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class="breadcrumbs__item breadcrumbs__item--active"><span class="breadcrumbs__link" itemprop="name">Subscription</span><meta itemprop="position" content="2"></li></ul></nav><span class="theme-doc-version-badge badge badge--secondary">Version: 5.0</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>Subscription</h1><p>This section describes the definition, model relationship, internal attributes, and usage notes for subscriptions in Apache RocketMQ.</p><h2 class="anchor anchorWithStickyNavbar_LWe7" id="definition">Definition<a href="#definition" class="hash-link" aria-label="Direct link to heading" title="Direct link to heading"></a></h2><p>A subscription is the rule and status settings for consumers to obtain and process messages in Apache RocketMQ.</p><p>Subscriptions are dynamically registered by consumer groups with brokers. Messages are then matched and consumed based on the filter rules defined by subscriptions.</p><p>By configuring subscriptions, you can control the following messaging behaviors:</p><ul><li><p>Message filter rules: These rules are used to define which messages in a topic are consumed by a consumer. By configuring message filter rules, consumers can effectively obtain messages that they want and specify message receiving ranges based on different business scenarios. For more information, see <a href="/docs/featureBehavior/07messagefilter">Message filtering</a>.</p></li><li><p>Consumption status: By default, the Apache RocketMQ broker provides persistent subscriptions. In other words, after a consumer group subscribes to a broker, consumers in the group can continue consuming messages from where the consumers left off after they reconnect.</p></li></ul><h2 class="anchor anchorWithStickyNavbar_LWe7" id="rules-for-determining-a-subscription">Rules for determining a subscription<a href="#rules-for-determining-a-subscription" class="hash-link" aria-label="Direct link to heading" title="Direct link to heading"></a></h2><p>The subscriptions of Apache RocketMQ are designed based on consumer groups and topics. Therefore, a subscription refers to the subscription of a specified consumer group to a topic. The following describes the rules for determining a subscription:</p><ul><li><p>One topic to many subscribersThe following figure shows two consumer groups (Group A and Group B) subscribed to Topic A. These two subscriptions are independent of each other and can be defined separately. |
| <img loading="lazy" alt="Subscription relationships are grouped differently" src="/assets/images/subscription_one_group-77bd92b987e8264ad3c5f27b29463942.png" width="1345" height="466" class="img_ev3q"></p></li></ul><h2 class="anchor anchorWithStickyNavbar_LWe7" id="model-relationship">Model relationship<a href="#model-relationship" class="hash-link" aria-label="Direct link to heading" title="Direct link to heading"></a></h2><p>The following figure shows the position of subscriptions in the domain model of Apache RocketMQ.<img loading="lazy" alt="Subscriptions" src="/assets/images/archiforsubsciption-a495c04e71ed64b9403b689f9413ed08.png" width="1977" height="877" class="img_ev3q"></p><ol><li><p>The message is initialized by the producer and sent to the Apache RocketMQ server.</p></li><li><p>Messages are stored in the specified queue of the topic in the order in which they arrive at the Apache RocketMQ server.</p></li><li><p>The consumer obtains and consumes messages from the Apache RocketMQ server based on the specified subscription relationship.</p></li></ol><h2 class="anchor anchorWithStickyNavbar_LWe7" id="internal-attributes">Internal attributes<a href="#internal-attributes" class="hash-link" aria-label="Direct link to heading" title="Direct link to heading"></a></h2><p><strong>Filter types</strong></p><ul><li><p>Definition: the type of a message filter rule. After a message filter rule is set for a subscription, the system matches the messages in a topic based on the filter rule. Only the messages that meet the conditions are delivered to consumers. This feature helps you classify messages sent to consumers based on your requirements.</p></li><li><p>Values:</p><ul><li><p>Tag filter: filters and matches the full text based on tag strings.</p></li><li><p>SQL92 filter: filters and matches message attributes based on SQL syntax.</p></li></ul></li></ul><p><strong>Filter expressions</strong></p><ul><li><p>Definition: the expression of a custom filter rule.</p></li><li><p>Values: For more information, see <a href="/docs/featureBehavior/07messagefilter">Syntax for filter expressions</a>.</p></li></ul><h2 class="anchor anchorWithStickyNavbar_LWe7" id="behavior-constraints">Behavior constraints<a href="#behavior-constraints" class="hash-link" aria-label="Direct link to heading" title="Direct link to heading"></a></h2><p><strong>Subscription consistency</strong></p><p>Apache RocketMQ manages subscriptions based on consumer groups. Therefore, consumers in the same consumer group must maintain the same consumption logic. Otherwise, consumption conflicts occur, which in turn causes some messages to be incorrectly consumed.</p><ul><li><p>Correct example</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">//Consumer c1</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">Consumer c1 = ConsumerBuilder.build(groupA);</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">c1.subscribe(topicA,"TagA");</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">//Consumer c2</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">Consumer c2 = ConsumerBuilder.build(groupA);</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">c2.subscribe(topicA,"TagA");</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></li></ul><ul><li><p>Incorrect example</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">//Consumer c1</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">Consumer c1 = ConsumerBuilder.build(groupA);</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">c1.subscribe(topicA,"TagA");</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">//Consumer c2Consumer </span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">c2 = ConsumerBuilder.build(groupA);</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">c2.subscribe(topicA,"TagB");</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></li></ul><h2 class="anchor anchorWithStickyNavbar_LWe7" id="usage-notes">Usage notes<a href="#usage-notes" class="hash-link" aria-label="Direct link to heading" title="Direct link to heading"></a></h2><p><strong>Do not frequently modify subscriptions.</strong></p><p>In Apache RocketMQ, subscriptions are associated with metadata and configurations such as filter rules and consumption progress. The system must also ensure that the consumption behavior, consumption logic, and load policy of all consumers in the consumer group are consistent. These factors result in a complex web of relationships that need to be managed. Therefore, we recommend that you do not regularly modify subscriptions to change the business logic in the production environment. 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