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<title>25. Programming Model</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 IV. Spring Cloud Stream"><link rel="prev" href="multi__main_concepts.html" title="24. Main Concepts"><link rel="next" href="multi__binders.html" title="26. Binders"></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">25. Programming Model</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="multi__main_concepts.html">Prev</a> </td><th width="60%" align="center">Part IV. Spring Cloud Stream</th><td width="20%" align="right"> <a accesskey="n" href="multi__binders.html">Next</a></td></tr></table><hr></div><div class="chapter"><div class="titlepage"><div><div><h2 class="title"><a name="_programming_model" href="#_programming_model"></a>25. Programming Model</h2></div></div></div><p>This section describes Spring Cloud Stream’s programming model.
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Spring Cloud Stream provides a number of predefined annotations for declaring bound input and output channels as well as how to listen to channels.</p><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_declaring_and_binding_producers_and_consumers" href="#_declaring_and_binding_producers_and_consumers"></a>25.1 Declaring and Binding Producers and Consumers</h2></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="_triggering_binding_via_literal_enablebinding_literal" href="#_triggering_binding_via_literal_enablebinding_literal"></a>25.1.1 Triggering Binding Via <code class="literal">@EnableBinding</code></h3></div></div></div><p>You can turn a Spring application into a Spring Cloud Stream application by applying the <code class="literal">@EnableBinding</code> annotation to one of the application’s configuration classes.
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<title>26. Programming Model</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__main_concepts.html" title="25. Main Concepts"><link rel="next" href="multi__binders.html" title="27. Binders"></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">26. Programming Model</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="multi__main_concepts.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__binders.html">Next</a></td></tr></table><hr></div><div class="chapter"><div class="titlepage"><div><div><h2 class="title"><a name="_programming_model" href="#_programming_model"></a>26. Programming Model</h2></div></div></div><p>This section describes Spring Cloud Stream’s programming model.
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Spring Cloud Stream provides a number of predefined annotations for declaring bound input and output channels as well as how to listen to channels.</p><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_declaring_and_binding_producers_and_consumers" href="#_declaring_and_binding_producers_and_consumers"></a>26.1 Declaring and Binding Producers and Consumers</h2></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="_triggering_binding_via_literal_enablebinding_literal" href="#_triggering_binding_via_literal_enablebinding_literal"></a>26.1.1 Triggering Binding Via <code class="literal">@EnableBinding</code></h3></div></div></div><p>You can turn a Spring application into a Spring Cloud Stream application by applying the <code class="literal">@EnableBinding</code> annotation to one of the application’s configuration classes.
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The <code class="literal">@EnableBinding</code> annotation itself is meta-annotated with <code class="literal">@Configuration</code> and triggers the configuration of Spring Cloud Stream infrastructure:</p><pre class="programlisting">...
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<em><span class="hl-annotation" style="color: gray">@Import(...)</span></em>
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<em><span class="hl-annotation" style="color: gray">@Configuration</span></em>
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@@ -10,7 +10,7 @@ The <code class="literal">@EnableBinding</code> annotation itself is meta-annota
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...
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Class<?>[] value() <span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">default</span> {};
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}</pre><p>The <code class="literal">@EnableBinding</code> annotation can take as parameters one or more interface classes that contain methods which represent bindable components (typically message channels).</p><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>The <code class="literal">@EnableBinding</code> annotation is only required on your <code class="literal">Configuration</code> classes, you can provide as many binding interfaces as you need, for instance: <code class="literal">@EnableBinding(value={Orders.class, Payment.class}</code>.
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Where both <code class="literal">Order</code> and <code class="literal">Payment</code> interfaces would declare <code class="literal">@Input</code> and <code class="literal">@Output</code> channels.</p></td></tr></table></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="__literal_input_literal_and_literal_output_literal" href="#__literal_input_literal_and_literal_output_literal"></a>25.1.2 <code class="literal">@Input</code> and <code class="literal">@Output</code></h3></div></div></div><p>A Spring Cloud Stream application can have an arbitrary number of input and output channels defined in an interface as <code class="literal">@Input</code> and <code class="literal">@Output</code> methods:</p><pre class="programlisting"><span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">public</span> <span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">interface</span> Barista {
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Where both <code class="literal">Order</code> and <code class="literal">Payment</code> interfaces would declare <code class="literal">@Input</code> and <code class="literal">@Output</code> channels.</p></td></tr></table></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="__literal_input_literal_and_literal_output_literal" href="#__literal_input_literal_and_literal_output_literal"></a>26.1.2 <code class="literal">@Input</code> and <code class="literal">@Output</code></h3></div></div></div><p>A Spring Cloud Stream application can have an arbitrary number of input and output channels defined in an interface as <code class="literal">@Input</code> and <code class="literal">@Output</code> methods:</p><pre class="programlisting"><span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">public</span> <span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">interface</span> Barista {
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<em><span class="hl-annotation" style="color: gray">@Input</span></em>
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SubscribableChannel orders();
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@@ -57,7 +57,7 @@ In this documentation, we will continue to refer to MessageChannels as the <code
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SubscribableChannel input();
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}</pre><p><code class="literal">Processor</code> can be used for an application which has both an inbound channel and an outbound channel.</p><pre class="programlisting"><span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">public</span> <span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">interface</span> Processor <span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">extends</span> Source, Sink {
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}</pre><p>Spring Cloud Stream provides no special handling for any of these interfaces; they are only provided out of the box.</p></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="_accessing_bound_channels" href="#_accessing_bound_channels"></a>25.1.3 Accessing Bound Channels</h3></div></div></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a name="_injecting_the_bound_interfaces" href="#_injecting_the_bound_interfaces"></a>Injecting the Bound Interfaces</h4></div></div></div><p>For each bound interface, Spring Cloud Stream will generate a bean that implements the interface.
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}</pre><p>Spring Cloud Stream provides no special handling for any of these interfaces; they are only provided out of the box.</p></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="_accessing_bound_channels" href="#_accessing_bound_channels"></a>26.1.3 Accessing Bound Channels</h3></div></div></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a name="_injecting_the_bound_interfaces" href="#_injecting_the_bound_interfaces"></a>Injecting the Bound Interfaces</h4></div></div></div><p>For each bound interface, Spring Cloud Stream will generate a bean that implements the interface.
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Invoking a <code class="literal">@Input</code>-annotated or <code class="literal">@Output</code>-annotated method of one of these beans will return the relevant bound channel.</p><p>The bean in the following example sends a message on the output channel when its <code class="literal">hello</code> method is invoked.
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It invokes <code class="literal">output()</code> on the injected <code class="literal">Source</code> bean to retrieve the target channel.</p><pre class="programlisting"><em><span class="hl-annotation" style="color: gray">@Component</span></em>
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<span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">public</span> <span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">class</span> SendingBean {
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@@ -103,7 +103,7 @@ Given the following declaration:</p><pre class="programlisting"><span xmlns:d="h
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<span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">public</span> <span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">void</span> sayHello(String name) {
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<span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">this</span>.output.send(MessageBuilder.withPayload(name).build());
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}
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}</pre></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="_producing_and_consuming_messages" href="#_producing_and_consuming_messages"></a>25.1.4 Producing and Consuming Messages</h3></div></div></div><p>You can write a Spring Cloud Stream application using either Spring Integration annotations or Spring Cloud Stream’s <code class="literal">@StreamListener</code> annotation.
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}</pre></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="_producing_and_consuming_messages" href="#_producing_and_consuming_messages"></a>26.1.4 Producing and Consuming Messages</h3></div></div></div><p>You can write a Spring Cloud Stream application using either Spring Integration annotations or Spring Cloud Stream’s <code class="literal">@StreamListener</code> annotation.
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The <code class="literal">@StreamListener</code> annotation is modeled after other Spring Messaging annotations (such as <code class="literal">@MessageMapping</code>, <code class="literal">@JmsListener</code>, <code class="literal">@RabbitListener</code>, etc.) but adds content type management and type coercion features.</p><div class="section"><div class="titlepage"><div><div><h4 class="title"><a name="_native_spring_integration_support" href="#_native_spring_integration_support"></a>Native Spring Integration Support</h4></div></div></div><p>Because Spring Cloud Stream is based on Spring Integration, Stream completely inherits Integration’s foundation and infrastructure as well as the component itself.
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For example, you can attach the output channel of a <code class="literal">Source</code> to a <code class="literal">MessageSource</code>:</p><pre class="programlisting"><em><span class="hl-annotation" style="color: gray">@EnableBinding(Source.class)</span></em>
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<span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">public</span> <span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">class</span> TimerSource {
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@@ -220,7 +220,7 @@ You can override that behavior, by taking responsibility for the acknowledgment,
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Map<String, Foo> payload = (Map<String, Foo>) received.getPayload();
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...
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}, <span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">new</span> ParameterizedTypeReference<Map<String, Foo>>() {});</pre></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="_reactive_programming_support" href="#_reactive_programming_support"></a>25.1.5 Reactive Programming Support</h3></div></div></div><p>Spring Cloud Stream also supports the use of reactive APIs where incoming and outgoing data is handled as continuous data flows.
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}, <span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">new</span> ParameterizedTypeReference<Map<String, Foo>>() {});</pre></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="_reactive_programming_support" href="#_reactive_programming_support"></a>26.1.5 Reactive Programming Support</h3></div></div></div><p>Spring Cloud Stream also supports the use of reactive APIs where incoming and outgoing data is handled as continuous data flows.
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Support for reactive APIs is available via the <code class="literal">spring-cloud-stream-reactive</code>, which needs to be added explicitly to your project.</p><p>The programming model with reactive APIs is declarative, where instead of specifying how each individual message should be handled, you can use operators that describe functional transformations from inbound to outbound data flows.</p><p>Spring Cloud Stream supports the following reactive APIs:</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">Reactor</li></ul></div><p>In the future, it is intended to support a more generic model based on Reactive Streams.</p><p>The reactive programming model is also using the <code class="literal">@StreamListener</code> annotation for setting up reactive handlers. The differences are that:</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">the <code class="literal">@StreamListener</code> annotation must not specify an input or output, as they are provided as arguments and return values from the method;</li><li class="listitem">the arguments of the method must be annotated with <code class="literal">@Input</code> and <code class="literal">@Output</code> indicating which input or output will the incoming and respectively outgoing data flows connect to;</li><li class="listitem">the return value of the method, if any, will be annotated with <code class="literal">@Output</code>, indicating the input where data shall be sent.</li></ul></div><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>Reactive programming support requires Java 1.8.</p></td></tr></table></div><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>As of Spring Cloud Stream 1.1.1 and later (starting with release train Brooklyn.SR2), reactive programming support requires the use of Reactor 3.0.4.RELEASE and higher.
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Earlier Reactor versions (including 3.0.1.RELEASE, 3.0.2.RELEASE and 3.0.3.RELEASE) are not supported.
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<code class="literal">spring-cloud-stream-reactive</code> will transitively retrieve the proper version, but it is possible for the project structure to manage the version of the <code class="literal">io.projectreactor:reactor-core</code> to an earlier release, especially when using Maven.
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@@ -295,7 +295,7 @@ The Publisher is still using Reactor Flux under the hood, but from an applicatio
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e -> e.poller(p -> p.fixedDelay(<span class="hl-number">1</span>)))
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.toReactivePublisher();
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}
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}</pre></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="_aggregation" href="#_aggregation"></a>25.1.6 Aggregation</h3></div></div></div><p>Spring Cloud Stream provides support for aggregating multiple applications together, connecting their input and output channels directly and avoiding the additional cost of exchanging messages via a broker.
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}</pre></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="_aggregation" href="#_aggregation"></a>26.1.6 Aggregation</h3></div></div></div><p>Spring Cloud Stream provides support for aggregating multiple applications together, connecting their input and output channels directly and avoiding the additional cost of exchanging messages via a broker.
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As of version 1.0 of Spring Cloud Stream, aggregation is supported only for the following types of applications:</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><span class="emphasis"><em>sources</em></span> - applications with a single output channel named <code class="literal">output</code>, typically having a single binding of the type <code class="literal">org.springframework.cloud.stream.messaging.Source</code></li><li class="listitem"><span class="emphasis"><em>sinks</em></span> - applications with a single input channel named <code class="literal">input</code>, typically having a single binding of the type <code class="literal">org.springframework.cloud.stream.messaging.Sink</code></li><li class="listitem"><span class="emphasis"><em>processors</em></span> - applications with a single input channel named <code class="literal">input</code> and a single output channel named <code class="literal">output</code>, typically having a single binding of the type <code class="literal">org.springframework.cloud.stream.messaging.Processor</code>.</li></ul></div><p>They can be aggregated together by creating a sequence of interconnected applications, in which the output channel of an element in the sequence is connected to the input channel of the next element, if it exists.
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A sequence can start with either a <span class="emphasis"><em>source</em></span> or a <span class="emphasis"><em>processor</em></span>, it can contain an arbitrary number of <span class="emphasis"><em>processors</em></span> and must end with either a <span class="emphasis"><em>processor</em></span> or a <span class="emphasis"><em>sink</em></span>.</p><p>Depending on the nature of the starting and ending element, the sequence may have one or more bindable channels, as follows:</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">if the sequence starts with a source and ends with a sink, all communication between the applications is direct and no channels will be bound</li><li class="listitem">if the sequence starts with a processor, then its input channel will become the <code class="literal">input</code> channel of the aggregate and will be bound accordingly</li><li class="listitem">if the sequence ends with a processor, then its output channel will become the <code class="literal">output</code> channel of the aggregate and will be bound accordingly</li></ul></div><p>Aggregation is performed using the <code class="literal">AggregateApplicationBuilder</code> utility class, as in the following example.
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Let’s consider a project in which we have source, processor and a sink, which may be defined in the project, or may be contained in one of the project’s dependencies.</p><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>Each component (source, sink or processor) in an aggregate application must be provided in a separate package if the configuration classes use <code class="literal">@SpringBootApplication</code>.
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@@ -376,4 +376,4 @@ For instance,</p><pre class="programlisting"><em><span class="hl-annotation" sty
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.from(SourceApplication.<span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">class</span>).namespace(<span xmlns:d="http://docbook.org/ns/docbook" class="hl-string">"source"</span>).args(<span xmlns:d="http://docbook.org/ns/docbook" class="hl-string">"--fixedDelay=5000"</span>)
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.via(ProcessorApplication.<span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">class</span>).namespace(<span xmlns:d="http://docbook.org/ns/docbook" class="hl-string">"processor1"</span>).args(<span xmlns:d="http://docbook.org/ns/docbook" class="hl-string">"--debug=true"</span>).run(args);
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}
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}</pre><p>The binding properties like <code class="literal">--spring.cloud.stream.bindings.output.destination=processor-output</code> need to be specified as one of the external configuration properties (cmdline arg etc.).</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__main_concepts.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__binders.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">24. Main Concepts </td><td width="20%" align="center"><a accesskey="h" href="multi_spring-cloud.html">Home</a></td><td width="40%" align="right" valign="top"> 26. Binders</td></tr></table></div></body></html>
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}</pre><p>The binding properties like <code class="literal">--spring.cloud.stream.bindings.output.destination=processor-output</code> need to be specified as one of the external configuration properties (cmdline arg etc.).</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__main_concepts.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__binders.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">25. Main Concepts </td><td width="20%" align="center"><a accesskey="h" href="multi_spring-cloud.html">Home</a></td><td width="40%" align="right" valign="top"> 27. Binders</td></tr></table></div></body></html>
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