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<h1 class="title">Spark Streaming Custom Receivers</h1>
<p>Spark Streaming can receive streaming data from any arbitrary data source beyond
the one’s for which it has in-built support (that is, beyond Flume, Kafka, files, sockets, etc.).
This requires the developer to implement a <em>receiver</em> that is customized for receiving data from
the concerned data source. This guide walks through the process of implementing a custom receiver
and using it in a Spark Streaming application.</p>
<h3 id="implementing-a-custom-receiver">Implementing a Custom Receiver</h3>
<p>This starts with implementing a <a href="api/scala/index.html#org.apache.spark.streaming.receiver.Receiver">Receiver</a>.
A custom receiver must extend this abstract class by implementing two methods
- <code>onStart()</code>: Things to do to start receiving data.
- <code>onStop()</code>: Things to do to stop receiving data.</p>
<p>Note that <code>onStart()</code> and <code>onStop()</code> must not block indefinitely. Typically, onStart() would start the threads
that responsible for receiving the data and <code>onStop()</code> would ensure that the receiving by those threads
are stopped. The receiving threads can also use <code>isStopped()</code>, a <code>Receiver</code> method, to check whether they
should stop receiving data.</p>
<p>Once the data is received, that data can be stored inside Spark
by calling <code>store(data)</code>, which is a method provided by the
<a href="api/scala/index.html#org.apache.spark.streaming.receiver.Receiver">Receiver</a> class.
There are number of flavours of <code>store()</code> which allow you store the received data
record-at-a-time or as whole collection of objects / serialized bytes.</p>
<p>Any exception in the receiving threads should be caught and handled properly to avoid silent
failures of the receiver. <code>restart(<exception>)</code> will restart the receiver by
asynchronously calling <code>onStop()</code> and then calling <code>onStart()</code> after a delay.
<code>stop(<exception>)</code> will call <code>onStop()</code> and terminate the receiver. Also, <code>reportError(<error>)</code>
reports a error message to the driver (visible in the logs and UI) without stopping / restarting
the receiver.</p>
<p>The following is a custom receiver that receives a stream of text over a socket. It treats
‘\n’ delimited lines in the text stream as records and stores them with Spark. If the receiving thread
has any error connecting or receiving, the receiver is restarted to make another attempt to connect.</p>
<div class="codetabs">
<div data-lang="scala">
<div class="highlight"><pre><code class="scala"><span class="k">class</span> <span class="nc">CustomReceiver</span><span class="o">(</span><span class="n">host</span><span class="k">:</span> <span class="kt">String</span><span class="o">,</span> <span class="n">port</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)</span>
<span class="k">extends</span> <span class="nc">Receiver</span><span class="o">[</span><span class="kt">String</span><span class="o">](</span><span class="nc">StorageLevel</span><span class="o">.</span><span class="nc">MEMORY_AND_DISK_2</span><span class="o">)</span> <span class="k">with</span> <span class="nc">Logging</span> <span class="o">{</span>
<span class="k">def</span> <span class="n">onStart</span><span class="o">()</span> <span class="o">{</span>
<span class="c1">// Start the thread that receives data over a connection</span>
<span class="k">new</span> <span class="nc">Thread</span><span class="o">(</span><span class="s">"Socket Receiver"</span><span class="o">)</span> <span class="o">{</span>
<span class="k">override</span> <span class="k">def</span> <span class="n">run</span><span class="o">()</span> <span class="o">{</span> <span class="n">receive</span><span class="o">()</span> <span class="o">}</span>
<span class="o">}.</span><span class="n">start</span><span class="o">()</span>
<span class="o">}</span>
<span class="k">def</span> <span class="n">onStop</span><span class="o">()</span> <span class="o">{</span>
<span class="c1">// There is nothing much to do as the thread calling receive()</span>
<span class="c1">// is designed to stop by itself isStopped() returns false</span>
<span class="o">}</span>
<span class="cm">/** Create a socket connection and receive data until receiver is stopped */</span>
<span class="k">private</span> <span class="k">def</span> <span class="n">receive</span><span class="o">()</span> <span class="o">{</span>
<span class="k">var</span> <span class="n">socket</span><span class="k">:</span> <span class="kt">Socket</span> <span class="o">=</span> <span class="kc">null</span>
<span class="k">var</span> <span class="n">userInput</span><span class="k">:</span> <span class="kt">String</span> <span class="o">=</span> <span class="kc">null</span>
<span class="k">try</span> <span class="o">{</span>
<span class="c1">// Connect to host:port</span>
<span class="n">socket</span> <span class="k">=</span> <span class="k">new</span> <span class="nc">Socket</span><span class="o">(</span><span class="n">host</span><span class="o">,</span> <span class="n">port</span><span class="o">)</span>
<span class="c1">// Until stopped or connection broken continue reading</span>
<span class="k">val</span> <span class="n">reader</span> <span class="k">=</span> <span class="k">new</span> <span class="nc">BufferedReader</span><span class="o">(</span><span class="k">new</span> <span class="nc">InputStreamReader</span><span class="o">(</span><span class="n">socket</span><span class="o">.</span><span class="n">getInputStream</span><span class="o">(),</span> <span class="s">"UTF-8"</span><span class="o">))</span>
<span class="n">userInput</span> <span class="k">=</span> <span class="n">reader</span><span class="o">.</span><span class="n">readLine</span><span class="o">()</span>
<span class="k">while</span><span class="o">(!</span><span class="n">isStopped</span> <span class="o">&&</span> <span class="n">userInput</span> <span class="o">!=</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
<span class="n">store</span><span class="o">(</span><span class="n">userInput</span><span class="o">)</span>
<span class="n">userInput</span> <span class="k">=</span> <span class="n">reader</span><span class="o">.</span><span class="n">readLine</span><span class="o">()</span>
<span class="o">}</span>
<span class="n">reader</span><span class="o">.</span><span class="n">close</span><span class="o">()</span>
<span class="n">socket</span><span class="o">.</span><span class="n">close</span><span class="o">()</span>
<span class="c1">// Restart in an attempt to connect again when server is active again</span>
<span class="n">restart</span><span class="o">(</span><span class="s">"Trying to connect again"</span><span class="o">)</span>
<span class="o">}</span> <span class="k">catch</span> <span class="o">{</span>
<span class="k">case</span> <span class="n">e</span><span class="k">:</span> <span class="kt">java.net.ConnectException</span> <span class="o">=></span>
<span class="c1">// restart if could not connect to server</span>
<span class="n">restart</span><span class="o">(</span><span class="s">"Error connecting to "</span> <span class="o">+</span> <span class="n">host</span> <span class="o">+</span> <span class="s">":"</span> <span class="o">+</span> <span class="n">port</span><span class="o">,</span> <span class="n">e</span><span class="o">)</span>
<span class="k">case</span> <span class="n">t</span><span class="k">:</span> <span class="kt">Throwable</span> <span class="o">=></span>
<span class="c1">// restart if there is any other error</span>
<span class="n">restart</span><span class="o">(</span><span class="s">"Error receiving data"</span><span class="o">,</span> <span class="n">t</span><span class="o">)</span>
<span class="o">}</span>
<span class="o">}</span>
<span class="o">}</span>
</code></pre></div>
</div>
<div data-lang="java">
<div class="highlight"><pre><code class="java"><span class="kd">public</span> <span class="kd">class</span> <span class="nc">JavaCustomReceiver</span> <span class="kd">extends</span> <span class="n">Receiver</span><span class="o"><</span><span class="n">String</span><span class="o">></span> <span class="o">{</span>
<span class="n">String</span> <span class="n">host</span> <span class="o">=</span> <span class="kc">null</span><span class="o">;</span>
<span class="kt">int</span> <span class="n">port</span> <span class="o">=</span> <span class="o">-</span><span class="mi">1</span><span class="o">;</span>
<span class="kd">public</span> <span class="nf">JavaCustomReceiver</span><span class="o">(</span><span class="n">String</span> <span class="n">host_</span> <span class="o">,</span> <span class="kt">int</span> <span class="n">port_</span><span class="o">)</span> <span class="o">{</span>
<span class="kd">super</span><span class="o">(</span><span class="n">StorageLevel</span><span class="o">.</span><span class="na">MEMORY_AND_DISK_2</span><span class="o">());</span>
<span class="n">host</span> <span class="o">=</span> <span class="n">host_</span><span class="o">;</span>
<span class="n">port</span> <span class="o">=</span> <span class="n">port_</span><span class="o">;</span>
<span class="o">}</span>
<span class="kd">public</span> <span class="kt">void</span> <span class="nf">onStart</span><span class="o">()</span> <span class="o">{</span>
<span class="c1">// Start the thread that receives data over a connection</span>
<span class="k">new</span> <span class="nf">Thread</span><span class="o">()</span> <span class="o">{</span>
<span class="nd">@Override</span> <span class="kd">public</span> <span class="kt">void</span> <span class="n">run</span><span class="o">()</span> <span class="o">{</span>
<span class="n">receive</span><span class="o">();</span>
<span class="o">}</span>
<span class="o">}.</span><span class="na">start</span><span class="o">();</span>
<span class="o">}</span>
<span class="kd">public</span> <span class="kt">void</span> <span class="nf">onStop</span><span class="o">()</span> <span class="o">{</span>
<span class="c1">// There is nothing much to do as the thread calling receive()</span>
<span class="c1">// is designed to stop by itself isStopped() returns false</span>
<span class="o">}</span>
<span class="cm">/** Create a socket connection and receive data until receiver is stopped */</span>
<span class="kd">private</span> <span class="kt">void</span> <span class="nf">receive</span><span class="o">()</span> <span class="o">{</span>
<span class="n">Socket</span> <span class="n">socket</span> <span class="o">=</span> <span class="kc">null</span><span class="o">;</span>
<span class="n">String</span> <span class="n">userInput</span> <span class="o">=</span> <span class="kc">null</span><span class="o">;</span>
<span class="k">try</span> <span class="o">{</span>
<span class="c1">// connect to the server</span>
<span class="n">socket</span> <span class="o">=</span> <span class="k">new</span> <span class="n">Socket</span><span class="o">(</span><span class="n">host</span><span class="o">,</span> <span class="n">port</span><span class="o">);</span>
<span class="n">BufferedReader</span> <span class="n">reader</span> <span class="o">=</span> <span class="k">new</span> <span class="n">BufferedReader</span><span class="o">(</span><span class="k">new</span> <span class="n">InputStreamReader</span><span class="o">(</span><span class="n">socket</span><span class="o">.</span><span class="na">getInputStream</span><span class="o">()));</span>
<span class="c1">// Until stopped or connection broken continue reading</span>
<span class="k">while</span> <span class="o">(!</span><span class="n">isStopped</span><span class="o">()</span> <span class="o">&&</span> <span class="o">(</span><span class="n">userInput</span> <span class="o">=</span> <span class="n">reader</span><span class="o">.</span><span class="na">readLine</span><span class="o">())</span> <span class="o">!=</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</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">"Received data '"</span> <span class="o">+</span> <span class="n">userInput</span> <span class="o">+</span> <span class="s">"'"</span><span class="o">);</span>
<span class="n">store</span><span class="o">(</span><span class="n">userInput</span><span class="o">);</span>
<span class="o">}</span>
<span class="n">reader</span><span class="o">.</span><span class="na">close</span><span class="o">();</span>
<span class="n">socket</span><span class="o">.</span><span class="na">close</span><span class="o">();</span>
<span class="c1">// Restart in an attempt to connect again when server is active again</span>
<span class="n">restart</span><span class="o">(</span><span class="s">"Trying to connect again"</span><span class="o">);</span>
<span class="o">}</span> <span class="k">catch</span><span class="o">(</span><span class="n">ConnectException</span> <span class="n">ce</span><span class="o">)</span> <span class="o">{</span>
<span class="c1">// restart if could not connect to server</span>
<span class="n">restart</span><span class="o">(</span><span class="s">"Could not connect"</span><span class="o">,</span> <span class="n">ce</span><span class="o">);</span>
<span class="o">}</span> <span class="k">catch</span><span class="o">(</span><span class="n">Throwable</span> <span class="n">t</span><span class="o">)</span> <span class="o">{</span>
<span class="c1">// restart if there is any other error</span>
<span class="n">restart</span><span class="o">(</span><span class="s">"Error receiving data"</span><span class="o">,</span> <span class="n">t</span><span class="o">);</span>
<span class="o">}</span>
<span class="o">}</span>
<span class="o">}</span>
</code></pre></div>
</div>
</div>
<h3 id="using-the-custom-receiver-in-a-spark-streaming-application">Using the custom receiver in a Spark Streaming application</h3>
<p>The custom receiver can be used in a Spark Streaming application by using
<code>streamingContext.receiverStream(<instance of custom receiver>)</code>. This will create
input DStream using data received by the instance of custom receiver, as shown below</p>
<div class="codetabs">
<div data-lang="scala">
<div class="highlight"><pre><code class="scala"><span class="c1">// Assuming ssc is the StreamingContext</span>
<span class="k">val</span> <span class="n">customReceiverStream</span> <span class="k">=</span> <span class="n">ssc</span><span class="o">.</span><span class="n">receiverStream</span><span class="o">(</span><span class="k">new</span> <span class="nc">CustomReceiver</span><span class="o">(</span><span class="n">host</span><span class="o">,</span> <span class="n">port</span><span class="o">))</span>
<span class="k">val</span> <span class="n">words</span> <span class="k">=</span> <span class="n">lines</span><span class="o">.</span><span class="n">flatMap</span><span class="o">(</span><span class="k">_</span><span class="o">.</span><span class="n">split</span><span class="o">(</span><span class="s">" "</span><span class="o">))</span>
<span class="o">...</span>
</code></pre></div>
<p>The full source code is in the example <a href="https://github.com/apache/spark/blob/master/examples/src/main/scala/org/apache/spark/streaming/examples/CustomReceiver.scala">CustomReceiver.scala</a>.</p>
</div>
<div data-lang="java">
<div class="highlight"><pre><code class="java"><span class="c1">// Assuming ssc is the JavaStreamingContext</span>
<span class="n">JavaDStream</span><span class="o"><</span><span class="n">String</span><span class="o">></span> <span class="n">customReceiverStream</span> <span class="o">=</span> <span class="n">ssc</span><span class="o">.</span><span class="na">receiverStream</span><span class="o">(</span><span class="k">new</span> <span class="n">JavaCustomReceiver</span><span class="o">(</span><span class="n">host</span><span class="o">,</span> <span class="n">port</span><span class="o">));</span>
<span class="n">JavaDStream</span><span class="o"><</span><span class="n">String</span><span class="o">></span> <span class="n">words</span> <span class="o">=</span> <span class="n">lines</span><span class="o">.</span><span class="na">flatMap</span><span class="o">(</span><span class="k">new</span> <span class="n">FlatMapFunction</span><span class="o"><</span><span class="n">String</span><span class="o">,</span> <span class="n">String</span><span class="o">>()</span> <span class="o">{</span> <span class="o">...</span> <span class="o">});</span>
<span class="o">...</span>
</code></pre></div>
<p>The full source code is in the example <a href="https://github.com/apache/spark/blob/master/examples/src/main/java/org/apache/spark/streaming/examples/JavaCustomReceiver.java">JavaCustomReceiver.java</a>.</p>
</div>
</div>
<h3 id="implementing-and-using-a-custom-actor-based-receiver">Implementing and Using a Custom Actor-based Receiver</h3>
<p>Custom <a href="http://doc.akka.io/docs/akka/2.2.4/scala/actors.html">Akka Actors</a> can also be used to
receive data. The <a href="api/scala/index.html#org.apache.spark.streaming.receiver.ActorHelper"><code>ActorHelper</code></a>
trait can be applied on any Akka actor, which allows received data to be stored in Spark using
<code>store(...)</code> methods. The supervisor strategy of this actor can be configured to handle failures, etc.</p>
<div class="highlight"><pre><code class="scala"><span class="k">class</span> <span class="nc">CustomActor</span> <span class="k">extends</span> <span class="nc">Actor</span> <span class="k">with</span> <span class="nc">ActorHelper</span> <span class="o">{</span>
<span class="k">def</span> <span class="n">receive</span> <span class="k">=</span> <span class="o">{</span>
<span class="k">case</span> <span class="n">data</span><span class="k">:</span> <span class="kt">String</span> <span class="o">=></span> <span class="n">store</span><span class="o">(</span><span class="n">data</span><span class="o">)</span>
<span class="o">}</span>
<span class="o">}</span>
</code></pre></div>
<p>And a new input stream can be created with this custom actor as</p>
<div class="highlight"><pre><code class="scala"><span class="c1">// Assuming ssc is the StreamingContext</span>
<span class="k">val</span> <span class="n">lines</span> <span class="k">=</span> <span class="n">ssc</span><span class="o">.</span><span class="n">actorStream</span><span class="o">[</span><span class="kt">String</span><span class="o">](</span><span class="nc">Props</span><span class="o">(</span><span class="k">new</span> <span class="nc">CustomActor</span><span class="o">()),</span> <span class="s">"CustomReceiver"</span><span class="o">)</span>
</code></pre></div>
<p>See <a href="https://github.com/apache/spark/blob/master/examples/src/main/scala/org/apache/spark/streaming/examples/ActorWordCount.scala">ActorWordCount.scala</a>
for an end-to-end example.</p>
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