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<title>34. Inter-Application Communication</title><link rel="stylesheet" type="text/css" href="css/manual-multipage.css"><meta name="generator" content="DocBook XSL Stylesheets V1.78.1"><link rel="home" href="multi_spring-cloud.html" title="Spring Cloud"><link rel="up" href="multi__spring_cloud_stream.html" title="Part V. Spring Cloud Stream"><link rel="prev" href="multi_schema-evolution.html" title="33. Schema evolution support"><link rel="next" href="multi__testing.html" title="35. Testing"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">34. Inter-Application Communication</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="multi_schema-evolution.html">Prev</a> </td><th width="60%" align="center">Part V. Spring Cloud Stream</th><td width="20%" align="right"> <a accesskey="n" href="multi__testing.html">Next</a></td></tr></table><hr></div><div class="chapter"><div class="titlepage"><div><div><h2 class="title"><a name="_inter_application_communication" href="#_inter_application_communication"></a>34. Inter-Application Communication</h2></div></div></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_connecting_multiple_application_instances" href="#_connecting_multiple_application_instances"></a>34.1 Connecting Multiple Application Instances</h2></div></div></div><p>While Spring Cloud Stream makes it easy for individual Spring Boot applications to connect to messaging systems, the typical scenario for Spring Cloud Stream is the creation of multi-application pipelines, where microservice applications send data to each other.
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You can achieve this scenario by correlating the input and output destinations of adjacent applications.</p><p>Supposing that a design calls for the Time Source application to send data to the Log Sink application, you can use a common destination named <code class="literal">ticktock</code> for bindings within both applications.</p><p>Time Source (that has the channel name <code class="literal">output</code>) will set the following property:</p><pre class="screen">spring.cloud.stream.bindings.output.destination=ticktock</pre><p>Log Sink (that has the channel name <code class="literal">input</code>) will set the following property:</p><pre class="screen">spring.cloud.stream.bindings.input.destination=ticktock</pre></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_instance_index_and_instance_count" href="#_instance_index_and_instance_count"></a>34.2 Instance Index and Instance Count</h2></div></div></div><p>When scaling up Spring Cloud Stream applications, each instance can receive information about how many other instances of the same application exist and what its own instance index is.
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Spring Cloud Stream does this through the <code class="literal">spring.cloud.stream.instanceCount</code> and <code class="literal">spring.cloud.stream.instanceIndex</code> properties.
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For example, if there are three instances of a HDFS sink application, all three instances will have <code class="literal">spring.cloud.stream.instanceCount</code> set to <code class="literal">3</code>, and the individual applications will have <code class="literal">spring.cloud.stream.instanceIndex</code> set to <code class="literal">0</code>, <code class="literal">1</code>, and <code class="literal">2</code>, respectively.</p><p>When Spring Cloud Stream applications are deployed via Spring Cloud Data Flow, these properties are configured automatically; when Spring Cloud Stream applications are launched independently, these properties must be set correctly.
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By default, <code class="literal">spring.cloud.stream.instanceCount</code> is <code class="literal">1</code>, and <code class="literal">spring.cloud.stream.instanceIndex</code> is <code class="literal">0</code>.</p><p>In a scaled-up scenario, correct configuration of these two properties is important for addressing partitioning behavior (see below) in general, and the two properties are always required by certain binders (e.g., the Kafka binder) in order to ensure that data are split correctly across multiple consumer instances.</p></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_partitioning" href="#_partitioning"></a>34.3 Partitioning</h2></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="_configuring_output_bindings_for_partitioning" href="#_configuring_output_bindings_for_partitioning"></a>34.3.1 Configuring Output Bindings for Partitioning</h3></div></div></div><p>An output binding is configured to send partitioned data by setting one and only one of its <code class="literal">partitionKeyExpression</code> or <code class="literal">partitionKeyExtractorName</code> (see next paragraph) properties, as well as its <code class="literal">partitionCount</code> property.</p><p>For example, the following is a valid and typical configuration:</p><pre class="screen">spring.cloud.stream.bindings.output.producer.partitionKeyExpression=payload.id
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spring.cloud.stream.bindings.output.producer.partitionCount=5</pre><p>Based on the above example configuration, data will be sent to the target partition using the following logic.</p><p>A partition key’s value is calculated for each message sent to a partitioned output channel based on the <code class="literal">partitionKeyExpression</code>.
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The <code class="literal">partitionKeyExpression</code> is a SpEL expression which is evaluated against the outbound message for extracting the partitioning key.</p><p>If a SpEL expression is not sufficient for your needs, you can instead calculate the partition key value by providing implementation of <code class="literal">org.springframework.cloud.stream.binder.PartitionKeyExtractorStrategy</code> and configuring it as a bean (i.e., <code class="literal">@Bean</code>). In the event you have more then one bean of type <code class="literal">org.springframework.cloud.stream.binder.PartitionKeyExtractorStrategy</code> available in the Application Context you can further filter it by specifying its name via <code class="literal">partitionKeyExtractorName</code> property:</p><pre class="screen">--spring.cloud.stream.bindings.output.producer.partitionKeyExtractorName=customPartitionKeyExtractor
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--spring.cloud.stream.bindings.output.producer.partitionCount=5
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. . .
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@Bean
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public CustomPartitionKeyExtractorClass customPartitionKeyExtractor() {
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return new CustomPartitionKeyExtractorClass();
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}</pre><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><table border="0" summary="Note"><tr><td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="images/note.png"></td><th align="left">Note</th></tr><tr><td align="left" valign="top"><p>In previous versions of Spring Cloud Stream you could specify the implementation of <code class="literal">org.springframework.cloud.stream.binder.PartitionKeyExtractorStrategy</code> as <code class="literal">spring.cloud.stream.bindings.output.producer.partitionKeyExtractorClass</code> property. Since version 2.0 this property is deprecated and support for it will be removed in a future version.</p></td></tr></table></div><p>Ones the message key is calculated, the partition selection process will determine the target partition as a value between <code class="literal">0</code> and <code class="literal">partitionCount - 1</code>.
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The default calculation, applicable in most scenarios, is based on the formula <code class="literal">key.hashCode() % partitionCount</code>.
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This can be customized on the binding, either by setting a SpEL expression to be evaluated against the 'key' (via the <code class="literal">partitionSelectorExpression</code> property) or by configuring an implementation of <code class="literal">org.springframework.cloud.stream.binder.PartitionSelectorStrategy</code> as a bean (i.e., @Bean). And similarly to the <code class="literal">PartitionKeyExtractorStrategy</code> you can further filter it using <code class="literal">spring.cloud.stream.bindings.output.producer.partitionSelectorName</code> property in the event there are more then one bean of this type is available in the Application Context.</p><pre class="screen">--spring.cloud.stream.bindings.output.producer.partitionSelectorName=customPartitionSelector
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. . .
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@Bean
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public CustomPartitionSelectorClass customPartitionSelector() {
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return new CustomPartitionSelectorClass();
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}</pre><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><table border="0" summary="Note"><tr><td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="images/note.png"></td><th align="left">Note</th></tr><tr><td align="left" valign="top"><p>In previous versions of Spring Cloud Stream you could specify the implementation of <code class="literal">org.springframework.cloud.stream.binder.PartitionSelectorStrategy</code> as <code class="literal">spring.cloud.stream.bindings.output.producer.partitionSelectorClass</code> property. Since version 2.0 this property is deprecated and support for it will be removed in a future version.</p></td></tr></table></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a name="_configuring_input_bindings_for_partitioning" href="#_configuring_input_bindings_for_partitioning"></a>Configuring Input Bindings for Partitioning</h4></div></div></div><p>An input binding (with the channel name <code class="literal">input</code>) is configured to receive partitioned data by setting its <code class="literal">partitioned</code> property, as well as the <code class="literal">instanceIndex</code> and <code class="literal">instanceCount</code> properties on the application itself, as in the following example:</p><pre class="screen">spring.cloud.stream.bindings.input.consumer.partitioned=true
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spring.cloud.stream.instanceIndex=3
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spring.cloud.stream.instanceCount=5</pre><p>The <code class="literal">instanceCount</code> value represents the total number of application instances between which the data need to be partitioned, and the <code class="literal">instanceIndex</code> must be a unique value across the multiple instances, between <code class="literal">0</code> and <code class="literal">instanceCount - 1</code>.
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The instance index helps each application instance to identify the unique partition(s) from which it receives data.
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It is required by binders using technology that doesn’t support partitioning natively, for example, with RabbitMQ, there is a queue for each partition, with the queue name containing the instance index.
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With Kafka, if <code class="literal">autoRebalanceEnabled</code> is <code class="literal">true</code> (default), Kafka will take care of distributing partitions across instances and these properties are not required.
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If <code class="literal">autoRebalanceEnabled</code> is set to false, the <code class="literal">instanceCount</code> and <code class="literal">instanceIndex</code> are used by the binder to determine which partition(s) the instance will subscribe to (you must have at least as many partitions as there are instances).
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The binder will allocate the partitions instead of Kafka.
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This might be useful if you want messages for a particular partition to always go to the same instance.
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When a binder configuration that requires them, it is important to set both values correctly in order to ensure that all of the data is consumed and that the application instances receive mutually exclusive datasets.</p><p>While a scenario which using multiple instances for partitioned data processing may be complex to set up in a standalone case, Spring Cloud Dataflow can simplify the process significantly by populating both the input and output values correctly as well as relying on the runtime infrastructure to provide information about the instance index and instance count.</p></div></div></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="multi_schema-evolution.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="multi__spring_cloud_stream.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="multi__testing.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">33. Schema evolution support </td><td width="20%" align="center"><a accesskey="h" href="multi_spring-cloud.html">Home</a></td><td width="40%" align="right" valign="top"> 35. Testing</td></tr></table></div></body></html> |