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Posted to commits@rocketmq.apache.org by li...@apache.org on 2017/06/08 10:10:05 UTC
[10/15] incubator-rocketmq-site git commit: Revert CSS
http://git-wip-us.apache.org/repos/asf/incubator-rocketmq-site/blob/af2718af/content/docs/faq/index.html
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+ <article class="page" itemscope itemtype="http://schema.org/CreativeWork">
+ <meta itemprop="headline" content="Frequently Asked Questions">
+ <meta itemprop="description" content="The following questions are frequently asked with regard to the RocketMQ project in general.General1. Why did we create rocketmq project instead of selecting other products?Please refer to Why RocketMQ">
+ <meta itemprop="datePublished" content="June 08, 2017">
+ <meta itemprop="dateModified" content="December 27, 2016">
+
+ <div class="page__inner-wrap">
+
+ <header>
+ <h1 class="page__title" itemprop="headline">Frequently Asked Questions
+</h1>
+
+ </header>
+
+
+ <section class="page__content" itemprop="text">
+ <p>The following questions are frequently asked with regard to the RocketMQ project in general.</p>
+
+<h2 id="general">General</h2>
+<h3 id="1-why-did-we-create-rocketmq-project-instead-of-selecting-other-products">1. Why did we create rocketmq project instead of selecting other products?</h3>
+<p>Please refer to <a href="/docs/motivation/">Why RocketMQ</a></p>
+
+<h3 id="2-do-i-have-to-install-other-softewares-such-as-zookeeper-to-use-rocketmq">2. Do I have to install other softewares, such as zookeeper, to use RocketMQ?</h3>
+<p>No. RocketMQ can run independently.</p>
+
+<h2 id="usage">Usage</h2>
+<h3 id="1-where-does-the-newly-created-consumer-id-start-consuming-messages">1. Where does the newly created Consumer ID start consuming messages?</h3>
+
+<ol>
+ <li>If the topic sends a message within three days, then the consumer start consuming messages from the first message saved in the server.</li>
+ <li>If the topic sends a message three days ago, the consumer starts to consume messages from the latest message in the server, in other words, starting from the tail of message queue.</li>
+ <li>If such consumer is rebooted, then it starts to consume messages from the last consumption location.</li>
+</ol>
+
+<h3 id="2-how-to-reconsume-message-when-consumption-fails">2. How to reconsume message when consumption fails?</h3>
+
+<ol>
+ <li>
+ <p>Cluster consumption pattern
+The consumer business logic code returns Action.ReconsumerLater, NULL, or throws an exception, if a message failed to be consumed, it will retry for up to 16 times, after that, the message would be descarded.</p>
+ </li>
+ <li>
+ <p>Broadcast consumption pattern
+The broadcaset consumption still ensures that a message is consumered at least once, but no resend option is provided.</p>
+ </li>
+</ol>
+
+<h3 id="3-how-to-query-the-failed-message-if-there-is-a-consumption-failure">3. How to query the failed message if there is a consumption failure?</h3>
+
+<ol>
+ <li>Using topic query by time, you can query messages within a period of time.</li>
+ <li>Using Topic and Message Id to accurately query the message.</li>
+ <li>Using Topic and Message Key accurately query a class of messages with the same Message Key.</li>
+</ol>
+
+<h3 id="4-are-messages-delivered-exactly-once">4. Are messages delivered exactly once?</h3>
+
+<p>RocketMQ ensures that all messages are delivered at least once. In most cases, the messages are not repeated.</p>
+
+<h3 id="5-how-to-add-a-new-broker">5. How to add a new broker?</h3>
+
+<ol>
+ <li>Start up a new broker and register it to the same list of name servers.</li>
+ <li>By default, only internal system topics and consumer groups are created automatically. If you would like to have your business topic and consumer groups on the new node, please replicate them from the existing broker. Admin tool and command lines are provided to handle this.</li>
+</ol>
+
+<h2 id="configuration-related">Configuration related</h2>
+<p>The following answers are all default values and can be modified by configuration.</p>
+<h3 id="1-how-long-are-the-messages-saved-on-the-server">1. How long are the messages saved on the server?</h3>
+
+<p>Stored messages are will be saved for up to 3 days, and messages that are not consumed for more than 3 days will be deleted.</p>
+
+<h3 id="2-what-is-the-size-limit-for-message-body">2. What is the size limit for message Body?</h3>
+<p>Generally 256KB.</p>
+
+<h3 id="3-how-to-set-the-number-of-consumer-threads">3. How to set the number of consumer threads?</h3>
+<p>When you start Consumer, set a ConsumeThreadNums property, example is as follows:</p>
+
+<div class="highlighter-rouge"><pre class="highlight"><code>consumer.setConsumeThreadMin(20);
+consumer.setConsumeThreadMax(20);
+</code></pre>
+</div>
+
+<h2 id="errors">Errors</h2>
+<h3 id="1-if-you-start-a-producer-or-consumer-failed-and-the-error-message-is-producer-group-or-consumer-repeat">1. If you start a producer or consumer failed and the error message is producer group or consumer repeat?</h3>
+<p>Reason：Using the same Producer /Consumer Group to launch multiple instances of Producer/Consumer in the same JVM may cause the client fail to start.</p>
+
+<p>Solution: Make sure that a JVM corresponding to one Producer /Consumer Group starts only with one Producer/Consumer instance.</p>
+
+<h3 id="2-if-consumer-failed-to-start-loading-json-file-in-broadcast-mode">2. If consumer failed to start loading json file in broadcast mode?</h3>
+<p>Reason: Fastjson version is too low to allow the broadcast consumer to load local offsets.json, causing the consumer boot failure. Damaged fastjson file can also cause the same problem.</p>
+
+<p>Solution: Fastjson version has to be upgraded to rocketmq client dependent version to ensure that the local offsets.json can be loaded. By default offsets.json file is in /home/{user}/.rocketmq_offsets. Or check the integrity of fastjson.</p>
+
+<h3 id="3-what-is-the-impact-of-a-broker-crash">3. What is the impact of a broker crash?</h3>
+
+<ol>
+ <li>Master crashes</li>
+</ol>
+
+<p>Messages can no longer be sent to this broker set, but if you have another broker set available, messages can still be sent given the topic is present. Messages can still be consumed from slaves.</p>
+
+<ol>
+ <li>Some slave crash</li>
+</ol>
+
+<p>As long as there is another working slave, there will be no impact on sending messages. There will also be no impact on consuming messages except when the consumer group is set to consume from this slave preferably. By default, comsumer group consumes from master.</p>
+
+<ol>
+ <li>All slaves crash</li>
+</ol>
+
+<p>There will be no impact on sending messages to master, but, if the master is SYNC_MASTER, producer will get a SLAVE_NOT_AVAILABLE indicating that the message is not sent to any slaves. There will also be no impact on consuming messages except that if the consumer group is set to consume from slave preferably. By default, comsumer group consumes from master.</p>
+
+<h3 id="4-producer-complains-no-topic-route-info-how-to-diagnose">4. Producer complains “No Topic Route Info”, how to diagnose?</h3>
+<p>This happens when you are trying to send messages to a topic whose routing info is not available to the producer.</p>
+
+<ol>
+ <li>Make sure that the producer can connect to a name server and is capable of fetching routing meta info from it.</li>
+ <li>Make sure that name servers do contain routing meta info of the topic. You may query the routing meta info from name server through topicRoute using admin tools or web console.</li>
+ <li>Make sure that your brokers are sending heartbeats to the same list of name servers your producer is connecting to.</li>
+ <li>Make sure that the topic’s permssion is 6(rw-), or at least 2(-w-).</li>
+</ol>
+
+<p>If you can’t find this topic, create it on a broker via admin tools command updateTopic or web console.</p>
+
+
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+ <meta itemprop="headline" content="Filter Example">
+ <meta itemprop="description" content="How to filter messages by SQL92 in Apache RocketMQ.">
+ <meta itemprop="datePublished" content="June 08, 2017">
+ <meta itemprop="dateModified" content="April 26, 2017">
+
+ <div class="page__inner-wrap">
+
+ <header>
+ <h1 class="page__title" itemprop="headline">Filter Example
+</h1>
+
+ </header>
+
+
+ <section class="page__content" itemprop="text">
+ <p>In most cases, tag is a simple and useful design to select message you want. For example:</p>
+
+<div class="language-java highlighter-rouge"><pre class="highlight"><code><span class="n">DefaultMQPushConsumer</span> <span class="n">consumer</span> <span class="o">=</span> <span class="k">new</span> <span class="n">DefaultMQPushConsumer</span><span class="o">(</span><span class="s">"CID_EXAMPLE"</span><span class="o">);</span>
+<span class="n">consumer</span><span class="o">.</span><span class="na">subscribe</span><span class="o">(</span><span class="s">"TOPIC"</span><span class="o">,</span> <span class="s">"TAGA || TAGB || TAGC"</span><span class="o">);</span>
+</code></pre>
+</div>
+
+<aside class="sidebar__right">
+<nav class="toc">
+ <header><h4 class="nav__title"><i class="fa fa-file-text"></i> On This Page</h4></header>
+<ul class="toc__menu" id="markdown-toc">
+ <li><a href="#principle" id="markdown-toc-principle">Principle</a></li>
+ <li><a href="#grammars" id="markdown-toc-grammars">Grammars</a></li>
+ <li><a href="#usage-constraints" id="markdown-toc-usage-constraints">Usage constraints</a></li>
+ <li><a href="#producer-example" id="markdown-toc-producer-example">Producer example</a></li>
+ <li><a href="#consumer-example" id="markdown-toc-consumer-example">Consumer example</a></li>
+</ul>
+
+ </nav>
+</aside>
+
+<p>The consumer will recieve messages that contains TAGA or TAGB or TAGC. But the limitation is that one message only can have one tag, and this may not work for sophisticated scenarios. In this case, you can use SQL expression to filter out messages.</p>
+
+<h3 id="principle">Principle</h3>
+
+<p>SQL feature could do some calculation through the properties you put in when sending messages. Under the grammars defined by RocketMQ, you can implement some interesting logic. Here is an example:</p>
+
+<pre>
+------------
+| message |
+|----------| a > 5 AND b = 'abc'
+| a = 10 | --------------------> Gotten
+| b = 'abc'|
+| c = true |
+------------
+------------
+| message |
+|----------| a > 5 AND b = 'abc'
+| a = 1 | --------------------> Missed
+| b = 'abc'|
+| c = true |
+------------
+</pre>
+
+<h3 id="grammars">Grammars</h3>
+
+<p>RocketMQ only defines some basic grammars to support this feature. You could also extend it easily.</p>
+
+<ol>
+ <li>Numeric comparison, like <code class="highlighter-rouge">></code>, <code class="highlighter-rouge">>=</code>, <code class="highlighter-rouge"><</code>, <code class="highlighter-rouge"><=</code>, <code class="highlighter-rouge">BETWEEN</code>, <code class="highlighter-rouge">=</code>;</li>
+ <li>Character comparison, like <code class="highlighter-rouge">=</code>, <code class="highlighter-rouge"><></code>, <code class="highlighter-rouge">IN</code>;</li>
+ <li><code class="highlighter-rouge">IS NULL</code> or <code class="highlighter-rouge">IS NOT NULL</code>;</li>
+ <li>Logical <code class="highlighter-rouge">AND</code>, <code class="highlighter-rouge">OR</code>, <code class="highlighter-rouge">NOT</code>;</li>
+</ol>
+
+<p>Constant types are:</p>
+
+<ol>
+ <li>Numeric, like 123, 3.1415;</li>
+ <li>Character, like ‘abc’, must be made with single quotes;</li>
+ <li><code class="highlighter-rouge">NULL</code>, special constant;</li>
+ <li>Boolean, <code class="highlighter-rouge">TRUE</code> or <code class="highlighter-rouge">FALSE</code>;</li>
+</ol>
+
+<h3 id="usage-constraints">Usage constraints</h3>
+
+<p>Only push consumer could select messages by SQL92. The interface is:</p>
+
+<p><code class="highlighter-rouge">public void subscribe(final String topic, final MessageSelector messageSelector)</code></p>
+
+<h3 id="producer-example">Producer example</h3>
+
+<p>You can put properties in message through method <code class="highlighter-rouge">putUserProperty</code> when sending.</p>
+
+<div class="language-java highlighter-rouge"><pre class="highlight"><code><span class="n">DefaultMQProducer</span> <span class="n">producer</span> <span class="o">=</span> <span class="k">new</span> <span class="n">DefaultMQProducer</span><span class="o">(</span><span class="s">"please_rename_unique_group_name"</span><span class="o">);</span>
+<span class="n">producer</span><span class="o">.</span><span class="na">start</span><span class="o">();</span>
+
+<span class="n">Message</span> <span class="n">msg</span> <span class="o">=</span> <span class="k">new</span> <span class="n">Message</span><span class="o">(</span><span class="s">"TopicTest"</span><span class="o">,</span>
+ <span class="n">tag</span><span class="o">,</span>
+ <span class="o">(</span><span class="s">"Hello RocketMQ "</span> <span class="o">+</span> <span class="n">i</span><span class="o">).</span><span class="na">getBytes</span><span class="o">(</span><span class="n">RemotingHelper</span><span class="o">.</span><span class="na">DEFAULT_CHARSET</span><span class="o">)</span>
+<span class="o">);</span>
+<span class="c1">// Set some properties.</span>
+<span class="n">msg</span><span class="o">.</span><span class="na">putUserProperty</span><span class="o">(</span><span class="s">"a"</span><span class="o">,</span> <span class="n">String</span><span class="o">.</span><span class="na">valueOf</span><span class="o">(</span><span class="n">i</span><span class="o">));</span>
+
+<span class="n">SendResult</span> <span class="n">sendResult</span> <span class="o">=</span> <span class="n">producer</span><span class="o">.</span><span class="na">send</span><span class="o">(</span><span class="n">msg</span><span class="o">);</span>
+
+<span class="n">producer</span><span class="o">.</span><span class="na">shutdown</span><span class="o">();</span>
+</code></pre>
+</div>
+
+<h3 id="consumer-example">Consumer example</h3>
+
+<p>Use <code class="highlighter-rouge">MessageSelector.bySql</code> to select messages through SQL92 when consuming.</p>
+
+<div class="language-java highlighter-rouge"><pre class="highlight"><code><span class="n">DefaultMQPushConsumer</span> <span class="n">consumer</span> <span class="o">=</span> <span class="k">new</span> <span class="n">DefaultMQPushConsumer</span><span class="o">(</span><span class="s">"please_rename_unique_group_name_4"</span><span class="o">);</span>
+
+<span class="c1">// only subsribe messages have property a, also a >=0 and a <= 3</span>
+<span class="n">consumer</span><span class="o">.</span><span class="na">subscribe</span><span class="o">(</span><span class="s">"TopicTest"</span><span class="o">,</span> <span class="n">MessageSelector</span><span class="o">.</span><span class="na">bySql</span><span class="o">(</span><span class="s">"a between 0 and 3"</span><span class="o">);</span>
+
+<span class="n">consumer</span><span class="o">.</span><span class="na">registerMessageListener</span><span class="o">(</span><span class="k">new</span> <span class="n">MessageListenerConcurrently</span><span class="o">()</span> <span class="o">{</span>
+ <span class="nd">@Override</span>
+ <span class="kd">public</span> <span class="n">ConsumeConcurrentlyStatus</span> <span class="nf">consumeMessage</span><span class="o">(</span><span class="n">List</span><span class="o"><</span><span class="n">MessageExt</span><span class="o">></span> <span class="n">msgs</span><span class="o">,</span> <span class="n">ConsumeConcurrentlyContext</span> <span class="n">context</span><span class="o">)</span> <span class="o">{</span>
+ <span class="k">return</span> <span class="n">ConsumeConcurrentlyStatus</span><span class="o">.</span><span class="na">CONSUME_SUCCESS</span><span class="o">;</span>
+ <span class="o">}</span>
+<span class="o">});</span>
+<span class="n">consumer</span><span class="o">.</span><span class="na">start</span><span class="o">();</span>
+</code></pre>
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+ <meta itemprop="headline" content="Motivation">
+ <meta itemprop="description" content="At early stages, we constructed our distributed messaging middleware based on ActiveMQ 5.x(prior to 5.3). Our multinational business uses it for asynchronous communication, search, social network activity stream, data pipeline,even in its trade processes. As our trade business throughput rises, pressure originating from our messaging clusteralso becomes urgent.">
+ <meta itemprop="datePublished" content="June 08, 2017">
+ <meta itemprop="dateModified" content="December 16, 2016">
+
+ <div class="page__inner-wrap">
+
+ <header>
+ <h1 class="page__title" itemprop="headline">Motivation
+</h1>
+
+ </header>
+
+
+ <section class="page__content" itemprop="text">
+ <p>At early stages, we constructed our distributed messaging middleware based on ActiveMQ 5.x(prior to 5.3). Our
+multinational business uses it for asynchronous communication, search, social network activity stream, data pipeline,
+even in its trade processes. As our trade business throughput rises, pressure originating from our messaging cluster
+also becomes urgent.</p>
+
+<aside class="sidebar__right">
+<nav class="toc">
+ <header><h4 class="nav__title"><i class="fa fa-file-text"></i> On This Page</h4></header>
+<ul class="toc__menu" id="markdown-toc">
+ <li><a href="#why-rocketmq-" id="markdown-toc-why-rocketmq-">Why RocketMQ ?</a></li>
+ <li><a href="#rocketmq-vs-activemq-vs-kafka" id="markdown-toc-rocketmq-vs-activemq-vs-kafka">RocketMQ vs. ActiveMQ vs. Kafka</a></li>
+</ul>
+
+ </nav>
+</aside>
+
+<h1 id="why-rocketmq-">Why RocketMQ ?</h1>
+
+<p>Based on our research, with increased queues and virtual topics in use, ActiveMQ IO module reaches a bottleneck. We
+tried our best to solve this problem through throttling, circuit breaker or degradation, but it did not work well. So
+we begin to focus on the popular messaging solution Kafka at that time. Unfortunately, Kafka can not meet our
+requirements especially in terms of low latency and high reliability, see <a href="/rocketmq/how-to-support-more-queues-in-rocketmq/">here</a> for details.</p>
+
+<p>In this context, we decided to invent a new messaging engine to handle a broader set of use cases, ranging from
+traditional pub/sub scenarios to high volume real-time zero-loss tolerance transaction system. We believe this solution
+can be beneficial, so we would like to open source it to the community. Today, more than 100 companies are using the
+open source version of RocketMQ in their business. We also published a commercial distribution based on RocketMQ, a PaaS
+ product called the <a href="https://intl.aliyun.com/">Alibaba Cloud Platform</a>.</p>
+
+<p>The following table demonstrates the comparison between RocketMQ, ActiveMQ and Kafka (Apache’s most popular messaging solutions according to <a href="https://github.com/akullpp/awesome-java">awesome-java</a>):</p>
+
+<h1 id="rocketmq-vs-activemq-vs-kafka">RocketMQ vs. ActiveMQ vs. Kafka</h1>
+
+<table>
+ <thead>
+ <tr>
+ <th>Messaging Product</th>
+ <th>Client SDK</th>
+ <th>Protocol and Specification</th>
+ <th>Order Message</th>
+ <th>Message Filter</th>
+ <th>Server Triggered Redelivery</th>
+ <th>Persistent Message</th>
+ <th>Retroactive Consumers</th>
+ <th>Message Priority</th>
+ <th>High Availability and Failover</th>
+ <th>Message Track</th>
+ <th>Configuration</th>
+ <th>Management and Operation Tools</th>
+ </tr>
+ </thead>
+ <tbody>
+ <tr>
+ <td>ActiveMQ</td>
+ <td>Java, .NET, C++ etc.</td>
+ <td>Push model, support OpenWire, STOMP, AMQP, MQTT, JMS</td>
+ <td>Exclusive Consumer or Exclusive Queues can ensure ordering</td>
+ <td>Supported</td>
+ <td>Not Supported</td>
+ <td>Supports very fast persistence using JDBC along with a high performance journal，such as levelDB, kahaDB</td>
+ <td>Supported</td>
+ <td>Supported</td>
+ <td>Supported, depending on storage,if using kahadb it requires a ZooKeeper server</td>
+ <td>Not Supported</td>
+ <td>The default configuration is low level, user need to optimize the configuration parameters</td>
+ <td>Supported</td>
+ </tr>
+ <tr>
+ <td>Kafka</td>
+ <td>Java, Scala etc.</td>
+ <td>Pull model, support TCP</td>
+ <td>Ensure ordering of messages within a partition</td>
+ <td>Supported, you can use Kafka Streams to filter messages</td>
+ <td>Not Supported</td>
+ <td>High performance file storage</td>
+ <td>Supported offset indicate</td>
+ <td>Not Supported</td>
+ <td>Supported, requires a ZooKeeper server</td>
+ <td>Not Supported</td>
+ <td>Kafka uses key-value pairs format for configuration. These values can be supplied either from a file or programmatically.</td>
+ <td>Supported, use terminal command to expose core metrics</td>
+ </tr>
+ <tr>
+ <td>RocketMQ</td>
+ <td>Java, .NET, C++</td>
+ <td>Pull model, support TCP, JMS</td>
+ <td>Ensure strict ordering of messages, have no hot spot problem,and can scale out gracefully</td>
+ <td>Supported, you can even upload yourself custom-built filter code snippets</td>
+ <td>Supported</td>
+ <td>High performance and low latency file storage</td>
+ <td>Supported timestamp and offset 2 indicates</td>
+ <td>Not Supported</td>
+ <td>Supported, Master-Slave model, without another kit</td>
+ <td>Supported</td>
+ <td>Work out of box,user only need to pay attention to a few configurations</td>
+ <td>Supported, rich web and terminal command to expose core metrics</td>
+ </tr>
+ </tbody>
+</table>
+
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