lukecwik commented on a change in pull request #13160:
URL: https://github.com/apache/beam/pull/13160#discussion_r510397882
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File path: website/www/site/content/en/documentation/programming-guide.md
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@@ -5143,3 +5143,282 @@ perUser.apply(ParDo.of(new DoFn<KV<String, ValueT>,
OutputT>() {
}
}));
{{< /highlight >}}
+
+## 12 Splittable DoFns {#splittable-dofns}
+
+Splittable DoFns (SDFs) enable users to create modular components containing
I/Os (and some advanced
+[non I/O use
cases](https://s.apache.org/splittable-do-fn#heading=h.5cep9s8k4fxv)). Having
modular
+I/O components that can be connected to each other simplify typical patterns
that users want.
+For example, a popular use case is to read filenames from a message queue
followed by parsing those
+files. Traditionally users were required to either write a single I/O
connector that contained the
+logic for the message queue and the file reader (increased complexity) or
choose to reuse a message
+queue I/O followed by a regular DoFn that read the file (decreased
performance). With splittable DoFns,
+we bring the richness of Apache Beam’s I/O APIs to DoFns enabling modularity
while maintaining the
+performance of traditional I/O connectors.
+
+### 12.1 Splittable DoFn basics {#splittable-dofn-basics}
+
+At a high level, a splittable DoFn is responsible for processing element and
restriction pairs. A
+restriction represents a subset of work that would have been necessary to have
been done when
+processing the element.
+
+Executing a splittable DoFn follows the following steps:
+
+1. Each element is paired with a restriction (e.g. filename is paired with
offset range representing the whole file).
+2. Each element and restriction pair is split (e.g. offset ranges are broken
up into smaller pieces).
+3. The runner redistributes the element and restriction pairs to several
workers.
+4. Element and restriction pairs are processed in parallel (e.g. the file is
read).
Review comment:
Below the diagram I have the explanation for the checkpoint/split. I
moved it to be part of the list.
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