Github user tzulitai commented on a diff in the pull request:
https://github.com/apache/flink/pull/4454#discussion_r131060102
--- Diff: docs/dev/testing.md ---
@@ -0,0 +1,102 @@
+---
+title: "Testing"
+nav-parent_id: dev
+nav-pos: 110
+---
+<!--
+Licensed to the Apache Software Foundation (ASF) under one
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+"License"); you may not use this file except in compliance
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+ http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing,
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+"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+KIND, either express or implied. See the License for the
+specific language governing permissions and limitations
+under the License.
+-->
+
+This page briefly discusses how to test Flink application in the local
environment.
+
+* This will be replaced by the TOC
+{:toc}
+
+## Unit testing
+
+It is encouraged to test your classes with unit tests as much as possible.
For example if one implement following `ReduceFunction`:
+~~~java
+public class SumReduce implements ReduceFunction<Long> {
+ @Override
+ public Long reduce(Long value1, Long value2) throws Exception {
+ return value1 + value2;
+ }
+}
+~~~
+it is very easy to unit test it with your favorite framework:
+~~~java
+public class SumReduceTest {
+ @Test
+ public void testSum() throws Exception {
+ SumReduce sumReduce = new SumReduce();
+
+ assertEquals(42L, sumReduce.reduce(40L, 2L));
+ }
+}
+~~~
+
+## Integration testing
+
+You also can write integration tests that are executed against local Flink
mini cluster.
+In order to do so add a test dependency `flink-test-utils`. For example if
you want to
+test following `MapFunction`:
+
+~~~java
+public class MultiplyByTwo implements MapFunction<Long, Long> {
+ @Override
+ public Long map(Long value) throws Exception {
+ return value * 2;
+ }
+}
+~~~
+
+You could write following integration test:
+
+~~~java
+public class ExampleIntegrationTest extends
StreamingMultipleProgramsTestBase {
+ @Test
+ public void testSum() throws Exception {
+ StreamExecutionEnvironment env =
StreamExecutionEnvironment.getExecutionEnvironment();
+ env.setParallelism(1);
+
+ // values are collected on a static variable
+ CollectSink.values.clear();
+
+ env.fromElements(1L, 21L, 22L)
+ .map(new MultiplyByTwo())
+ .addSink(new CollectSink());
+ env.execute();
+
+ assertEquals(Lists.newArrayList(2L, 42L, 44L), CollectSink.values);
+ }
+
+ private static class CollectSink implements SinkFunction<Long> {
+ // must be static
+ public static final List<Long> values = new ArrayList<>();
+
+ @Override
+ public void invoke(Long value) throws Exception {
+ values.add(value);
+ }
+ }
+}
+~~~
+
+Static variable in `CollectSink` is required because Flink serializes all
operators before distributing them across a cluster.
--- End diff --
I find this instruction a bit too specific. There's other ways to achieve
this, such as just initializing it in the open() method.
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