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<h1 class="post-title-main">Changing a polled source stream's period</h1>
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<a target="_blank" href="https://github.com/apache/incubator-edgent-website/blob/master/site/recipes/recipe_adaptable_polling_source.md" class="btn btn-default githubEditButton" role="button"><i class="fa fa-github fa-lg"></i> Edit me</a>
<p>The <a href="recipe_source_function.html">Writing a source function</a> recipe introduced the basics of creating a source stream by polling a data source periodically.</p>
<p>Oftentimes, a user wants the poll frequency to be adaptable rather than static. For example, an event such as a sudden rise in a temperature sensor may motivate more frequent polling of the sensor and analysis of the data until the condition subsides. A change in the poll frequency may be driven by locally performed analytics or via a command from an external source.</p>
<p>An Edgent <code>IotProvider</code> and <code>IoTDevice</code> with its command streams would be a natural way to control the application. In this recipe we will just simulate a &quot;set poll period&quot; command stream.</p>
<p>The <code>Topology.poll()</code> <a href="/javadoc/latest/org/apache/edgent/topology/Topology.html#poll-org.apache.edgent.function.Supplier-long-java.util.concurrent.TimeUnit-">documentation</a> describes how the poll period may be changed at runtime.</p>
<p>The mechanism is based on a more general Edgent runtime <code>edgent.execution.services.ControlService</code> service. The runtime registers &quot;control beans&quot; for entities that are controllable. These controls can be retrieved at runtime via the service.</p>
<p>At runtime, <code>Topology.poll()</code> registers a <code>edgent.execution.mbeans.PeriodMXBean</code> control. <strong>Retrieving the control at runtime requires setting an alias on the poll generated stream using <code>TStream.alias()</code>.</strong></p>
<h2 id="create-the-polled-stream-and-set-its-alias">Create the polled stream and set its alias</h2>
<div class="highlight"><pre><code class="language-java" data-lang="java"><span class="n">Topology</span> <span class="n">top</span> <span class="o">=</span> <span class="o">...;</span>
<span class="n">SimulatedTemperatureSensor</span> <span class="n">tempSensor</span> <span class="o">=</span> <span class="k">new</span> <span class="n">SimulatedTemperatureSensor</span><span class="o">();</span>
<span class="n">TStream</span><span class="o">&lt;</span><span class="n">Double</span><span class="o">&gt;</span> <span class="n">engineTemp</span> <span class="o">=</span> <span class="n">top</span><span class="o">.</span><span class="na">poll</span><span class="o">(</span><span class="n">tempSensor</span><span class="o">,</span> <span class="mi">1</span><span class="o">,</span> <span class="n">TimeUnit</span><span class="o">.</span><span class="na">SECONDS</span><span class="o">)</span>
<span class="o">.</span><span class="na">alias</span><span class="o">(</span><span class="s">"engineTemp"</span><span class="o">)</span>
<span class="o">.</span><span class="na">tag</span><span class="o">(</span><span class="s">"engineTemp"</span><span class="o">);</span>
</code></pre></div>
<p>It&#39;s also a good practice to add tags to streams to improve the usability of the development mode Edgent console.</p>
<h2 id="define-a-set-poll-period-method">Define a &quot;set poll period&quot; method</h2>
<div class="highlight"><pre><code class="language-java" data-lang="java"><span class="kd">static</span> <span class="o">&lt;</span><span class="n">T</span><span class="o">&gt;</span> <span class="kt">void</span> <span class="nf">setPollPeriod</span><span class="o">(</span><span class="n">TStream</span><span class="o">&lt;</span><span class="n">T</span><span class="o">&gt;</span> <span class="n">pollStream</span><span class="o">,</span> <span class="kt">long</span> <span class="n">period</span><span class="o">,</span> <span class="n">TimeUnit</span> <span class="n">unit</span><span class="o">)</span> <span class="o">{</span>
<span class="c1">// get the topology's runtime ControlService service</span>
<span class="n">ControlService</span> <span class="n">cs</span> <span class="o">=</span> <span class="n">pollStream</span><span class="o">.</span><span class="na">topology</span><span class="o">().</span><span class="na">getRuntimeServiceSupplier</span><span class="o">()</span>
<span class="o">.</span><span class="na">get</span><span class="o">().</span><span class="na">getService</span><span class="o">(</span><span class="n">ControlService</span><span class="o">.</span><span class="na">class</span><span class="o">);</span>
<span class="c1">// using the the stream's alias, get its PeriodMXBean control</span>
<span class="n">PeriodMXBean</span> <span class="n">control</span> <span class="o">=</span> <span class="n">cs</span><span class="o">.</span><span class="na">getControl</span><span class="o">(</span><span class="n">TStream</span><span class="o">.</span><span class="na">TYPE</span><span class="o">,</span> <span class="n">pollStream</span><span class="o">.</span><span class="na">getAlias</span><span class="o">(),</span> <span class="n">PeriodMXBean</span><span class="o">.</span><span class="na">class</span><span class="o">);</span>
<span class="c1">// change the poll period using the control</span>
<span class="n">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">println</span><span class="o">(</span><span class="s">"Setting period="</span><span class="o">+</span><span class="n">period</span><span class="o">+</span><span class="s">" "</span><span class="o">+</span><span class="n">unit</span><span class="o">+</span><span class="s">" stream="</span><span class="o">+</span><span class="n">pollStream</span><span class="o">);</span>
<span class="n">control</span><span class="o">.</span><span class="na">setPeriod</span><span class="o">(</span><span class="n">period</span><span class="o">,</span> <span class="n">unit</span><span class="o">);</span>
<span class="o">}</span>
</code></pre></div>
<h2 id="process-the-set-poll-period-command-stream">Process the &quot;set poll period&quot; command stream</h2>
<p>Our commands are on the <code>TStream&lt;JsonObject&gt; cmds</code> stream. Each <code>JsonObject</code> tuple is a command with the properties &quot;period&quot; and &quot;unit&quot;.</p>
<div class="highlight"><pre><code class="language-java" data-lang="java"><span class="n">cmds</span><span class="o">.</span><span class="na">sink</span><span class="o">(</span><span class="n">json</span> <span class="o">-&gt;</span> <span class="n">setPollPeriod</span><span class="o">(</span><span class="n">engineTemp</span><span class="o">,</span>
<span class="n">json</span><span class="o">.</span><span class="na">getAsJsonPrimitive</span><span class="o">(</span><span class="s">"period"</span><span class="o">).</span><span class="na">getAsLong</span><span class="o">(),</span>
<span class="n">TimeUnit</span><span class="o">.</span><span class="na">valueOf</span><span class="o">(</span><span class="n">json</span><span class="o">.</span><span class="na">getAsJsonPrimitive</span><span class="o">(</span><span class="s">"unit"</span><span class="o">).</span><span class="na">getAsString</span><span class="o">())));</span>
</code></pre></div>
<h2 id="the-final-application">The final application</h2>
<div class="highlight"><pre><code class="language-java" data-lang="java"><span class="kn">import</span> <span class="nn">java.util.Date</span><span class="o">;</span>
<span class="kn">import</span> <span class="nn">java.util.concurrent.TimeUnit</span><span class="o">;</span>
<span class="kn">import</span> <span class="nn">java.util.concurrent.atomic.AtomicInteger</span><span class="o">;</span>
<span class="kn">import</span> <span class="nn">com.google.gson.JsonObject</span><span class="o">;</span>
<span class="kn">import</span> <span class="nn">org.apache.edgent.execution.mbeans.PeriodMXBean</span><span class="o">;</span>
<span class="kn">import</span> <span class="nn">org.apache.edgent.execution.services.ControlService</span><span class="o">;</span>
<span class="kn">import</span> <span class="nn">org.apache.edgent.providers.development.DevelopmentProvider</span><span class="o">;</span>
<span class="kn">import</span> <span class="nn">org.apache.edgent.providers.direct.DirectProvider</span><span class="o">;</span>
<span class="kn">import</span> <span class="nn">org.apache.edgent.samples.utils.sensor.SimulatedTemperatureSensor</span><span class="o">;</span>
<span class="kn">import</span> <span class="nn">org.apache.edgent.topology.TStream</span><span class="o">;</span>
<span class="kn">import</span> <span class="nn">org.apache.edgent.topology.Topology</span><span class="o">;</span>
<span class="cm">/**
* A recipe for a polled source stream with an adaptable poll period.
*/</span>
<span class="kd">public</span> <span class="kd">class</span> <span class="nc">AdaptablePolledSource</span> <span class="o">{</span>
<span class="cm">/**
* Poll a temperature sensor to periodically obtain temperature readings.
* Respond to a simulated command stream to change the poll period.
*/</span>
<span class="kd">public</span> <span class="kd">static</span> <span class="kt">void</span> <span class="nf">main</span><span class="o">(</span><span class="n">String</span><span class="o">[]</span> <span class="n">args</span><span class="o">)</span> <span class="kd">throws</span> <span class="n">Exception</span> <span class="o">{</span>
<span class="n">DirectProvider</span> <span class="n">dp</span> <span class="o">=</span> <span class="k">new</span> <span class="n">DirectProvider</span><span class="o">();</span>
<span class="n">Topology</span> <span class="n">top</span> <span class="o">=</span> <span class="n">dp</span><span class="o">.</span><span class="na">newTopology</span><span class="o">(</span><span class="s">"TemperatureSensor"</span><span class="o">);</span>
<span class="c1">// Generate a polled temperature sensor stream and set its alias</span>
<span class="n">SimulatedTemperatureSensor</span> <span class="n">tempSensor</span> <span class="o">=</span> <span class="k">new</span> <span class="n">SimulatedTemperatureSensor</span><span class="o">();</span>
<span class="n">TStream</span><span class="o">&lt;</span><span class="n">Double</span><span class="o">&gt;</span> <span class="n">engineTemp</span> <span class="o">=</span> <span class="n">top</span><span class="o">.</span><span class="na">poll</span><span class="o">(</span><span class="n">tempSensor</span><span class="o">,</span> <span class="mi">1</span><span class="o">,</span> <span class="n">TimeUnit</span><span class="o">.</span><span class="na">SECONDS</span><span class="o">)</span>
<span class="o">.</span><span class="na">alias</span><span class="o">(</span><span class="s">"engineTemp"</span><span class="o">)</span>
<span class="o">.</span><span class="na">tag</span><span class="o">(</span><span class="s">"engineTemp"</span><span class="o">);</span>
<span class="c1">// Report the time each temperature reading arrives and the value</span>
<span class="n">engineTemp</span><span class="o">.</span><span class="na">peek</span><span class="o">(</span><span class="n">tuple</span> <span class="o">-&gt;</span> <span class="n">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">println</span><span class="o">(</span><span class="k">new</span> <span class="n">Date</span><span class="o">()</span> <span class="o">+</span> <span class="s">" temp="</span> <span class="o">+</span> <span class="n">tuple</span><span class="o">));</span>
<span class="c1">// Generate a simulated "set poll period" command stream</span>
<span class="n">TStream</span><span class="o">&lt;</span><span class="n">JsonObject</span><span class="o">&gt;</span> <span class="n">cmds</span> <span class="o">=</span> <span class="n">simulatedSetPollPeriodCmds</span><span class="o">(</span><span class="n">top</span><span class="o">);</span>
<span class="c1">// Process the commands to change the poll period</span>
<span class="n">cmds</span><span class="o">.</span><span class="na">sink</span><span class="o">(</span><span class="n">json</span> <span class="o">-&gt;</span> <span class="n">setPollPeriod</span><span class="o">(</span><span class="n">engineTemp</span><span class="o">,</span>
<span class="n">json</span><span class="o">.</span><span class="na">getAsJsonPrimitive</span><span class="o">(</span><span class="s">"period"</span><span class="o">).</span><span class="na">getAsLong</span><span class="o">(),</span>
<span class="n">TimeUnit</span><span class="o">.</span><span class="na">valueOf</span><span class="o">(</span><span class="n">json</span><span class="o">.</span><span class="na">getAsJsonPrimitive</span><span class="o">(</span><span class="s">"unit"</span><span class="o">).</span><span class="na">getAsString</span><span class="o">())));</span>
<span class="n">dp</span><span class="o">.</span><span class="na">submit</span><span class="o">(</span><span class="n">top</span><span class="o">);</span>
<span class="o">}</span>
<span class="kd">static</span> <span class="o">&lt;</span><span class="n">T</span><span class="o">&gt;</span> <span class="kt">void</span> <span class="nf">setPollPeriod</span><span class="o">(</span><span class="n">TStream</span><span class="o">&lt;</span><span class="n">T</span><span class="o">&gt;</span> <span class="n">pollStream</span><span class="o">,</span> <span class="kt">long</span> <span class="n">period</span><span class="o">,</span> <span class="n">TimeUnit</span> <span class="n">unit</span><span class="o">)</span> <span class="o">{</span>
<span class="c1">// get the topology's runtime ControlService service</span>
<span class="n">ControlService</span> <span class="n">cs</span> <span class="o">=</span> <span class="n">pollStream</span><span class="o">.</span><span class="na">topology</span><span class="o">().</span><span class="na">getRuntimeServiceSupplier</span><span class="o">()</span>
<span class="o">.</span><span class="na">get</span><span class="o">().</span><span class="na">getService</span><span class="o">(</span><span class="n">ControlService</span><span class="o">.</span><span class="na">class</span><span class="o">);</span>
<span class="c1">// using the the stream's alias, get its PeriodMXBean control</span>
<span class="n">PeriodMXBean</span> <span class="n">control</span> <span class="o">=</span> <span class="n">cs</span><span class="o">.</span><span class="na">getControl</span><span class="o">(</span><span class="n">TStream</span><span class="o">.</span><span class="na">TYPE</span><span class="o">,</span> <span class="n">pollStream</span><span class="o">.</span><span class="na">getAlias</span><span class="o">(),</span> <span class="n">PeriodMXBean</span><span class="o">.</span><span class="na">class</span><span class="o">);</span>
<span class="c1">// change the poll period using the control</span>
<span class="n">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">println</span><span class="o">(</span><span class="s">"Setting period="</span><span class="o">+</span><span class="n">period</span><span class="o">+</span><span class="s">" "</span><span class="o">+</span><span class="n">unit</span><span class="o">+</span><span class="s">" stream="</span><span class="o">+</span><span class="n">pollStream</span><span class="o">);</span>
<span class="n">control</span><span class="o">.</span><span class="na">setPeriod</span><span class="o">(</span><span class="n">period</span><span class="o">,</span> <span class="n">unit</span><span class="o">);</span>
<span class="o">}</span>
<span class="kd">static</span> <span class="n">TStream</span><span class="o">&lt;</span><span class="n">JsonObject</span><span class="o">&gt;</span> <span class="nf">simulatedSetPollPeriodCmds</span><span class="o">(</span><span class="n">Topology</span> <span class="n">top</span><span class="o">)</span> <span class="o">{</span>
<span class="n">AtomicInteger</span> <span class="n">lastPeriod</span> <span class="o">=</span> <span class="k">new</span> <span class="n">AtomicInteger</span><span class="o">(</span><span class="mi">1</span><span class="o">);</span>
<span class="n">TStream</span><span class="o">&lt;</span><span class="n">JsonObject</span><span class="o">&gt;</span> <span class="n">cmds</span> <span class="o">=</span> <span class="n">top</span><span class="o">.</span><span class="na">poll</span><span class="o">(()</span> <span class="o">-&gt;</span> <span class="o">{</span>
<span class="c1">// toggle between 1 and 2 sec period</span>
<span class="kt">int</span> <span class="n">newPeriod</span> <span class="o">=</span> <span class="n">lastPeriod</span><span class="o">.</span><span class="na">get</span><span class="o">()</span> <span class="o">==</span> <span class="mi">1</span> <span class="o">?</span> <span class="mi">2</span> <span class="o">:</span> <span class="mi">1</span><span class="o">;</span>
<span class="n">lastPeriod</span><span class="o">.</span><span class="na">set</span><span class="o">(</span><span class="n">newPeriod</span><span class="o">);</span>
<span class="n">JsonObject</span> <span class="n">jo</span> <span class="o">=</span> <span class="k">new</span> <span class="n">JsonObject</span><span class="o">();</span>
<span class="n">jo</span><span class="o">.</span><span class="na">addProperty</span><span class="o">(</span><span class="s">"period"</span><span class="o">,</span> <span class="n">newPeriod</span><span class="o">);</span>
<span class="n">jo</span><span class="o">.</span><span class="na">addProperty</span><span class="o">(</span><span class="s">"unit"</span><span class="o">,</span> <span class="n">TimeUnit</span><span class="o">.</span><span class="na">SECONDS</span><span class="o">.</span><span class="na">toString</span><span class="o">());</span>
<span class="k">return</span> <span class="n">jo</span><span class="o">;</span>
<span class="o">},</span> <span class="mi">5</span><span class="o">,</span> <span class="n">TimeUnit</span><span class="o">.</span><span class="na">SECONDS</span><span class="o">)</span>
<span class="o">.</span><span class="na">tag</span><span class="o">(</span><span class="s">"cmds"</span><span class="o">);</span>
<span class="k">return</span> <span class="n">cmds</span><span class="o">;</span>
<span class="o">}</span>
<span class="o">}</span>
</code></pre></div>
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