davidwrede commented on a change in pull request #13317:
URL: https://github.com/apache/beam/pull/13317#discussion_r523299584
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File path: website/www/site/content/en/documentation/programming-guide.md
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@@ -5431,4 +5431,280 @@ use case.
{{< highlight py >}}
{{< code_sample "sdks/python/apache_beam/examples/snippets/snippets.py"
BundleFinalize >}}
-{{< /highlight >}}
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+{{< /highlight >}}
+
+## 13. Multi-language pipelines {#mulit-language-pipelines}
+
+Beam allows you to combine transforms written in any supported SDK language
(currently, Java and Python) and use them in one multi-language pipeline. This
capability makes it easy to provide new functionality simultaneously in
different Apache Beam SDKs through a single cross-language transform. For
example, the [Apache Kafka
connector](https://github.com/apache/beam/blob/master/sdks/python/apache_beam/io/kafka.py)
and [SQL
transform](https://github.com/apache/beam/blob/master/sdks/python/apache_beam/transforms/sql.py)
from the Java SDK can be used in Python streaming pipelines.
+
+Pipelines that use transforms from more than one SDK-language are known as
*multi-language pipelines*.
+
+### 13.1. Creating cross-language transforms {#create-x-lang-transforms}
+
+To make transforms written in one language available to pipelines written in
another language, an *expansion service* for that transform is used to create
and inject the appropriate language-specific pipeline fragments into your
pipeline.
+
+In the following example, a Python pipeline written the Apache Beam SDK for
Python starts up a local Java expansion service on your computer to create and
inject the appropriate Java pipeline fragments for executing the Java Kafka
cross-language transform into your Python pipeline. The SDK then downloads and
stages the necessary Java dependencies needed to execute these transforms.
+
+
+
+At runtime, the Beam runner will execute both Python and Java transforms to
execute your pipeline.
+
+In this section, we will use
[KafkaIO.Read](https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/io/kafka/KafkaIO.Read.html)
to illustrate how to create a cross-language transform for Java and a test
example for Python.
+
+#### 13.1.1. Creating cross-language Java transforms
+
+To make your Apache Beam Java SDK transform portable across SDK languages, you
must implement two interfaces:
[ExternalTransformBuilder](https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/ExternalTransformBuilder.java)
and
[ExternalTransformRegistrar](https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/expansion/ExternalTransformRegistrar.java).
The `ExternalTransformBuilder` interface constructs the cross-language
transform using configuration values passed in from the pipeline and the
`ExternalTransformRegistrar` interface registers the cross-language transform
for use with the expansion service.
+
+**Implementing the interfaces**
+
+1. Define a Builder class for your transform that implements the
`ExternalTransformBuilder` interface and overrides the `buildExternal` method
that will be used to build your transform object. Initial configuration values
for your transform should be defined in the `buildExternal` method. In most
cases, it is convenient to make the Java transform builder class implement
`ExternalTransformBuilder`.
+
+ > **Note:** `ExternalTransformBuilder` requires you to define a
configuration object (a simple POJO) to capture a set of parameters sent by
external SDKs to initiate the Java transform. Usually these parameters directly
map to constructor parameters of the Java transform.
+
+ {{< highlight >}}
[email protected]
+abstract static class Builder<K, V>
+ implements ExternalTransformBuilder<External.Configuration, PBegin,
PCollection<KV<K, V>>> {
+ abstract Builder<K, V> setConsumerConfig(Map<String, Object> config);
+
+ abstract Builder<K, V> setTopics(List<String> topics);
+
+ /** Remaining property declarations omitted for clarity. */
+
+ abstract Read<K, V> build();
+
+ @Override
+ public PTransform<PBegin, PCollection<KV<K, V>>> buildExternal(
+ External.Configuration config) {
+ ImmutableList.Builder<String> listBuilder = ImmutableList.builder();
+ for (String topic : config.topics) {
+ listBuilder.add(topic);
+ }
+ setTopics(listBuilder.build());
+
+ /** Remaining property defaults omitted for clarity. */
+ }
+}
+ {{< /highlight >}}
+2. Register the transform as an external cross-language transform by defining
a class that implements `ExternalTransformRegistrar`. You must annotate your
class with the `AutoService` annotation to ensure that your transform is
registered and instantiated properly by the expansion service.
+3. In your registrar class, define a Uniform Resource Name (URN) for your
transform. The URN must be a unique string that identifies your transform with
the expansion service.
+4. From within your registrar class, define a configuration class for the
parameters used during the initialization of your transform by the external SDK.
+
+ The following example from the KafkaIO transform shows how to implement
steps two through four:
+
+ {{< highlight >}}
+@AutoService(ExternalTransformRegistrar.class)
+public static class External implements ExternalTransformRegistrar {
+
+ public static final String URN = "beam:external:java:kafka:read:v1";
+
+ @Override
+ public Map<String, Class<? extends ExternalTransformBuilder<?, ?, ?>>>
knownBuilders() {
+ return ImmutableMap.of(
+ URN,
+ (Class<? extends ExternalTransformBuilder<?, ?, ?>>)
+ (Class<?>) AutoValue_KafkaIO_Read.Builder.class);
+ }
+
+ /** Parameters class to expose the Read transform to an external SDK. */
+ public static class Configuration {
+ private Map<String, String> consumerConfig;
+ private List<String> topics;
+
+
+ public void setConsumerConfig(Map<String, String> consumerConfig) {
+ this.consumerConfig = consumerConfig;
+ }
+
+ public void setTopics(List<String> topics) {
+ this.topics = topics;
+ }
+
+ /** Remaining properties omitted for clarity. */
+ }
+}
+ {{< /highlight >}}
+
+After you have implemented the `ExternalTransformBuilder` and
`ExternalTransformRegistrar` interfaces, your transform can be registered and
created successfully by the default Java expansion service.
+
+**Using the expansion service**
Review comment:
Good point. Done.
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