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<h1 class="title">Spark Streaming Custom Receivers</h1>
<p>A “Spark Streaming” receiver can be a simple network stream, streams of messages from a message queue, files etc. A receiver can also assume roles more than just receiving data like filtering, preprocessing, to name a few of the possibilities. The api to plug-in any user defined custom receiver is thus provided to encourage development of receivers which may be well suited to ones specific need.</p>
<p>This guide shows the programming model and features by walking through a simple sample receiver and corresponding Spark Streaming application.</p>
<h3 id="writing-a-simple-receiver">Writing a Simple Receiver</h3>
<p>This starts with implementing <a href="api/streaming/index.html#org.apache.spark.streaming.dstream.NetworkReceiver">NetworkReceiver</a>.</p>
<p>The following is a simple socket text-stream receiver.</p>
<div class="highlight"><pre><code class="scala"> <span class="k">class</span> <span class="nc">SocketTextStreamReceiver</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="kt">extends</span> <span class="kt">NetworkReceiver</span><span class="o">[</span><span class="kt">String</span><span class="o">]</span>
<span class="o">{</span>
<span class="kt">protected</span> <span class="kt">lazy</span> <span class="kt">val</span> <span class="kt">blocksGenerator:</span> <span class="kt">BlockGenerator</span> <span class="o">=</span>
<span class="kt">new</span> <span class="kt">BlockGenerator</span><span class="o">(</span><span class="kt">StorageLevel.MEMORY_ONLY_SER_2</span><span class="o">)</span>
<span class="kt">protected</span> <span class="kt">def</span> <span class="kt">onStart</span><span class="o">()</span> <span class="o">=</span> <span class="o">{</span>
<span class="n">blocksGenerator</span><span class="o">.</span><span class="n">start</span><span class="o">()</span>
<span class="k">val</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="k">val</span> <span class="n">dataInputStream</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="k">var</span> <span class="n">data</span><span class="k">:</span> <span class="kt">String</span> <span class="o">=</span> <span class="n">dataInputStream</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">data</span> <span class="o">!=</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
<span class="n">blocksGenerator</span> <span class="o">+=</span> <span class="n">data</span>
<span class="n">data</span> <span class="k">=</span> <span class="n">dataInputStream</span><span class="o">.</span><span class="n">readLine</span><span class="o">()</span>
<span class="o">}</span>
<span class="o">}</span>
<span class="k">protected</span> <span class="k">def</span> <span class="n">onStop</span><span class="o">()</span> <span class="o">{</span>
<span class="n">blocksGenerator</span><span class="o">.</span><span class="n">stop</span><span class="o">()</span>
<span class="o">}</span>
<span class="o">}</span>
</code></pre></div>
<p>All we did here is extended NetworkReceiver and called blockGenerator’s API method (i.e. +=) to push our blocks of data. Please refer to scala-docs of NetworkReceiver for more details.</p>
<h3 id="an-actor-as-receiver">An Actor as Receiver</h3>
<p>This starts with implementing <a href="#References">Actor</a></p>
<p>Following is a simple socket text-stream receiver, which is appearently overly simplified using Akka’s socket.io api.</p>
<div class="highlight"><pre><code class="scala"> <span class="k">class</span> <span class="nc">SocketTextStreamReceiver</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="n">bytesToString</span><span class="k">:</span> <span class="kt">ByteString</span> <span class="o">=></span> <span class="nc">String</span><span class="o">)</span> <span class="k">extends</span> <span class="nc">Actor</span> <span class="k">with</span> <span class="nc">Receiver</span> <span class="o">{</span>
<span class="k">override</span> <span class="k">def</span> <span class="n">preStart</span> <span class="k">=</span> <span class="nc">IOManager</span><span class="o">(</span><span class="n">context</span><span class="o">.</span><span class="n">system</span><span class="o">).</span><span class="n">connect</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">def</span> <span class="n">receive</span> <span class="k">=</span> <span class="o">{</span>
<span class="k">case</span> <span class="nc">IO</span><span class="o">.</span><span class="nc">Read</span><span class="o">(</span><span class="n">socket</span><span class="o">,</span> <span class="n">bytes</span><span class="o">)</span> <span class="k">=></span> <span class="n">pushBlock</span><span class="o">(</span><span class="n">bytesToString</span><span class="o">(</span><span class="n">bytes</span><span class="o">))</span>
<span class="o">}</span>
<span class="o">}</span>
</code></pre></div>
<p>All we did here is mixed in trait Receiver and called pushBlock api method to push our blocks of data. Please refer to scala-docs of Receiver for more details.</p>
<h3 id="a-sample-spark-application">A Sample Spark Application</h3>
<ul>
<li>First create a Spark streaming context with master url and batchduration.</li>
</ul>
<div class="highlight"><pre><code class="scala"> <span class="k">val</span> <span class="n">ssc</span> <span class="k">=</span> <span class="k">new</span> <span class="nc">StreamingContext</span><span class="o">(</span><span class="n">master</span><span class="o">,</span> <span class="s">"WordCountCustomStreamSource"</span><span class="o">,</span>
<span class="nc">Seconds</span><span class="o">(</span><span class="n">batchDuration</span><span class="o">))</span>
</code></pre></div>
<ul>
<li>Plug-in the custom receiver into the spark streaming context and create a DStream.</li>
</ul>
<div class="highlight"><pre><code class="scala"> <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">networkStream</span><span class="o">[</span><span class="kt">String</span><span class="o">](</span><span class="k">new</span> <span class="nc">SocketTextStreamReceiver</span><span class="o">(</span>
<span class="s">"localhost"</span><span class="o">,</span> <span class="mi">8445</span><span class="o">))</span>
</code></pre></div>
<ul>
<li>OR Plug-in the actor as receiver into the spark streaming context and create a DStream.</li>
</ul>
<div class="highlight"><pre><code class="scala"> <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">SocketTextStreamReceiver</span><span class="o">(</span>
<span class="s">"localhost"</span><span class="o">,</span><span class="mi">8445</span><span class="o">,</span> <span class="n">z</span> <span class="k">=></span> <span class="n">z</span><span class="o">.</span><span class="n">utf8String</span><span class="o">)),</span><span class="s">"SocketReceiver"</span><span class="o">)</span>
</code></pre></div>
<ul>
<li>Process it.</li>
</ul>
<div class="highlight"><pre><code class="scala"> <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="k">val</span> <span class="n">wordCounts</span> <span class="k">=</span> <span class="n">words</span><span class="o">.</span><span class="n">map</span><span class="o">(</span><span class="n">x</span> <span class="k">=></span> <span class="o">(</span><span class="n">x</span><span class="o">,</span> <span class="mi">1</span><span class="o">)).</span><span class="n">reduceByKey</span><span class="o">(</span><span class="k">_</span> <span class="o">+</span> <span class="k">_</span><span class="o">)</span>
<span class="n">wordCounts</span><span class="o">.</span><span class="n">print</span><span class="o">()</span>
<span class="n">ssc</span><span class="o">.</span><span class="n">start</span><span class="o">()</span>
</code></pre></div>
<ul>
<li>
<p>After processing it, stream can be tested using the netcat utility.</p>
<p>$ nc -l localhost 8445
hello world
hello hello</p>
</li>
</ul>
<h2 id="multiple-homogeneousheterogeneous-receivers">Multiple Homogeneous/Heterogeneous Receivers.</h2>
<p>A DStream union operation is provided for taking union on multiple input streams.</p>
<div class="highlight"><pre><code class="scala"> <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">SocketTextStreamReceiver</span><span class="o">(</span>
<span class="s">"localhost"</span><span class="o">,</span><span class="mi">8445</span><span class="o">,</span> <span class="n">z</span> <span class="k">=></span> <span class="n">z</span><span class="o">.</span><span class="n">utf8String</span><span class="o">)),</span><span class="s">"SocketReceiver"</span><span class="o">)</span>
<span class="c1">// Another socket stream receiver</span>
<span class="k">val</span> <span class="n">lines2</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">SocketTextStreamReceiver</span><span class="o">(</span>
<span class="s">"localhost"</span><span class="o">,</span><span class="mi">8446</span><span class="o">,</span> <span class="n">z</span> <span class="k">=></span> <span class="n">z</span><span class="o">.</span><span class="n">utf8String</span><span class="o">)),</span><span class="s">"SocketReceiver"</span><span class="o">)</span>
<span class="k">val</span> <span class="n">union</span> <span class="k">=</span> <span class="n">lines</span><span class="o">.</span><span class="n">union</span><span class="o">(</span><span class="n">lines2</span><span class="o">)</span>
</code></pre></div>
<p>Above stream can be easily process as described earlier.</p>
<p><em>A more comprehensive example is provided in the spark streaming examples</em></p>
<h2 id="references">References</h2>
<p>1.<a href="http://doc.akka.io/docs/akka/2.0.5/scala/actors.html">Akka Actor documentation</a>
2.<a href="api/streaming/index.html#org.apache.spark.streaming.dstream.NetworkReceiver">NetworkReceiver</a></p>
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