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<h1>A Guide for Unit Testing in Apache Flink</h1>
<p><i></i></p>
<article>
<p>07 Feb 2020 Kartik Khare (<a href="https://twitter.com/khare_khote">@khare_khote</a>)</p>
<p>Writing unit tests is one of the essential tasks of designing a production-grade application. Without tests, a single change in code can result in cascades of failure in production. Thus unit tests should be written for all types of applications, be it a simple job cleaning data and training a model or a complex multi-tenant, real-time data processing system. In the following sections, we provide a guide for unit testing of Apache Flink applications.
Apache Flink provides a robust unit testing framework to make sure your applications behave in production as expected during development. You need to include the following dependencies to utilize the provided framework.</p>
<div class="highlight"><pre><code class="language-xml"><span class="nt">&lt;dependency&gt;</span>
<span class="nt">&lt;groupId&gt;</span>org.apache.flink<span class="nt">&lt;/groupId&gt;</span>
<span class="nt">&lt;artifactId&gt;</span>flink-test-utils_${scala.binary.version}<span class="nt">&lt;/artifactId&gt;</span>
<span class="nt">&lt;version&gt;</span>${flink.version}<span class="nt">&lt;/version&gt;</span>
<span class="nt">&lt;scope&gt;</span>test<span class="nt">&lt;/scope&gt;</span>
<span class="nt">&lt;/dependency&gt;</span>
<span class="nt">&lt;dependency&gt;</span>
<span class="nt">&lt;groupId&gt;</span>org.apache.flink<span class="nt">&lt;/groupId&gt;</span>
<span class="nt">&lt;artifactId&gt;</span>flink-runtime_2.11<span class="nt">&lt;/artifactId&gt;</span>
<span class="nt">&lt;version&gt;</span>1.9.0<span class="nt">&lt;/version&gt;</span>
<span class="nt">&lt;scope&gt;</span>test<span class="nt">&lt;/scope&gt;</span>
<span class="nt">&lt;classifier&gt;</span>tests<span class="nt">&lt;/classifier&gt;</span>
<span class="nt">&lt;/dependency&gt;</span>
<span class="nt">&lt;dependency&gt;</span>
<span class="nt">&lt;groupId&gt;</span>org.apache.flink<span class="nt">&lt;/groupId&gt;</span>
<span class="nt">&lt;artifactId&gt;</span>flink-streaming-java_2.11<span class="nt">&lt;/artifactId&gt;</span>
<span class="nt">&lt;version&gt;</span>1.9.0<span class="nt">&lt;/version&gt;</span>
<span class="nt">&lt;scope&gt;</span>test<span class="nt">&lt;/scope&gt;</span>
<span class="nt">&lt;classifier&gt;</span>tests<span class="nt">&lt;/classifier&gt;</span>
<span class="nt">&lt;/dependency&gt;</span></code></pre></div>
<p>The strategy of writing unit tests differs for various operators. You can break down the strategy into the following three buckets:</p>
<ul>
<li>Stateless Operators</li>
<li>Stateful Operators</li>
<li>Timed Process Operators</li>
</ul>
<h1 id="stateless-operators">Stateless Operators</h1>
<p>Writing unit tests for a stateless operator is a breeze. You need to follow the basic norm of writing a test case, i.e., create an instance of the function class and test the appropriate methods. Let’s take an example of a simple <code>Map</code> operator.</p>
<div class="highlight"><pre><code class="language-java"><span class="kd">public</span> <span class="kd">class</span> <span class="nc">MyStatelessMap</span> <span class="kd">implements</span> <span class="n">MapFunction</span><span class="o">&lt;</span><span class="n">String</span><span class="o">,</span> <span class="n">String</span><span class="o">&gt;</span> <span class="o">{</span>
<span class="nd">@Override</span>
<span class="kd">public</span> <span class="n">String</span> <span class="nf">map</span><span class="o">(</span><span class="n">String</span> <span class="n">in</span><span class="o">)</span> <span class="kd">throws</span> <span class="n">Exception</span> <span class="o">{</span>
<span class="n">String</span> <span class="n">out</span> <span class="o">=</span> <span class="s">&quot;hello &quot;</span> <span class="o">+</span> <span class="n">in</span><span class="o">;</span>
<span class="k">return</span> <span class="n">out</span><span class="o">;</span>
<span class="o">}</span>
<span class="o">}</span></code></pre></div>
<p>The test case for the above operator should look like</p>
<div class="highlight"><pre><code class="language-java"><span class="nd">@Test</span>
<span class="kd">public</span> <span class="kt">void</span> <span class="nf">testMap</span><span class="o">()</span> <span class="kd">throws</span> <span class="n">Exception</span> <span class="o">{</span>
<span class="n">MyStatelessMap</span> <span class="n">statelessMap</span> <span class="o">=</span> <span class="k">new</span> <span class="nf">MyStatelessMap</span><span class="o">();</span>
<span class="n">String</span> <span class="n">out</span> <span class="o">=</span> <span class="n">statelessMap</span><span class="o">.</span><span class="na">map</span><span class="o">(</span><span class="s">&quot;world&quot;</span><span class="o">);</span>
<span class="n">Assert</span><span class="o">.</span><span class="na">assertEquals</span><span class="o">(</span><span class="s">&quot;hello world&quot;</span><span class="o">,</span> <span class="n">out</span><span class="o">);</span>
<span class="o">}</span></code></pre></div>
<p>Pretty simple, right? Let’s take a look at one for the <code>FlatMap</code> operator.</p>
<div class="highlight"><pre><code class="language-java"><span class="kd">public</span> <span class="kd">class</span> <span class="nc">MyStatelessFlatMap</span> <span class="kd">implements</span> <span class="n">FlatMapFunction</span><span class="o">&lt;</span><span class="n">String</span><span class="o">,</span> <span class="n">String</span><span class="o">&gt;</span> <span class="o">{</span>
<span class="nd">@Override</span>
<span class="kd">public</span> <span class="kt">void</span> <span class="nf">flatMap</span><span class="o">(</span><span class="n">String</span> <span class="n">in</span><span class="o">,</span> <span class="n">Collector</span><span class="o">&lt;</span><span class="n">String</span><span class="o">&gt;</span> <span class="n">collector</span><span class="o">)</span> <span class="kd">throws</span> <span class="n">Exception</span> <span class="o">{</span>
<span class="n">String</span> <span class="n">out</span> <span class="o">=</span> <span class="s">&quot;hello &quot;</span> <span class="o">+</span> <span class="n">in</span><span class="o">;</span>
<span class="n">collector</span><span class="o">.</span><span class="na">collect</span><span class="o">(</span><span class="n">out</span><span class="o">);</span>
<span class="o">}</span>
<span class="o">}</span></code></pre></div>
<p><code>FlatMap</code> operators require a <code>Collector</code> object along with the input. For the test case, we have two options:</p>
<ol>
<li>Mock the <code>Collector</code> object using Mockito</li>
<li>Use the <code>ListCollector</code> provided by Flink</li>
</ol>
<p>I prefer the second method as it requires fewer lines of code and is suitable for most of the cases.</p>
<div class="highlight"><pre><code class="language-java"><span class="nd">@Test</span>
<span class="kd">public</span> <span class="kt">void</span> <span class="nf">testFlatMap</span><span class="o">()</span> <span class="kd">throws</span> <span class="n">Exception</span> <span class="o">{</span>
<span class="n">MyStatelessFlatMap</span> <span class="n">statelessFlatMap</span> <span class="o">=</span> <span class="k">new</span> <span class="nf">MyStatelessFlatMap</span><span class="o">();</span>
<span class="n">List</span><span class="o">&lt;</span><span class="n">String</span><span class="o">&gt;</span> <span class="n">out</span> <span class="o">=</span> <span class="k">new</span> <span class="n">ArrayList</span><span class="o">&lt;&gt;();</span>
<span class="n">ListCollector</span><span class="o">&lt;</span><span class="n">String</span><span class="o">&gt;</span> <span class="n">listCollector</span> <span class="o">=</span> <span class="k">new</span> <span class="n">ListCollector</span><span class="o">&lt;&gt;(</span><span class="n">out</span><span class="o">);</span>
<span class="n">statelessFlatMap</span><span class="o">.</span><span class="na">flatMap</span><span class="o">(</span><span class="s">&quot;world&quot;</span><span class="o">,</span> <span class="n">listCollector</span><span class="o">);</span>
<span class="n">Assert</span><span class="o">.</span><span class="na">assertEquals</span><span class="o">(</span><span class="n">Lists</span><span class="o">.</span><span class="na">newArrayList</span><span class="o">(</span><span class="s">&quot;hello world&quot;</span><span class="o">),</span> <span class="n">out</span><span class="o">);</span>
<span class="o">}</span></code></pre></div>
<h1 id="stateful-operators">Stateful Operators</h1>
<p>Writing test cases for stateful operators requires more effort. You need to check whether the operator state is updated correctly and if it is cleaned up properly along with the output of the operator.</p>
<p>Let’s take an example of stateful <code>FlatMap</code> function</p>
<div class="highlight"><pre><code class="language-java"><span class="kd">public</span> <span class="kd">class</span> <span class="nc">StatefulFlatMap</span> <span class="kd">extends</span> <span class="n">RichFlatMapFunction</span><span class="o">&lt;</span><span class="n">String</span><span class="o">,</span> <span class="n">String</span><span class="o">&gt;</span> <span class="o">{</span>
<span class="n">ValueState</span><span class="o">&lt;</span><span class="n">String</span><span class="o">&gt;</span> <span class="n">previousInput</span><span class="o">;</span>
<span class="nd">@Override</span>
<span class="kd">public</span> <span class="kt">void</span> <span class="nf">open</span><span class="o">(</span><span class="n">Configuration</span> <span class="n">parameters</span><span class="o">)</span> <span class="kd">throws</span> <span class="n">Exception</span> <span class="o">{</span>
<span class="n">previousInput</span> <span class="o">=</span> <span class="n">getRuntimeContext</span><span class="o">().</span><span class="na">getState</span><span class="o">(</span>
<span class="k">new</span> <span class="n">ValueStateDescriptor</span><span class="o">&lt;</span><span class="n">String</span><span class="o">&gt;(</span><span class="s">&quot;previousInput&quot;</span><span class="o">,</span> <span class="n">Types</span><span class="o">.</span><span class="na">STRING</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="nf">flatMap</span><span class="o">(</span><span class="n">String</span> <span class="n">in</span><span class="o">,</span> <span class="n">Collector</span><span class="o">&lt;</span><span class="n">String</span><span class="o">&gt;</span> <span class="n">collector</span><span class="o">)</span> <span class="kd">throws</span> <span class="n">Exception</span> <span class="o">{</span>
<span class="n">String</span> <span class="n">out</span> <span class="o">=</span> <span class="s">&quot;hello &quot;</span> <span class="o">+</span> <span class="n">in</span><span class="o">;</span>
<span class="k">if</span><span class="o">(</span><span class="n">previousInput</span><span class="o">.</span><span class="na">value</span><span class="o">()</span> <span class="o">!=</span> <span class="kc">null</span><span class="o">){</span>
<span class="n">out</span> <span class="o">=</span> <span class="n">out</span> <span class="o">+</span> <span class="s">&quot; &quot;</span> <span class="o">+</span> <span class="n">previousInput</span><span class="o">.</span><span class="na">value</span><span class="o">();</span>
<span class="o">}</span>
<span class="n">previousInput</span><span class="o">.</span><span class="na">update</span><span class="o">(</span><span class="n">in</span><span class="o">);</span>
<span class="n">collector</span><span class="o">.</span><span class="na">collect</span><span class="o">(</span><span class="n">out</span><span class="o">);</span>
<span class="o">}</span>
<span class="o">}</span></code></pre></div>
<p>The intricate part of writing tests for the above class is to mock the configuration as well as the runtime context of the application. Flink provides TestHarness classes so that users don’t have to create the mock objects themselves. Using the <code>KeyedOperatorHarness</code>, the test looks like:</p>
<div class="highlight"><pre><code class="language-java"><span class="kn">import</span> <span class="nn">org.apache.flink.streaming.api.operators.StreamFlatMap</span><span class="o">;</span>
<span class="kn">import</span> <span class="nn">org.apache.flink.streaming.runtime.streamrecord.StreamRecord</span><span class="o">;</span>
<span class="kn">import</span> <span class="nn">org.apache.flink.streaming.util.KeyedOneInputStreamOperatorTestHarness</span><span class="o">;</span>
<span class="kn">import</span> <span class="nn">org.apache.flink.streaming.util.OneInputStreamOperatorTestHarness</span><span class="o">;</span>
<span class="nd">@Test</span>
<span class="kd">public</span> <span class="kt">void</span> <span class="nf">testFlatMap</span><span class="o">()</span> <span class="kd">throws</span> <span class="n">Exception</span><span class="o">{</span>
<span class="n">StatefulFlatMap</span> <span class="n">statefulFlatMap</span> <span class="o">=</span> <span class="k">new</span> <span class="nf">StatefulFlatMap</span><span class="o">();</span>
<span class="c1">// OneInputStreamOperatorTestHarness takes the input and output types as type parameters </span>
<span class="n">OneInputStreamOperatorTestHarness</span><span class="o">&lt;</span><span class="n">String</span><span class="o">,</span> <span class="n">String</span><span class="o">&gt;</span> <span class="n">testHarness</span> <span class="o">=</span>
<span class="c1">// KeyedOneInputStreamOperatorTestHarness takes three arguments:</span>
<span class="c1">// Flink operator object, key selector and key type</span>
<span class="k">new</span> <span class="n">KeyedOneInputStreamOperatorTestHarness</span><span class="o">&lt;&gt;(</span>
<span class="k">new</span> <span class="n">StreamFlatMap</span><span class="o">&lt;&gt;(</span><span class="n">statefulFlatMap</span><span class="o">),</span> <span class="n">x</span> <span class="o">-&gt;</span> <span class="s">&quot;1&quot;</span><span class="o">,</span> <span class="n">Types</span><span class="o">.</span><span class="na">STRING</span><span class="o">);</span>
<span class="n">testHarness</span><span class="o">.</span><span class="na">open</span><span class="o">();</span>
<span class="c1">// test first record</span>
<span class="n">testHarness</span><span class="o">.</span><span class="na">processElement</span><span class="o">(</span><span class="s">&quot;world&quot;</span><span class="o">,</span> <span class="mi">10</span><span class="o">);</span>
<span class="n">ValueState</span><span class="o">&lt;</span><span class="n">String</span><span class="o">&gt;</span> <span class="n">previousInput</span> <span class="o">=</span>
<span class="n">statefulFlatMap</span><span class="o">.</span><span class="na">getRuntimeContext</span><span class="o">().</span><span class="na">getState</span><span class="o">(</span>
<span class="k">new</span> <span class="n">ValueStateDescriptor</span><span class="o">&lt;&gt;(</span><span class="s">&quot;previousInput&quot;</span><span class="o">,</span> <span class="n">Types</span><span class="o">.</span><span class="na">STRING</span><span class="o">));</span>
<span class="n">String</span> <span class="n">stateValue</span> <span class="o">=</span> <span class="n">previousInput</span><span class="o">.</span><span class="na">value</span><span class="o">();</span>
<span class="n">Assert</span><span class="o">.</span><span class="na">assertEquals</span><span class="o">(</span>
<span class="n">Lists</span><span class="o">.</span><span class="na">newArrayList</span><span class="o">(</span><span class="k">new</span> <span class="n">StreamRecord</span><span class="o">&lt;&gt;(</span><span class="s">&quot;hello world&quot;</span><span class="o">,</span> <span class="mi">10</span><span class="o">)),</span>
<span class="n">testHarness</span><span class="o">.</span><span class="na">extractOutputStreamRecords</span><span class="o">());</span>
<span class="n">Assert</span><span class="o">.</span><span class="na">assertEquals</span><span class="o">(</span><span class="s">&quot;world&quot;</span><span class="o">,</span> <span class="n">stateValue</span><span class="o">);</span>
<span class="c1">// test second record</span>
<span class="n">testHarness</span><span class="o">.</span><span class="na">processElement</span><span class="o">(</span><span class="s">&quot;parallel&quot;</span><span class="o">,</span> <span class="mi">20</span><span class="o">);</span>
<span class="n">Assert</span><span class="o">.</span><span class="na">assertEquals</span><span class="o">(</span>
<span class="n">Lists</span><span class="o">.</span><span class="na">newArrayList</span><span class="o">(</span>
<span class="k">new</span> <span class="n">StreamRecord</span><span class="o">&lt;&gt;(</span><span class="s">&quot;hello world&quot;</span><span class="o">,</span> <span class="mi">10</span><span class="o">),</span>
<span class="k">new</span> <span class="n">StreamRecord</span><span class="o">&lt;&gt;(</span><span class="s">&quot;hello parallel world&quot;</span><span class="o">,</span> <span class="mi">20</span><span class="o">)),</span>
<span class="n">testHarness</span><span class="o">.</span><span class="na">extractOutputStreamRecords</span><span class="o">());</span>
<span class="n">Assert</span><span class="o">.</span><span class="na">assertEquals</span><span class="o">(</span><span class="s">&quot;parallel&quot;</span><span class="o">,</span> <span class="n">previousInput</span><span class="o">.</span><span class="na">value</span><span class="o">());</span>
<span class="o">}</span></code></pre></div>
<p>The test harness provides many helper methods, three of which are being used here:</p>
<ol>
<li><code>open</code>: calls the open of the <code>FlatMap</code> function with relevant parameters. It also initializes the context.</li>
<li><code>processElement</code>: allows users to pass an input element as well as the timestamp associated with the element.</li>
<li><code>extractOutputStreamRecords</code>: gets the output records along with their timestamps from the <code>Collector</code>.</li>
</ol>
<p>The test harness simplifies the unit testing for the stateful functions to a large extent.</p>
<p>You might also need to check whether the state value is being set correctly. You can get the state value directly from the operator using a mechanism similar to the one used while creating the state. This is also demonstrated in the previous example.</p>
<h1 id="timed-process-operators">Timed Process Operators</h1>
<p>Writing tests for process functions, that work with time, is quite similar to writing tests for stateful functions because you can also use test harness.
However, you need to take care of another aspect, which is providing timestamps for events and controlling the current time of the application. By setting the current (processing or event) time, you can trigger registered timers, which will call the <code>onTimer</code> method of the function</p>
<div class="highlight"><pre><code class="language-java"><span class="kd">public</span> <span class="kd">class</span> <span class="nc">MyProcessFunction</span> <span class="kd">extends</span> <span class="n">KeyedProcessFunction</span><span class="o">&lt;</span><span class="n">String</span><span class="o">,</span> <span class="n">String</span><span class="o">,</span> <span class="n">String</span><span class="o">&gt;</span> <span class="o">{</span>
<span class="nd">@Override</span>
<span class="kd">public</span> <span class="kt">void</span> <span class="nf">processElement</span><span class="o">(</span><span class="n">String</span> <span class="n">in</span><span class="o">,</span> <span class="n">Context</span> <span class="n">context</span><span class="o">,</span> <span class="n">Collector</span><span class="o">&lt;</span><span class="n">String</span><span class="o">&gt;</span> <span class="n">collector</span><span class="o">)</span> <span class="kd">throws</span> <span class="n">Exception</span> <span class="o">{</span>
<span class="n">context</span><span class="o">.</span><span class="na">timerService</span><span class="o">().</span><span class="na">registerProcessingTimeTimer</span><span class="o">(</span><span class="mi">50</span><span class="o">);</span>
<span class="n">String</span> <span class="n">out</span> <span class="o">=</span> <span class="s">&quot;hello &quot;</span> <span class="o">+</span> <span class="n">in</span><span class="o">;</span>
<span class="n">collector</span><span class="o">.</span><span class="na">collect</span><span class="o">(</span><span class="n">out</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="nf">onTimer</span><span class="o">(</span><span class="kt">long</span> <span class="n">timestamp</span><span class="o">,</span> <span class="n">OnTimerContext</span> <span class="n">ctx</span><span class="o">,</span> <span class="n">Collector</span><span class="o">&lt;</span><span class="n">String</span><span class="o">&gt;</span> <span class="n">out</span><span class="o">)</span> <span class="kd">throws</span> <span class="n">Exception</span> <span class="o">{</span>
<span class="n">out</span><span class="o">.</span><span class="na">collect</span><span class="o">(</span><span class="n">String</span><span class="o">.</span><span class="na">format</span><span class="o">(</span><span class="s">&quot;Timer triggered at timestamp %d&quot;</span><span class="o">,</span> <span class="n">timestamp</span><span class="o">));</span>
<span class="o">}</span>
<span class="o">}</span></code></pre></div>
<p>We need to test both the methods in the <code>KeyedProcessFunction</code>, i.e., <code>processElement</code> as well as <code>onTimer</code>. Using a test harness, we can control the current time of the function. Thus, we can trigger the timer at will rather than waiting for a specific time.</p>
<p>Let’s take a look at the test case</p>
<div class="highlight"><pre><code class="language-java"><span class="nd">@Test</span>
<span class="kd">public</span> <span class="kt">void</span> <span class="nf">testProcessElement</span><span class="o">()</span> <span class="kd">throws</span> <span class="n">Exception</span><span class="o">{</span>
<span class="n">MyProcessFunction</span> <span class="n">myProcessFunction</span> <span class="o">=</span> <span class="k">new</span> <span class="nf">MyProcessFunction</span><span class="o">();</span>
<span class="n">OneInputStreamOperatorTestHarness</span><span class="o">&lt;</span><span class="n">String</span><span class="o">,</span> <span class="n">String</span><span class="o">&gt;</span> <span class="n">testHarness</span> <span class="o">=</span>
<span class="k">new</span> <span class="n">KeyedOneInputStreamOperatorTestHarness</span><span class="o">&lt;&gt;(</span>
<span class="k">new</span> <span class="n">KeyedProcessOperator</span><span class="o">&lt;&gt;(</span><span class="n">myProcessFunction</span><span class="o">),</span> <span class="n">x</span> <span class="o">-&gt;</span> <span class="s">&quot;1&quot;</span><span class="o">,</span> <span class="n">Types</span><span class="o">.</span><span class="na">STRING</span><span class="o">);</span>
<span class="c1">// Function time is initialized to 0</span>
<span class="n">testHarness</span><span class="o">.</span><span class="na">open</span><span class="o">();</span>
<span class="n">testHarness</span><span class="o">.</span><span class="na">processElement</span><span class="o">(</span><span class="s">&quot;world&quot;</span><span class="o">,</span> <span class="mi">10</span><span class="o">);</span>
<span class="n">Assert</span><span class="o">.</span><span class="na">assertEquals</span><span class="o">(</span>
<span class="n">Lists</span><span class="o">.</span><span class="na">newArrayList</span><span class="o">(</span><span class="k">new</span> <span class="n">StreamRecord</span><span class="o">&lt;&gt;(</span><span class="s">&quot;hello world&quot;</span><span class="o">,</span> <span class="mi">10</span><span class="o">)),</span>
<span class="n">testHarness</span><span class="o">.</span><span class="na">extractOutputStreamRecords</span><span class="o">());</span>
<span class="o">}</span>
<span class="nd">@Test</span>
<span class="kd">public</span> <span class="kt">void</span> <span class="nf">testOnTimer</span><span class="o">()</span> <span class="kd">throws</span> <span class="n">Exception</span> <span class="o">{</span>
<span class="n">MyProcessFunction</span> <span class="n">myProcessFunction</span> <span class="o">=</span> <span class="k">new</span> <span class="nf">MyProcessFunction</span><span class="o">();</span>
<span class="n">OneInputStreamOperatorTestHarness</span><span class="o">&lt;</span><span class="n">String</span><span class="o">,</span> <span class="n">String</span><span class="o">&gt;</span> <span class="n">testHarness</span> <span class="o">=</span>
<span class="k">new</span> <span class="n">KeyedOneInputStreamOperatorTestHarness</span><span class="o">&lt;&gt;(</span>
<span class="k">new</span> <span class="n">KeyedProcessOperator</span><span class="o">&lt;&gt;(</span><span class="n">myProcessFunction</span><span class="o">),</span> <span class="n">x</span> <span class="o">-&gt;</span> <span class="s">&quot;1&quot;</span><span class="o">,</span> <span class="n">Types</span><span class="o">.</span><span class="na">STRING</span><span class="o">);</span>
<span class="n">testHarness</span><span class="o">.</span><span class="na">open</span><span class="o">();</span>
<span class="n">testHarness</span><span class="o">.</span><span class="na">processElement</span><span class="o">(</span><span class="s">&quot;world&quot;</span><span class="o">,</span> <span class="mi">10</span><span class="o">);</span>
<span class="n">Assert</span><span class="o">.</span><span class="na">assertEquals</span><span class="o">(</span><span class="mi">1</span><span class="o">,</span> <span class="n">testHarness</span><span class="o">.</span><span class="na">numProcessingTimeTimers</span><span class="o">());</span>
<span class="c1">// Function time is set to 50</span>
<span class="n">testHarness</span><span class="o">.</span><span class="na">setProcessingTime</span><span class="o">(</span><span class="mi">50</span><span class="o">);</span>
<span class="n">Assert</span><span class="o">.</span><span class="na">assertEquals</span><span class="o">(</span>
<span class="n">Lists</span><span class="o">.</span><span class="na">newArrayList</span><span class="o">(</span>
<span class="k">new</span> <span class="n">StreamRecord</span><span class="o">&lt;&gt;(</span><span class="s">&quot;hello world&quot;</span><span class="o">,</span> <span class="mi">10</span><span class="o">),</span>
<span class="k">new</span> <span class="n">StreamRecord</span><span class="o">&lt;&gt;(</span><span class="s">&quot;Timer triggered at timestamp 50&quot;</span><span class="o">)),</span>
<span class="n">testHarness</span><span class="o">.</span><span class="na">extractOutputStreamRecords</span><span class="o">());</span>
<span class="o">}</span></code></pre></div>
<p>The mechanism to test the multi-input stream operators such as CoProcess functions is similar to the ones described in this article. You should use the TwoInput variant of the harness for these operators, such as <code>TwoInputStreamOperatorTestHarness</code>.</p>
<h1 id="summary">Summary</h1>
<p>In the previous sections we showcased how unit testing in Apache Flink works for stateless, stateful and times-aware-operators. We hope you found the steps easy to follow and execute while developing your Flink applications. If you have any questions or feedback you can reach out to me <a href="https://www.kharekartik.dev/about/">here</a> or contact the community on the <a href="https://flink.apache.org/community.html">Apache Flink user mailing list</a>.</p>
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