davidwrede commented on a change in pull request #13317:
URL: https://github.com/apache/beam/pull/13317#discussion_r523243100
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File path: website/www/site/content/en/documentation/programming-guide.md
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@@ -5431,4 +5431,282 @@ 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 and SQL transform 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);
Review comment:
I noted this in line 5452.
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