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href="/docs/4.x/contributionGuide/01how-to-contribute">Contribution Guide</a></div></li></ul></nav></div></div></aside><main class="docMainContainer_gTbr"><div class="container padding-top--md padding-bottom--lg"><div class="row"><div class="col docItemCol_VOVn"><div class="docItemContainer_Djhp"><article><nav class="theme-doc-breadcrumbs breadcrumbsContainer_Z_bl" aria-label="Breadcrumbs"><ul class="breadcrumbs" itemscope="" itemtype="https://schema.org/BreadcrumbList"><li class="breadcrumbs__item"><a aria-label="Home page" class="breadcrumbs__link" href="/"><svg viewBox="0 0 24 24" class="breadcrumbHomeIcon_YNFT"><path d="M10 19v-5h4v5c0 .55.45 1 1 1h3c.55 0 1-.45 1-1v-7h1.7c.46 0 .68-.57.33-.87L12.67 3.6c-.38-.34-.96-.34-1.34 0l-8.36 7.53c-.34.3-.13.87.33.87H5v7c0 .55.45 1 1 1h3c.55 0 1-.45 1-1z" fill="currentColor"></path></svg></a></li><li class="breadcrumbs__item"><span class="breadcrumbs__link">Producer</span><meta itemprop="position" content="1"></li><li itemscope="" itemprop="itemListElement" itemtype="https://schema.org/ListItem" class="breadcrumbs__item breadcrumbs__item--active"><span class="breadcrumbs__link" itemprop="name">Core Concept</span><meta itemprop="position" content="2"></li></ul></nav><span class="theme-doc-version-badge badge badge--secondary">Version: 4.x</span><div class="tocCollapsible_ETCw theme-doc-toc-mobile tocMobile_ITEo"><button type="button" class="clean-btn tocCollapsibleButton_TO0P">On this page</button></div><div class="theme-doc-markdown markdown"><h1>Core Concept</h1><p>Introduction to the basic concepts of the Producer section, including <strong>Message, Tag, Keys, Message Queue and Producer</strong>.</p><h2 class="anchor anchorWithStickyNavbar_LWe7" id="message">Message<a href="#message" class="hash-link" aria-label="Direct link to heading" title="Direct link to heading"></a></h2><p>The composition of RocketMQ messages is simple, as shown in the following figure.</p><ul><li><strong>topic</strong>: the topic of the message to be sent.</li><li><strong>body</strong>: the storage content of the message.</li><li><strong>properties</strong>: the message properties.</li><li><strong>transactionId</strong>: the id of the transaction message.</li></ul><div class="theme-admonition theme-admonition-tip alert alert--success admonition_LlT9"><div class="admonitionHeading_tbUL"><span class="admonitionIcon_kALy"><svg viewBox="0 0 12 16"><path fill-rule="evenodd" d="M6.5 0C3.48 0 1 2.19 1 5c0 .92.55 2.25 1 3 1.34 2.25 1.78 2.78 2 4v1h5v-1c.22-1.22.66-1.75 2-4 .45-.75 1-2.08 1-3 0-2.81-2.48-5-5.5-5zm3.64 7.48c-.25.44-.47.8-.67 1.11-.86 1.41-1.25 2.06-1.45 3.23-.02.05-.02.11-.02.17H5c0-.06 0-.13-.02-.17-.2-1.17-.59-1.83-1.45-3.23-.2-.31-.42-.67-.67-1.11C2.44 6.78 2 5.65 2 5c0-2.2 2.02-4 4.5-4 1.22 0 2.36.42 3.22 1.19C10.55 2.94 11 3.94 11 5c0 .66-.44 1.78-.86 2.48zM4 14h5c-.23 1.14-1.3 2-2.5 2s-2.27-.86-2.5-2z"></path></svg></span>tip</div><div class="admonitionContent_S0QG"><ul><li><p>Tag: Whether it is RocketMQ Tag filtering or delayed message feature, etc., the capabilities of Properties will be used.</p></li><li><p>Keys: The server will create a hash index based on Keys. You are able to query messages based on Topic and Keys in the Console after setting. Please ensure that the keys (e.g. order number, product ID, etc) are unique since it is a hash index. |
| :::</li></ul><p>The relationship between Topic and Tag is shown in the following figure.</p><p><img loading="lazy" alt="Tag" src="/assets/images/Tag-d3d114dcf3f174d165a3749d34c27f7a.png" width="1574" height="472" class="img_ev3q"></p><h3 class="anchor anchorWithStickyNavbar_LWe7" id="when-to-use-topictag">When to use Topic/Tag?<a href="#when-to-use-topictag" class="hash-link" aria-label="Direct link to heading" title="Direct link to heading"></a></h3><p>It can be determined from the following aspects:</p><ul><li><p>Whether the message types are consistent: Such as simple messages, transaction messages, timed (delayed) messages, and ordered messages. Different message types use different Topics, which cannot be distinguished by Tags.</p></li><li><p>Whether the business is related: The messages that are not directly related, such as Taobao messages and JD Logistics messages, are distinguished by different Topics. In contrast, the messages belonging to Tmall transaction, including electrical order, women's clothing order, cosmetics order messages could be distinguished by Tags.</p></li><li><p>Whether the message priority is identical：For example, as logistics message, Hema must be delivered within an hour, Tmall supermarket must be delivered within 24 hours, and Taobao logistics is relatively slower. Messages with different priorities could be distinguished by different topics.</p></li><li><p>Whether the message volume is equivalent: Some business messages are small in volume but require high real-time performance. If they stay under the same Topic with trillion-level messages, it may be "starve" due to the long waiting time. Therefore, it is necessary to split messages of different volumes into different Topics.</p></li></ul><p>In general, you can choose to create multiple Topics, or create multiple Tags under a single Topic for message classification. There is no necessary connection between messages under different Topics, and Tags are used to distinguish interrelated messages under the same topic, such as the complete sets and subsets, or the sequence of processes.</p><h2 class="anchor anchorWithStickyNavbar_LWe7" id="keys">Keys<a href="#keys" class="hash-link" aria-label="Direct link to heading" title="Direct link to heading"></a></h2><p>Each message of Apache RocketMQ can place a unique identification —— Keys field at the business level, which is convenient for locating the problem of message loss in the future. The broker side will create an index (hash index) for each message so that the client can query the content of the message through Topic and Key, as well as who consumes the message. Since it is a hash index, please make sure that the key is as unique as possible to avoid potential hash collisions.</p><div class="language-java codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-java codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token plain"> // Order Id</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> String orderId = "20034568923546";</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> message.setKeys(orderId);</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><h2 class="anchor anchorWithStickyNavbar_LWe7" id="message-queue">Message Queue<a href="#message-queue" class="hash-link" aria-label="Direct link to heading" title="Direct link to heading"></a></h2><p>To support high concurrency and horizontal expansion, Topic needs to be partitioned, which is called Message Queue in RocketMQ. A Topic may have multiple queues and may be distributed on different Brokers.</p><p><img loading="lazy" alt="MessageQueue" src="/assets/images/MessageQueue-fb79728bdee77764e50382620552495d.png" width="1390" height="1152" class="img_ev3q"></p><p>In general, a message will only exist in one of the queues under a Topic if it is not sent repeatedly (e.g., a client resents messages since the server does not respond). The message will be stored in a queue according to the principle of FIFO (First In, First Out). Each message will have its own position, and each queue will calculate the total number of the messages, which is called MaxOffset; the position corresponding to the starting point of the queue is called MinOffset. Message Queue can improve the concurrency of message production and consumption.</p><h2 class="anchor anchorWithStickyNavbar_LWe7" id="producer">Producer<a href="#producer" class="hash-link" aria-label="Direct link to heading" title="Direct link to heading"></a></h2><p>The Producer is the sender of the message. Apache RocketMQ owns rich message types and is able to support various scenarios.</p><p>For instance, an order will be closed due to the payment timeout in an e-commerce transaction, so a delayed message should be sent when the order is created. This message will be delivered to the Consumer after 30 minutes. After receiving the message, the Consumer needs to determine whether the corresponding order has been paid. If the payment is not completed, the order will be closed. If the payment has been completed, then ignore it.</p><p>In the e-commerce scenario, the business requires the messages of the same order to be kept in strict sequence, the ordered messages could therefore be applied.</p><p>In the log processing scenario, a relatively large sending delay is acceptable, but it has a high throughput requirement. It is expected that millions of logs need to be processed within a second. In this case, the batch messages could be sent.</p><p>In the bank deduction scenarios, in order to keep the upstream deduction operation consistent with the downstream SMS notification, transaction messages could be utilized.</p><p>The next section will introduce the sending of various types of messages.</p></div><footer class="theme-doc-footer docusaurus-mt-lg"><div class="theme-doc-footer-edit-meta-row row"><div class="col"><a href="https://github.com/apache/rocketmq-site/tree/new-official-website/docs/02-producer/01concept1.md" target="_blank" rel="noreferrer noopener" class="theme-edit-this-page"><svg fill="currentColor" height="20" width="20" viewBox="0 0 40 40" class="iconEdit_Z9Sw" aria-hidden="true"><g><path d="m34.5 11.7l-3 3.1-6.3-6.3 3.1-3q0.5-0.5 1.2-0.5t1.1 0.5l3.9 3.9q0.5 0.4 0.5 1.1t-0.5 1.2z m-29.5 17.1l18.4-18.5 6.3 6.3-18.4 18.4h-6.3v-6.2z"></path></g></svg>Edit this page</a></div><div class="col lastUpdated_vwxv"></div></div></footer></article><nav 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