"</span><span class="o">)</span> <span class="nc">Date</span> <span class="n">expireTime</span><span class="o">,</span>
<span class="nd">@Param</span><span class="o">(</span><span class="s">"instanceId"</span><span class="o">)</span> <span class="nc">String</span> <span class="n">instanceId</span><span class="o">);</span>
</code></pre></div> </div>
</li>
<li>
<p>UnLock</p>
<p>Reserved interface for extending other ways of locking</p>
</li>
</ul>
<h2 id="other-lock-implementations">Other lock implementations</h2>
<p>Other types of locks can be implemented through the <code class="language-plaintext highlighter-rouge">LockProvider.java</code> and <code class="language-plaintext highlighter-rouge">LockProviderPersistence.java</code> interfaces, such as zookeeper or redis etc.</p>
<h2 id="note">Note</h2>
<ul>
<li>
<p>Need to synchronize time across multiple servers</p>
</li>
<li>
<p>Need to configure the correct time zone parameters when using MySQL database</p>
</li>
</ul>
</section>
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<p class="page__date"><strong><i class="fa fa-fw fa-calendar" aria-hidden="true"></i> Updated:</strong> <time datetime="2019-04-11">April 11, 2019</time></p>
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