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| Exclusive Queues |
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| Metrics |
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| Security |
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| Masking Passwords |
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| Broker Plugins |
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| Resource Limits |
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| The JMS Bridge |
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| Client Reconnection and Session Reattachment |
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| Diverting and Splitting Message Flows |
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| Core Bridges |
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| Transformers |
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| Duplicate Message Detection |
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| Clusters |
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| Federation |
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| Address Federation |
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| Queue Federation |
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| High Availability and Failover |
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| Graceful Server Shutdown |
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| Libaio Native Libraries |
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| Thread management |
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| Embedded Web Server |
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| Logging |
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| REST Interface |
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| Embedding the Broker |
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| Apache Karaf |
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| Apache Tomcat |
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| Spring Integration |
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| CDI Integration |
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| Intercepting Operations |
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| Data Tools |
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| <h1> |
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| <a href="." >Ring Queues</a> |
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| <section class="normal markdown-section"> |
| |
| <h1 id="ring-queue">Ring Queue</h1> |
| <p>Queues operate with first-in, first-out (FIFO) semantics which means that |
| messages, in general, are added to the "tail" of the queue and removed from the |
| "head." A "ring" queue is a special type of queue with a <em>fixed</em> size. The |
| fixed size is maintained by removing the message at the head of the queue when |
| the number of messages on the queue reaches the configured size.</p> |
| <p>For example, consider a queue configured with a ring size of 3 and a producer |
| which sends the messages <code>A</code>, <code>B</code>, <code>C</code>, & <code>D</code> in that order. Once <code>C</code> is sent |
| the number of messages in the queue will be 3 which is the same as the |
| configured ring size. We can visualize the queue growth like this...</p> |
| <p>After <code>A</code> is sent:</p> |
| <pre><code> |---| |
| head/tail -> | A | |
| |---| |
| </code></pre><p>After <code>B</code> is sent:</p> |
| <pre><code> |---| |
| head -> | A | |
| |---| |
| tail -> | B | |
| |---| |
| </code></pre><p>After <code>C</code> is sent:</p> |
| <pre><code> |---| |
| head -> | A | |
| |---| |
| | B | |
| |---| |
| tail -> | C | |
| |---| |
| </code></pre><p>When <code>D</code> is sent it will be added to the tail of the queue and the message at |
| the head of the queue (i.e. <code>A</code>) will be removed so the queue will look like |
| this:</p> |
| <pre><code> |---| |
| head -> | B | |
| |---| |
| | C | |
| |---| |
| tail -> | D | |
| |---| |
| </code></pre><p>This example covers the most basic use case with messages being added to the |
| tail of the queue. However, there are a few other important use cases |
| involving:</p> |
| <ul> |
| <li>Messages in delivery & rollbacks</li> |
| <li>Scheduled messages</li> |
| <li>Paging</li> |
| </ul> |
| <p>However, before we get to those use cases let's look at the basic configuration |
| of a ring queue.</p> |
| <h2 id="configuration">Configuration</h2> |
| <p>There are 2 parameters related to ring queue configuration.</p> |
| <p>The <code>ring-size</code> parameter can be set directly on the <code>queue</code> element. The |
| default value comes from the <code>default-ring-size</code> <code>address-setting</code> (see below).</p> |
| <pre><code class="lang-xml"><span class="hljs-tag"><<span class="hljs-name">addresses</span>></span> |
| <span class="hljs-tag"><<span class="hljs-name">address</span> <span class="hljs-attr">name</span>=<span class="hljs-string">"myRing"</span>></span> |
| <span class="hljs-tag"><<span class="hljs-name">anycast</span>></span> |
| <span class="hljs-tag"><<span class="hljs-name">queue</span> <span class="hljs-attr">name</span>=<span class="hljs-string">"myRing"</span> <span class="hljs-attr">ring-size</span>=<span class="hljs-string">"3"</span> /></span> |
| <span class="hljs-tag"></<span class="hljs-name">anycast</span>></span> |
| <span class="hljs-tag"></<span class="hljs-name">address</span>></span> |
| <span class="hljs-tag"></<span class="hljs-name">addresses</span>></span> |
| </code></pre> |
| <p>The <code>default-ring-size</code> is an <code>address-setting</code> which applies to queues on |
| matching addresses which don't have an explicit <code>ring-size</code> set. This is |
| especially useful for auto-created queues. The default value is <code>-1</code> (i.e. |
| no limit).</p> |
| <pre><code class="lang-xml"><span class="hljs-tag"><<span class="hljs-name">address-settings</span>></span> |
| <span class="hljs-tag"><<span class="hljs-name">address-setting</span> <span class="hljs-attr">match</span>=<span class="hljs-string">"ring.#"</span>></span> |
| <span class="hljs-tag"><<span class="hljs-name">default-ring-size</span>></span>3<span class="hljs-tag"></<span class="hljs-name">default-ring-size</span>></span> |
| <span class="hljs-tag"></<span class="hljs-name">address-setting</span>></span> |
| <span class="hljs-tag"></<span class="hljs-name">address-settings</span>></span> |
| </code></pre> |
| <p>The <code>ring-size</code> may be updated at runtime. If the new <code>ring-size</code> is set |
| <em>lower</em> than the previous <code>ring-size</code> the broker will not immediately delete |
| enough messages from the head of the queue to enforce the new size. New |
| messages sent to the queue will force the deletion of old messages (i.e. the |
| queue won't grow any larger), but the queue will not reach its new size until |
| it does so <em>naturally</em> through the normal consumption of messages by |
| clients.</p> |
| <h2 id="messages-in-delivery--rollbacks">Messages in Delivery & Rollbacks</h2> |
| <p>When messages are "in delivery" they are in an in-between state where they are |
| not technically on the queue but they are also not yet acknowledged. The |
| broker is at the consumer’s mercy to either acknowledge such messages or not. |
| In the context of a ring queue, messages which are in-delivery cannot be |
| removed from the queue.</p> |
| <p>This presents a few dilemmas.</p> |
| <p>Due to the nature of messages in delivery a client can actually send more |
| messages to a ring queue than it would otherwise permit. This can make it |
| appear that the ring-size is not being enforced properly. Consider this |
| simple scenario:</p> |
| <ul> |
| <li>Queue <code>foo</code> with <code>ring-size="3"</code></li> |
| <li>1 Consumer on queue <code>foo</code></li> |
| <li>Message <code>A</code> sent to <code>foo</code> & dispatched to consumer</li> |
| <li><code>messageCount</code>=1, <code>deliveringCount</code>=1</li> |
| <li>Message <code>B</code> sent to <code>foo</code> & dispatched to consumer</li> |
| <li><code>messageCount</code>=2, <code>deliveringCount</code>=2</li> |
| <li>Message <code>C</code> sent to <code>foo</code> & dispatched to consumer</li> |
| <li><code>messageCount</code>=3, <code>deliveringCount</code>=3</li> |
| <li>Message <code>D</code> sent to <code>foo</code> & dispatched to consumer</li> |
| <li><code>messageCount</code>=4, <code>deliveringCount</code>=4</li> |
| </ul> |
| <p>The <code>messageCount</code> for <code>foo</code> is now 4, one <em>greater</em> than the <code>ring-size</code> |
| of 3! However, the broker has no choice but to allow this because it cannot |
| remove messages from the queue which are in delivery.</p> |
| <p>Now consider that the consumer is closed without actually acknowledging any |
| of these 4 messages. These 4 in-delivery, unacknowledged messages will be |
| cancelled back to the broker and added to the <em>head</em> of the queue in the |
| reverse order from which they were consumed. This, of course, will put the |
| queue over its configured <code>ring-size</code>. Therefore, since a ring queue |
| prefers messages at the tail of the queue over messages at the head it will |
| keep <code>B</code>, <code>C</code>, & <code>D</code> and delete <code>A</code> (since <code>A</code> was the last message added |
| to the head of the queue).</p> |
| <p>Transaction or core session rollbacks are treated the same way.</p> |
| <p>If you wish to avoid these kinds of situations and you're using the core |
| client directly or the core JMS client you can minimize messages in delivery |
| by reducing the size of <code>consumerWindowSize</code> (1024 * 1024 bytes by default).</p> |
| <h2 id="scheduled-messages">Scheduled Messages</h2> |
| <p>When a scheduled message is sent to a queue it isn't immediately added to the |
| tail of the queue like normal messages. It is held in an intermediate buffer |
| and scheduled for delivery onto the <em>head</em> of the queue according to the |
| details of the message. However, scheduled messages are nevertheless reflected |
| in the message count of the queue. As with messages which are in delivery this |
| can make it appear that the ring queue's size is not being enforced. Consider |
| this simple scenario:</p> |
| <ul> |
| <li>Queue <code>foo</code> with <code>ring-size="3"</code></li> |
| <li>At 12:00 message <code>A</code> sent to <code>foo</code> scheduled for 12:05</li> |
| <li><code>messageCount</code>=1, <code>scheduledCount</code>=1</li> |
| <li>At 12:01 message <code>B</code> sent to <code>foo</code></li> |
| <li><code>messageCount</code>=2, <code>scheduledCount</code>=1</li> |
| <li>At 12:02 message <code>C</code> sent to <code>foo</code></li> |
| <li><code>messageCount</code>=3, <code>scheduledCount</code>=1</li> |
| <li>At 12:03 message <code>D</code> sent to <code>foo</code></li> |
| <li><code>messageCount</code>=4, <code>scheduledCount</code>=1</li> |
| </ul> |
| <p>The <code>messageCount</code> for <code>foo</code> is now 4, one <em>greater</em> than the <code>ring-size</code> of 3! |
| However, the scheduled message is not technically on the queue yet (i.e. it is |
| on the broker and scheduled to be put on the queue). When the scheduled |
| delivery time for 12:05 comes the message will put on the head of the queue, |
| but since the ring queue's size has already been reach the scheduled message |
| <code>A</code> will be removed.</p> |
| <h2 id="paging">Paging</h2> |
| <p>Similar to scheduled messages and messages in delivery, paged messages don't |
| count against a ring queue's size because messages are actually paged at the |
| <em>address</em> level, not the queue level. A paged message is not technically on a |
| queue although it is reflected in a queue's <code>messageCount</code>.</p> |
| <p>It is recommended that paging is not used for addresses with ring queues. In |
| other words, ensure that the entire address will be able to fit into memory or |
| use the <code>DROP</code>, <code>BLOCK</code> or <code>FAIL</code> <code>address-full-policy</code>.</p> |
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