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| Guarantees of sends and commits |
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| Exclusive Queues |
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| Message Grouping |
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| Consumer Priority |
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| Management Console |
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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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| </a> |
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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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| <!-- Title --> |
| <h1> |
| <i class="fa fa-circle-o-notch fa-spin"></i> |
| <a href="." >Flow Control</a> |
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| <h1 id="flow-control">Flow Control</h1> |
| <p>Flow control is used to limit the flow of data between a client and |
| server, or a server and another server in order to prevent the client or |
| server being overwhelmed with data.</p> |
| <h2 id="consumer-flow-control">Consumer Flow Control</h2> |
| <p>This controls the flow of data between the server and the client as the |
| client consumes messages. For performance reasons clients normally |
| buffer messages before delivering to the consumer via the <code>receive()</code> |
| method or asynchronously via a message listener. If the consumer cannot |
| process messages as fast as they are being delivered and stored in the |
| internal buffer, then you could end up with a situation where messages |
| would keep building up possibly causing out of memory on the client if |
| they cannot be processed in time.</p> |
| <h2 id="window-based-flow-control">Window-Based Flow Control</h2> |
| <p>By default, Apache ActiveMQ Artemis consumers buffer messages from the server in a |
| client side buffer before the client consumes them. This improves |
| performance: otherwise every time the client consumes a message, |
| Apache ActiveMQ Artemis would have to go the server to request the next message. In |
| turn, this message would then get sent to the client side, if one was |
| available.</p> |
| <p>A network round trip would be involved for <em>every</em> message and |
| considerably reduce performance.</p> |
| <p>To prevent this, Apache ActiveMQ Artemis pre-fetches messages into a buffer on each |
| consumer. The total maximum size of messages (in bytes) that will be |
| buffered on each consumer is determined by the <code>consumerWindowSize</code> |
| parameter.</p> |
| <p>By default, the <code>consumerWindowSize</code> is set to 1 MiB (1024 * 1024 |
| bytes) unless overridden via (<a href="address-model.html#configuring-addresses-and-queues-via-address-settings">Address Settings</a>)</p> |
| <p>The value can be:</p> |
| <ul> |
| <li><p><code>-1</code> for an <em>unbounded</em> buffer</p> |
| </li> |
| <li><p><code>0</code> to not buffer any messages.</p> |
| </li> |
| <li><p><code>>0</code> for a buffer with the given maximum size in bytes.</p> |
| </li> |
| </ul> |
| <p>Setting the consumer window size can considerably improve performance |
| depending on the messaging use case. As an example, let's consider the |
| two extremes:</p> |
| <h3 id="fast-consumers">Fast consumers</h3> |
| <p>Fast consumers can process messages as fast as they consume them (or |
| even faster)</p> |
| <p>To allow fast consumers, set the <code>consumerWindowSize</code> to -1. This |
| will allow <em>unbounded</em> message buffering on the client side.</p> |
| <p>Use this setting with caution: it can overflow the client memory if |
| the consumer is not able to process messages as fast as it receives |
| them.</p> |
| <h3 id="slow-consumers">Slow consumers</h3> |
| <p>Slow consumers takes significant time to process each message and it |
| is desirable to prevent buffering messages on the client side so |
| that they can be delivered to another consumer instead.</p> |
| <p>Consider a situation where a queue has 2 consumers; 1 of which is |
| very slow. Messages are delivered in a round robin fashion to both |
| consumers, the fast consumer processes all of its messages very |
| quickly until its buffer is empty. At this point there are still |
| messages awaiting to be processed in the buffer of the slow consumer |
| thus preventing them being processed by the fast consumer. The fast |
| consumer is therefore sitting idle when it could be processing the |
| other messages.</p> |
| <p>To allow slow consumers, set <code>consumerWindowSize</code> on the URI to 0 |
| (for no buffer at all). This will prevent the slow consumer from |
| buffering any messages on the client side. Messages will remain on |
| the server side ready to be consumed by other consumers.</p> |
| <p>Setting this to 0 can give deterministic distribution between |
| multiple consumers on a queue.</p> |
| <p>Most of the consumers cannot be clearly identified as fast or slow |
| consumers but are in-between. In that case, setting the value of |
| <code>consumerWindowSize</code> to optimize performance depends on the messaging |
| use case and requires benchmarks to find the optimal value, but a value |
| of 1MiB is fine in most cases.</p> |
| <p>Please see <a href="examples.html">the examples chapter</a> for an example which shows |
| how to configure ActiveMQ Artemis to prevent consumer buffering when dealing |
| with slow consumers.</p> |
| <h2 id="rate-limited-flow-control">Rate limited flow control</h2> |
| <p>It is also possible to control the <em>rate</em> at which a consumer can |
| consume messages. This is a form of throttling and can be used to make |
| sure that a consumer never consumes messages at a rate faster than the |
| rate specified. This is configured using the <code>consumerMaxRate</code> URI |
| parameter.</p> |
| <p>The rate must be a positive integer to enable this functionality and is |
| the maximum desired message consumption rate specified in units of |
| messages per second. Setting this to <code>-1</code> disables rate limited flow |
| control. The default value is <code>-1</code>.</p> |
| <p>Please see <a href="examples.html">the examples chapter</a> for a working example of |
| limiting consumer rate.</p> |
| <blockquote> |
| <p><strong>Note:</strong></p> |
| <p>Rate limited flow control can be used in conjunction with window based |
| flow control. Rate limited flow control only effects how many messages |
| a client can consume in a second and not how many messages are in its |
| buffer. So if you had a slow rate limit and a high window based limit |
| the clients internal buffer would soon fill up with messages.</p> |
| </blockquote> |
| <h2 id="producer-flow-control">Producer flow control</h2> |
| <p>Apache ActiveMQ Artemis also can limit the amount of data sent from a client to a |
| server to prevent the server being overwhelmed.</p> |
| <h3 id="window-based-flow-control">Window based flow control</h3> |
| <p>In a similar way to consumer window based flow control, Apache ActiveMQ Artemis |
| producers, by default, can only send messages to an address as long as |
| they have sufficient credits to do so. The amount of credits required to |
| send a message is given by the size of the message.</p> |
| <p>As producers run low on credits they request more from the server, when |
| the server sends them more credits they can send more messages.</p> |
| <p>The amount of credits a producer requests in one go is known as the |
| <em>window size</em> and it is controlled by the <code>producerWindowSize</code> URI |
| parameter.</p> |
| <p>The window size therefore determines the amount of bytes that can be |
| in-flight at any one time before more need to be requested - this |
| prevents the remoting connection from getting overloaded.</p> |
| <h4 id="blocking-core-producers">Blocking CORE Producers</h4> |
| <p>When using the CORE protocol (used by both the Artemis Core Client and Artemis JMS Client) |
| the server will always aim give the same number of credits as have been requested. |
| However, it is also possible to set a maximum size on any address, and the server |
| will never send more credits to any one producer than what is available according to |
| the address's upper memory limit. Although a single producer will be issued more |
| credits than available (at the time of issue) it is possible that more than 1 |
| producer be associated with the same address and so it is theoretically possible |
| that more credits are allocated across total producers than what is available. |
| It is therefore possible to go over the address limit by approximately:</p> |
| <pre><code>total number of producers on address * producer window size |
| </code></pre><p>For example, if I have a queue called "myqueue", I could set the |
| maximum memory size to 10MiB, and the the server will control the number |
| of credits sent to any producers which are sending any messages to |
| myqueue such that the total messages in the queue never exceeds 10MiB.</p> |
| <p>When the address gets full, producers will block on the client side |
| until more space frees up on the address, i.e. until messages are |
| consumed from the queue thus freeing up space for more messages to be |
| sent.</p> |
| <p>We call this blocking producer flow control, and it's an efficient way |
| to prevent the server running out of memory due to producers sending |
| more messages than can be handled at any time.</p> |
| <p>It is an alternative approach to paging, which does not block producers |
| but instead pages messages to storage.</p> |
| <p>To configure an address with a maximum size and tell the server that you |
| want to block producers for this address if it becomes full, you need to |
| define an AddressSettings (<a href="address-model.html#configuring-addresses-and-queues-via-address-settings">Configuring Queues Via Address Settings</a>) block for the address and specify |
| <code>max-size-bytes</code> and <code>address-full-policy</code></p> |
| <p>The address block applies to all queues registered to that address. I.e. |
| the total memory for all queues bound to that address will not exceed |
| <code>max-size-bytes</code>. In the case of JMS topics this means the <em>total</em> |
| memory of all subscriptions in the topic won't exceed max-size-bytes.</p> |
| <p>Here's an example:</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">"exampleQueue"</span>></span> |
| <span class="hljs-tag"><<span class="hljs-name">max-size-bytes</span>></span>100000<span class="hljs-tag"></<span class="hljs-name">max-size-bytes</span>></span> |
| <span class="hljs-tag"><<span class="hljs-name">address-full-policy</span>></span>BLOCK<span class="hljs-tag"></<span class="hljs-name">address-full-policy</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 above example would set the max size of the queue "exampleQueue" |
| to be 100000 bytes and would block any producers sending to that address |
| to prevent that max size being exceeded.</p> |
| <p>Note the policy must be set to <code>BLOCK</code> to enable blocking producer flow |
| control.</p> |
| <blockquote> |
| <p><strong>Note:</strong></p> |
| <p>Note that in the default configuration all addresses are set to block |
| producers after 10 MiB of message data is in the address. This means |
| you cannot send more than 10MiB of message data to an address without |
| it being consumed before the producers will be blocked. If you do not |
| want this behaviour increase the <code>max-size-bytes</code> parameter or change |
| the address full message policy.</p> |
| <p><strong>Note:</strong></p> |
| <p>Producer credits are allocated from the broker to the client. Flow control |
| credit checking (i.e. checking a producer has enough credit) is done on the |
| client side only. It is possible for the broker to over allocate credits, like |
| in the multiple producer scenario outlined above. It is also possible for |
| a misbehaving client to ignore the flow control credits issued by the broker |
| and continue sending with out sufficient credit.</p> |
| </blockquote> |
| <h4 id="blocking-amqp-producers">Blocking AMQP Producers</h4> |
| <p>Apache ActiveMQ Artemis ships with out of the box with 2 protocols that support flow control. Artemis CORE protocol and |
| AMQP. Both protocols implement flow control slightly differently and therefore address full BLOCK policy behaves slightly |
| different for clients that use each protocol respectively.</p> |
| <p>As explained earlier in this chapter the CORE protocol uses a producer window size flow control system. Where credits |
| (representing bytes) are allocated to producers, if a producer wants to send a message it should wait until it has |
| enough byte credits available for it to send. AMQP flow control credits are not representative of bytes but instead |
| represent the number of messages a producer is permitted to send (regardless of the message size).</p> |
| <p>BLOCK for AMQP works mostly in the same way as the producer window size mechanism above. Artemis will issue 100 credits |
| to a client at a time and refresh them when the clients credits reaches 30. The broker will stop issuing credits once an |
| address is full. However, since AMQP credits represent whole messages and not bytes, it would be possible in some |
| scenarios for an AMQP client to significantly exceed an address upper bound should the broker continue accepting |
| messages until the clients credits are exhausted. For this reason there is an additional parameter available on address |
| settings that specifies an upper bound on an address size in bytes. Once this upper bound is reach Artemis will start |
| rejecting AMQP messages. This limit is the max-size-bytes-reject-threshold and is by default set to -1 (or no limit). |
| This is additional parameter allows a kind of soft and hard limit, in normal circumstances the broker will utilize the |
| max-size-bytes parameter using using flow control to put back pressure on the client, but will protect the broker by |
| rejecting messages once the address size is reached.</p> |
| <h3 id="rate-limited-flow-control">Rate limited flow control</h3> |
| <p>Apache ActiveMQ Artemis also allows the rate a producer can emit message to be limited, |
| in units of messages per second. By specifying such a rate, Apache ActiveMQ Artemis |
| will ensure that producer never produces messages at a rate higher than |
| that specified. This is controlled by the <code>producerMaxRate</code> URL parameter.</p> |
| <p>The <code>producerMaxRate</code> must be a positive integer to enable this functionality and is |
| the maximum desired message production rate specified in units of |
| messages per second. Setting this to <code>-1</code> disables rate limited flow |
| control. The default value is <code>-1</code>.</p> |
| <p>Please see <a href="examples.html">the examples chapter</a> for a working example of limiting |
| producer rate.</p> |
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