[GitHub] [flink] kl0u commented on a change in pull request #10833: [FLINK-15535][documentation] Add usage of ProcessFunctionTestHarnesses for testing documentation

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kl0u commented on a change in pull request #10833: [FLINK-15535][documentation] Add usage of ProcessFunctionTestHarnesses for testing documentation
URL: https://github.com/apache/flink/pull/10833#discussion_r366269830
 
 

 ##########
 File path: docs/dev/stream/testing.md
 ##########
 @@ -332,6 +332,87 @@ Many more examples for the usage of these test harnesses can be found in the Fli
 
 <span class="label label-info">Note</span> Be aware that `AbstractStreamOperatorTestHarness` and its derived classes are currently not part of the public API and can be subject to change.
 
+### Unit Testing ProcessFunction
+
+The `ProcessFunction` is a widely used low-level and powerful stream processing operation, giving access to the basic building blocks of all (acyclic) streaming applications.
+Flink provides a test harness named `ProcessFunctionTestHarnesses` that can be used to test your `ProcessFunction`. Considering this example:
+
+<div class="codetabs" markdown="1">
+<div data-lang="java" markdown="1">
+{% highlight java %}
+public static class PassThroughProcessFunction extends ProcessFunction<Integer, Integer> {
+
+	@Override
+	public void processElement(Integer value, Context ctx, Collector<Integer> out) throws Exception {
+        out.collect(value);
+	}
+}
+{% endhighlight %}
+</div>
+
+<div data-lang="scala" markdown="1">
+{% highlight scala %}
+class PassThroughProcessFunction extends ProcessFunction[Integer, Integer] {
+
+    @throws[Exception]
+    override def processElement(value: Integer, ctx: ProcessFunction[Integer, Integer]#Context, out: Collector[Integer]): Unit = {
+      out.collect(value)
+    }
+}
+{% endhighlight %}
+</div>
+</div>
+
+It is very easy to unit test such a function with `ProcessFunctionTestHarnesses` by passing suitable arguments and verifying the output.
+
+<div class="codetabs" markdown="1">
+<div data-lang="java" markdown="1">
+{% highlight java %}
+public class PassThroughProcessFunctionTest {
+
+    @Test
+    public void testPassThrough() throws Exception {
+
+        //instantiate user-defined function
+        PassThroughProcessFunction processFunction = new PassThroughProcessFunction();
+
+        // wrap user defined function into a the corresponding operator
+        OneInputStreamOperatorTestHarness<Integer, Integer> harness = ProcessFunctionTestHarnesses
+        	.forProcessFunction(processFunction);
+
+        //push (timestamped) elements into the operator (and hence user defined function)
+        harness.processElement(1, 10);
+
+        //retrieve list of emitted records for assertions
+        assertEquals(harness.extractOutputValues(), Collections.singletonList(1));
+    }
+}
+{% endhighlight %}
+</div>
+
+<div data-lang="scala" markdown="1">
+{% highlight scala %}
+class PassThroughProcessFunctionTest extends FlatSpec with Matchers {
+
+  "PassThroughProcessFunction" should "forward values" in {
+
+    //instantiate user-defined function
+    val processFunction = new PassThroughProcessFunction
+
+    // wrap user defined function into a the corresponding operator
+    val harness = ProcessFunctionTestHarnesses.forProcessFunction(processFunction)
+
+    //push (timestamped) elements into the operator (and hence user defined function)
+    harness.processElement(1, 10)
+
+    //retrieve list of emitted records for assertions
+    harness.extractOutputValues() should contain (1)
+  }
+}
+{% endhighlight %}
+</div>
+</div>
+
 
 Review comment:
   Here I would add something like: "For more examples on how to use the `ProcessFunctionTestHarnesses` in order to test the different flavours of the `ProcessFunction`, e.g. `KeyedProcessFunction`, `KeyedCoProcessFunction`, `BroadcastProcessFunction`, etc, the user is encouraged to look at the `ProcessFunctionTestHarnessesTest`." 

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